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Q&A Spotlight

Just How Big Should a Photovoltaic Array Be?

Some residential PV systems will produce substantially more power than the house needs

This Massachusetts house has photovoltaic capacity of 18 kW. According to the builder, R. Carter Scott, the PV array is large enough to power a plug-in car as well as the house.
Image Credit: Transformations Inc.

Kevin Dickson has come across an article about a high-performance house in Massachusetts that has got him wondering whether big photovoltaic systems are overtaking Passivhaus to become the next big trend in high-efficiency building.

The house is the work of R. Carter Scott and a design team that included Betsy Pettit and Joe Lstiburek of Building Science Corp., among a number of other experts. It was one of eight in Devens, Mass., that Scott’s company, Transformations Inc., was chosen to build for MassDevelopment, the state’s finance and development authority.

Among the many energy features of this net-zero energy home is an 18-kW photovoltaic array that completely covers the roof on the back of the house, including the porch and garage.

“Wow,” Dickson writes in a post at GreenBuildingAdvisor’s Q&A forum. “Does anyone agree with me that this is a stronger trend than Passivhuas requirements for new construction?”

Grid-tied PV is a standard part of net-zero designs, but at 18 kW, this system would dwarf most residential installations. The question at the heart of this Q&A Spotlight is whether falling PV prices and a variety of incentives such as Solar Renewable Energy Certificates have affected the way builders and homeowners make decisions about solar electricity.

Should PV arrays be designed solely to get a house across the net-zero energy threshold, or can PV serve another other purposes?

More power than a Passivhaus design would need

The PV panels on the Devens house should generate an estimated 10,200 kWh more electricity per year than it consumes, enough to power a Nissan Leaf or Chevy Volt for 30,000 miles, Scott has estimated.

As Mike Eliason points out, if the house were built to the Passivhaus standard and met its energy consumption requirement, it would consume no more than 6,828 kWh in a year.

So is the PV array overkill?

“Low PV prices, good PV incentives, and favorable feed-in tariffs allow homeowners to consider the installation of a very large PV array,” writes GBA senior editor Martin Holladay. “And the investment has a very quick financial return. If you can afford it, why not?”

“When it comes to Passivhaus certification,” Holladay adds, “it certainly seems optional or unnecessary if you already have a net-zero energy house. Why bother?”

Not a case of PV vs. Passivhaus

To John Semmelhack, Scott has found a “sweet spot” with “superinsulated saltboxes just a notch below Passivhaus efficiency.”

“At this point,” Semmelhack writes, “the giant PV systems here seem to be just as much a function of distorted incentives as low PV prices. The slow, regulated utilities (financed by Massachusetts ratepayers) are overpaying for the ‘solar attribute’ through high SREC prices and the state is kicking in unnecessary tax credits and rebates. Small, nimble businesses, such as Transformations, are stepping in to make some money while the gettin’ is good. I would do the same in their shoes.”

Semmelhack thinks the “relatively low” prices for the houses are made possible in part by the homeowner turning over all tax credits, rebates and SREC income from the PV system to the builder or, alternately, leasing the PV system from the builder.

“Brilliant strategy, I think. In addition, as a clever builder and business owner, Scott has integrated the PV installation into his own company, instead of outsourcing. Cheers to Carter.”

More PV, less conservation

Sonny Chatum’s answer to Kevin Dickson’s original question would be ‘yes.’

“Throwing up PV is a stronger trend than ANY sort of substantial, residential energy footprint reduction, I would bet, at least in the new residential/retrofit mainstream,” Chatum writes.

“For my state, I have personally and literally seen stack upon stack of applications for SRECs. There’s no way most of these people are substantially reducing their electricity demand (forget Pasivhaus) prior to silicon glazing their roofs. I would also bet that many of these people feel comfortable using MORE electricity after PV installation, since some of their previous bill has been offset by the PV.

“Sure,” he adds, “the PV helps the utilities meet renewable energy portfolio requirements, but it’s drops in a bucket with holes in it.”

An array as large as 18 kW also requires special approval from the utility, Dennis Heidner surmises, because it’s greater than the 10 kW normally allowed for a neighborhood transformer.

“There are 50+ panels on the house, probably 260w each, set up at different angles,” he says.

“Since this is larger than normally allowed net metering agreements, the house was designed…for the purpose of selling the power commercially back to the utility,” Heidner writes. “Perhaps taking advantage of good feed-in tariffs.

“Using the roof top as a source of revenue is actually a very clever and good idea,” he continues. “The solar on the roof was never about driving the consumption in the house to zero. It would be to compensate for the swimming pool, a car, and bring in extra revenue, etc.”

Our expert’s opinion

Here’s how GBA technical director Peter Yost views it:

I think pitting enclosure and HVAC efficiency against PV is inherently problematic. It’s not an either/or situation.

We all know that it’s Efficiency First, that efficiency dollars yield the quickest and best return on the investment, for the individual home and for the greater public. And we should have policies and incentives in place to promote and encourage efficiency as much if not more than renewables.

But if PV arrays don’t preclude subsequent efficiency work, and the opportunity for any size array presents itself, it should be squarely on the table. You can often interest a client in a non-invasive rooftop PV system over gutting the interior, but again, this is really an apples and oranges choice in my opinion.

Frankly, I favor centralized, typically ground-mount systems: large arrays (500 kW or more) perfectly situated and oriented for maximum gain, with three tilt-angle settings easily manually adjusted, seasonally. These community or large commercial arrays have much better economies of scale overall. Our town school system just signed an agreement for such a system, with land being leased from local landowners and investors with “tax appetite” financing the system to take advantage of tax underwriting that our (non-profit) municipality can’t.

So, residential roof top PV systems should be as big as the project and roof can bear, but for my money, let’s have sun harvesting more like what plants do: large fields that feed the grid more mightily than many tiny green PV roofs!

34 Comments

  1. Eric Sandeen | | #1

    One downside to mega-installs
    One downside to mega-installs, depending on the incentive structure, is that it concentrates all that public money on one individual. I've seen it around here; there's a limited pot, and one warehouse company gobbled up 3 or 4 x 40kW installs-worth of incentives in one year, because hey why not it's (almost) free? Meanwhile small residential owners missed the boat - sorry, money's gone.

    I guess there's nothing technically wrong with that; certainly it's within the letter of the law. But I'd rather see solar sprinkled out in a distributed fashion, with maybe 100 new arrays in 20 residential neighborhoods, rather than 4 giant invisible arrays on warehouses. Same amount of clean energy, but less momentum.

    Regarding consumption, speaking for myself at least, PV has made me more power-aware, but maybe that's because our array currently covers only 70% of our use annually, and I'm always chasing that elusive net-positive month.

  2. Jin Kazama | | #2

    Eric Sandeen : problem with
    Eric Sandeen : problem with spreading smaller arrays is the installation costs and payback gets thrown off ...

  3. Anne Decker | | #3

    PV farms....
    While I agree with the expert's opinion generally, I disagree with his last point. My husband and I are having the same argument right now as we consider the extent of our planned solar array (on our planned Passive House). He, too, believes that solar arrays ought to be centralized; however, I'm of the mind that open land is a valuable resource and that rooftops are perfectly suited to the task of collecting solar electricity. Why should we put giant PV arrays across landscapes that could be used for community gardening or pastures for livestock? You can't grow vegetables or raise animals on the average rooftop, but you can harvest the sun. Why shouldn't we crowd-source our electricity?

  4. Jan Juran | | #4

    Rooftop PV
    Hi Anne: good points. I will be installing a 4.8KW PV system on my Southampton, NY rooftop soon, awaiting warmer weather. My local utility LIPA charges more for Transmission and Distribution (9+ cents per KWHr) than for electricity production. My new rooftop PV array will help lower local grid distribution costs in our growing, congested, low excess peak capacity local market esp during peak usage summer afternoons. Rooftop PV can lower these T&D costs, which often exceed generation cost in the large, densely populated coastal US markets. Remote PV arrays generally do not lower these significant T&D costs, although to be fair PV production is higher during peak usage hours in most US (non-northern tier) markets irrespective of mount location. Remote large PV arrays may still be cost effective in areas such as the Arizona desert, where land inappropriate for agriculture has a very high solar resource and is close enough to major markets such as Phoenix, Las Vegas and Southern Cal.
    (Unrelated item: RMI reports that in Germany, residential rooftop PV installed costs are much lower than in the US, and German rooftop racking systems do not penetrate the house roof. Is anyone familiar with the German racking systems? I would be interested in learning more re my pending installation).

  5. David Coote | | #5

    Rooftop vs utility scale PV deployment
    It's simply not the case that large centralised PV systems are cheaper than rooftop mounted systems; at least in more mature PV markets.

    At a seminar I attended given by a German manufacturer late last year the company stated that current total installation costs in Europe were about $4/W for utility scale installations and $2/W for residential.

    The installation cost for a utility scale system has to include land, access roads, expensive free-standing mounts, an expensive grid-interconnect possibly with new transmission lines, a legal structure and staff to operate and maintain the system, and so on. And having all the generation concentrated in one spot with a large system makes the supply more unreliable with respect to intermittent insolation, terrorism, a bloke with a backhoe cutting a cable etc compared to dotting PV's around a more dispersed deployment.

    And as mentioned above rooftop installations reduce the need to choof power around the country with most rooftop PV generation up until high penetrations consumed onsite or locally. Out of a currrent peak cost of about 25c/kWh in Melbourne a bit over half is for transmission and distribution.

    Germany is up to about 25GW of PV's. Most of this is rooftop and/or smaller scale installations privately owned. On some sunny days in May 2012 PV's were meeting up to 30-50% of German demand.

    The utilities are, of course, pushing utility scale PV deployments. This is perhaps not surprising :) How to integrate decentralised renewables into centralised fossil systems and keep on making money is something of a challenge for current utilities.

  6. David Coote | | #6

    Adding some storage to residential PV systems
    Ths current issue of ReNew - published by the ATA in Melbourne; see http://renew.org.au/ - has a useful article looking at some aspects of what's involved to set up some storage using Li batteries at a home. The Li article isn't available online but copies can be ordered from the ATA.

    Having even a small amount of storage can reduce grid dependence considerably. And the new technologies are a lot easier to maintain, have better discharge characteristics etc

  7. Curt Kinder | | #7

    Economies of scale and shade avoidance
    Does Peter Y really favor arrays of 500 MW? That's truly huge and perpetuates all the inefficiencies and impacts of massive transmission lines.

    Perhaps he meant 500 kW... Arrays of that size garner substantial economies of scale and can be deployed on large flat commercial roofs (think super Walmarts and similar buildings) with little worry about shading without requiring major changes to power distribution networks.

    Excessively opulent feed-in tariffs should be phased down to avoid both windfall profits by PV owners as well as ratepayer backlash against excessive subsidies.

    I also have to wonder if David Coote's numbers are backward - "current total installation costs in Europe were about $4/W for utility scale installations and $2/W for residential." Should that read $2 utility scale, $4 residential?

    I also suspect that:

    "The PV panels on the Devens house should generate an estimated 10.2 kWh more electricity per year than it consumes, enough to power a Nissan Leaf or Chevy Volt for 30,000 miles, Scott has estimated."

    is also wrong...perhaps 10.2 kWh surplus power is anticipated per DAY, which would offset quite a bit of EV charging.

  8. Joe Pandolfo | | #8

    adding some storage to residential PV?
    Scott G's initial question, "Should PV arrays be designed solely to get a house across the net-zero energy threshold, or can PV serve another other purposes?", and DavidC's comment above, both tie to a question we've been puzzling over.

    We're in design phase for our 2000sq ft new home in northeast CT, and puzzling in particular over dhw system choices. Strongly considering the heat-pump dhw units, as an alternative to the ST panel+gas back-up option I first had in mind.

    So, this all leads me to ask, How can I efficiently (read, 'cheaply') add some battery storage+'extra' PV to our grid-tied PV, for the purpose of emergency/complementary supply?

    The design load (and primary purpose) for this add-on would be to run the heat pump dhw off the grid; in part to keep that vital function immune from grid blackouts (already an annual, multi-day to week-long occurrence in our forested area, and sure to become more frequent as dramatic weather events increase).

    A little background to the question:
    We'll be super-insulating the shell; small wood stove on main floor; decent amount passive solar gain, natural venting & day lighting through the window scheme. The basement floor will be full walkout on south and east, and enough around the southwest corner to add a west window... so there will be some solar mass down there (in conjunction with the underground northwest corner) to feed the heat pump.

    The alternatives to compare against, I suppose, are:
    1. ST panel+gas back-up dhw system that can run without electricity (reduction in overall PV?)
    2. blackout gasoline generator to power the dhw electric load long enough for a tankful of water
    3. hot wash-water off the wood stove and some candle-lit nights after the blizzard or hurricane.

    Any & all thoughts are welcome! Thanks

  9. User avater GBA Editor
    Martin Holladay | | #9

    Response to Joe Pandolfo
    Joe,
    If I were you, I wouldn't spend the many thousands of dollars it would require for a large lead-acid battery bank to run your electric water heater for a few days.

    I live off grid. During the winter, most of my domestic hot water is heated by my wood stove. There is a coil in the fire box, connected by thermosyphon to a tank on the second floor.

    My backup consists of a propane-fired tankless water heater that requires no electricity.

  10. Joe Pandolfo | | #10

    thanks Martin
    Looks like low tech wins the day, Thanks Martin.

    In any event, would be interested to hear what's on the horizon for battery technology; will it ever be cheap?

  11. User avater GBA Editor
    Martin Holladay | | #11

    Response to Joe Pandolfo
    Joe,
    Q. "Would be interested to hear what's on the horizon for battery technology; will it ever be cheap?"

    A. My crystal ball is a little cloudy. No, wait! Now it's coming into focus. I see the answer. No.

  12. John Bailey | | #12

    this house is two investments
    First of all, it's a house. Second of all, it's a power plant. You can invest in either separately.

    In this case, the power plant is integrated with the house, but only weakly. You could build the same portfolio for only a little more in the back yard with some extra framing. A neighborhood of houses could collectively build a much larger plant in a separate location. A state could set aside areas for such large scale plants in good locations around demand. An owner of a house outside of one of these collectively acting communities could buy into their project and take her return in the form of dollars to pay her own local utility for power.

    It's this last option for "going solar" that seems to be totally neglected in the green press. Why should it make more sense for a person to invest in a power plant on her roof than to buy in to a remote operation and collect financial returns. If putting the investment on the roof necessarily makes more sense, well, I just say, it's not clear to me this must be so.

  13. Derek Roff | | #13

    Response to Joe Pandolfo (and Martin)
    If you are completely off-grid, you need batteries. For others, who are considering batteries as an option in a grid-tied PV installation, it's worth calculating the cost of this energy storage. Batteries have a limited lifetime. If you calculate the total number of kWh stored in and retrieved from a battery bank during its life, you can calculate the cost per kilowatt hour. The calculations that I have seen say that batteries, by themselves, will add $.05-$.20 per kWh to the cost of a solar system. I would be interested in whether Martin has calculated his battery costs in this way.

    If the batteries are used only for emergencies, the cost per useful kilowatt hour could be ten times as much, since the batteries will still be degrading even under very light use.

    I tend to agree with Martin's crystal ball on upcoming battery price and performance. However, there are a few promising possibilities. One is super capacitors. Another is the liquid metal battery, invented by Donald Sadoway of MIT. My favorite quote from him is, "If you want batteries to be dirt cheap, make them out of dirt!" He has batteries beyond the prototype stage, so we might see something interesting in the next ten years. Check out his TED Talk. http://www.ted.com/talks/donald_sadoway_the_missing_link_to_renewable_energy.html

  14. David Coote | | #14

    Residential electricity storage using Li batteries
    The ATA article I mentioned says $10k for 25.6kWh of Li storage with this doubled for a total cost of $20k including balance of system costs. With a 70% discharge this would give you about 18kWh of storage. Those are Australian prices. I suspect that this configuration might be cheaper to put together in the US. And teh ATA tends to be conservative with pricing these systems. It might be possible to find a cheaper inverter.

    Li batteries are a lot easier to look after than lead acid and have much better discharge lifetime characteristics. The price of Li batteries has been dropping with the growing market for server UPS systems increasing so scale of production, learning curve and so on. EV's are also increasing uptake. I was chatting with a guy from a battery control company a month or so ago. They make utility scale battery based storages some of which are now in operation in various remote and island grids in the US. He thinks that used EV battery packs with some life left will be a useful source of cheap residential storage.

    It's possible to wire a residential grid-tied PV and storage system with a bit of switching so that it can supply when the grid is not present. This gives you some security of supply through grid outages. We only use about 3MWH/annum so with storage as above we could last for several days of grid outage even if our PV's weren't producing. During an outage you could restrict usage to some lighting, a refrigerator and freezer and perhaps a PC. If you had one of the very high CoP heatpumps I guess you could put some power into that appliance.

    As to Curt's query, the utility scale and residential scale total system costs are the right way around. Once you have a large residential PV market the installation companies can buy in bulk and take advantage of falling prices as against getting locked in to a price with a long lead-time utility scale project. The other factors I mentioned contribute to lower overall residential system costs compared to utility scale.

  15. Dennis Heidner | | #15

    Response to Jan Juran
    Jan, you had asked about German installations and how they could do it without roof penetrations..

    Most German roofs are tile. The tiles interlock -- but they are easy to lift out and remove. The common method to mount the rails is to lift out either individual tiles or rows, add clips that grab the top of the now exposed row, then set the tiles back on the roof. The rails then mount to the clips. Removing a tile and re-installing them is not considered "penetrating" the roof by the Germans.

    Title roofs often have specially shaped and fired roof vents. The German installers typically run the wires through the vents, down above the insulation (if there is any) to the inverter which may be mounted on a upper floor room wall just below the roof. The wires (no conduit used) come through the sheet rock to the inverters where they are combined. The 240VAC cable from the inverter is then run down stairs to the common area for electrical meters.

    Unlike US systems, most German systems do not use metal conduit. If they are worried about wear and tear on the cable, they will route only the portions of the wires they need -- through gray plastic electrical conduits. EVEN OUTSIDE.

    I have solar on the roof, I've spent a great deal of time the last several - going through the design details of US vs German systems -- the often quoted price of $2/W or $3/W for German installs -- well, in my opinion they do not match the actual cost for those installs. German installers/manufacturers are also quick to point out that in the US - we are crazy and use flammable materials on the roof and the siding. They use tile and stucco.

  16. Gene DeJoannis | | #16

    How Large Should a PV System Be
    In Connecticut, electric ratepayers pay into a clean energy fund which is used to pay incentives for commercial and residential conservation and PV programs. Funds are dispensed by the state's Green Bank (Clean Energy Finance and Investment Authority - CEFIA). Their policy is to only subsidize systems that supply about 95% of the family's annual electric use, so that they are never net exporters under the state's Net Metering regulation (if they did export they receive only the wholesale generation rate). I have argued that this policy is not the best for the growth of the residential PV market. I propose we change the policy to do what was done on this Massachusetts house: cover every square foot of roof that has a good solar exposure with PV panels. In addition we should institute a virtual net metering policy so that other nearby houses that do not have good solar access could share the output of this solar roof. CEFIA would sign a power purchase agreement with the homeowner so they would receive a fixed annual kWH allotment based on their past use, perhaps with reductions for some CEFIA subsidized conservation upgrades. The remainder of the roof's PV output would belong to CEFIA and would be sold to other homeowners or apartment dwellers who invest in the excess portion of that PV array (less the CEFIA subsidy).
    This idea occurred to me after hearing from a solar installer that more than half the homes they survey in CT just do not have good solar access. So why should we "waste" the solar resource we have when CEFIA approves a home for PV installation when so many others would like to invest in that solar roof ? It would be more efficient for the installer to do the whole roof, and if he doesn't, it will be much more costly to come back later to expand it and add a second inverter. The utility would be paid a small amount per KWh by the buyers to transmit power between homes. CEFIA could easily find local buyers for the PV power generated and build market growth much faster by reaching out to an expanded pool of customers. In any case, basing a PV capacity on the present electric use is short-sighted, because electric use varies directly with family size and family size changes over time as children leave home or the house is sold to a new family. It would be much more flexible if the homeowners with the PV on their roof could just sign a new power purchase agreement with CEFIA for more or less annual kWh as their family size and electric use changes. In addition to flexibility the PV hosts get a cooler home and a longer roofing life because of the shading by the PV panels.
    So should public money be used to maximize the area of every solar installation? I say YES, but limit the host site to a power budget based on family size to insure they take conservation measures , and distribute the excess power to other local homes who choose to invest in a virtual solar roof through the state Green Bank, CEFIA. The host site benefits, area residents without solar access benefit, the utility makes a small profit, and CEFIA expands the market by at least a factor of 2.

  17. Dennis Heidner | | #17

    Centralized arrays.
    I disagree with Peter Yost's preference for centralized arrays. We need them, but I believe equally as important is a good distribution smaller scale roof top solar. There are several reasons:

    1) Given a choice of open fields, forest, or farmland - few people will look for property to build their house next to a megawatt solar array. Nothing nasty or evil about them - just people seem to prefer green grass over the acre of high tech panels. Even with the electrical dna and mindset I have, I would still select open field or green space. I do like looking at arrays, I literally have thousands of pictures of solar arrays from around the world. But I would still chose green space. So the first problem I see is even though the solar farm may not be "polluting" there is a (not in my back yard) NIMBY problem that goes with them.

    2) In part because of NIMBY, large scale solar arrays are built away from the urban areas. (less pollution does also help). Generally the solar farms are placed near large high voltage lines. Good Medium Voltage (or High Voltage) grid access for the very large arrays is a requirement. So this means the power is often some distance from the consumers. Moving power across transmission lines also implies power loss. All large power plants (coal, nuclear, gas, wind) have the same transmission loss problems - but society has generally come to accept the losses -- the other choice is "in my back yard".

    3) Moving solar PV to roof tops of residences (not all of them of course) offers several possible benefits. These include much lower transmission and distribution losses. Another benefit is that roof top solar PV can help provide voltage support to local distribution grids (if carefully planned). The high incentives for renewable generation (solar PV) is generally based on the public policy to reduce CO2 and incentives that are in theory approximately equal to the avoided costs of building new plants and transmission lines.

    4) I wish I could say that all large scale solar PV plants are built with the environment in mind, but unfortunately some are not. Some developers like clearing the land under the panels and replace the topsoil with landscaping fabric and gravel. While the power may be from the sun, the carbon sink ability of the growing plants are gone.

    The bad: https://vimeo.com/62009308

    The alternative: http://www.abakus-solar.us/blog/solar-farm-pv-power-plant-grounds-management-vegetation-control/

    5) Roof top space - if it has good solar insolation - is space already in possession of the home owner. If it can be used, and it works with the building architecture great. Roof top solar avoids the nasty discussions about changing land from farm use to industrial use.

  18. Jin Kazama | | #18

    Let's work numbers again
    Let's work numbers again :)

    Unfortunately, locally we do not have any incentives for PV, and will probably never have
    ( residential ) because of the clean hydro we have and the cheap price we pay.
    But still , worth calculating some.

    Let's look at PV as both investment and a move to green energy.

    It seems that investing in PV can be quite profitable in the long run, for stable families
    ( if you plan on staying in the house 20years or more .. )
    I don't know what are the payback periods in USA with the incentives,
    but i just run a few calculations based on current system costs
    ( without installation ) and i am getting an approximately same payback period for approx ANY size of PV system.

    a 6KW system gives a 600$ "earnings" after payback of 10-12 years
    a 20KW system gives a 2000$ "earnings" after payback of 10-12 years also

    and you could scale up to the limits of your utility conditions ...

    Now that is some good investment ?

    My current house will use approx 30K Kwh /year once i am done with everything.
    We can't currently resale any KWh here, but they give you credits for netmetering surplus,
    which are applicable to your yearly account.
    So if i would like to invest, i could go up to near 30K KW installation if the utility rules permits it,
    and get back something in the 3000$/year worth of credit after payback,
    so actually getting totally "free" electricity.
    And that is if the KWh price doesn't surpass 0.1$/kwh , which it probably will within 10 years or so,
    giving a shorter payback and higher "returns" .

    Am i getting this right ??
    Even without incentives, as i will be personally getting a license for electricity work in the near future
    ( would cost me almost nothing in installation ) , i would be temped to invest ...
    even if my personal payback would be in the 12-14 years ... i know i am here for 20 years+.

    Then, let's look at it from a different perspective.
    Let's say that i am a "green house" builder ( which is what i wish to do in the near future also )
    And i build houses for sale ( not building for a specific client ... )
    How can i "market" or "sale" this kind of investment to a potential client ??
    If a certain building with performance near passivhaus standards, would require let's say 15K KWh of yearly energy, it would be pretty cheap and easy to bring that down to 0 using PV.

    Is it reasonable to believe that it is possible to build houses of a certain dimension
    ( something smaller than 30'X30' is not very well accepted here ) in our cold climate
    ( zone 7 ) that would consume only ~15K KWh of total energy per year ??

    just a few thaughts .. again i could be all off at 1:40am :p

  19. Dennis Heidner | | #19

    John Bailey makes good points and sizing
    John's comment about the house as two investments is absolutely on target. A place to live-in and a house that produces power.

    Not all houses can take advantage of this two purpose use. You need the solar insolation, the desired to do so, and the energy market conditions that make it work. Not all energy markets are created equal! Early German FiT was a reason to invest. Germans do not have net metering. In the US while we have net metering - a FiT is generally not available.

    With netmetering incentives for the solar PV may only equal what you consume and no more. For these homes it often makes sense to size the generation capacity such that you do not exceed 60% or 75% of your consumption. An owner may still choose to build a larger array than needed - not for the financial incentives - but because they want to reduce CO2 world wide. Instead of buying "green renewable credits" they over build - because they have the ability and choose to make an impact. The Active House Alliance projects are examples of owners purposely choosing to be net positive.

    It doesn't mean that "efficiency first" is ignored. Nor does it mean that passive house envelop requirements are wrong -- it is simply a choice to produce more energy, recognizing other houses can not. If the production capability lines up with the ability to generate income -- why not? Yes I can hear the arguments - the are better ways to spend the money. But that really is up to the owner and the values they place on items that may not be easily monetized. Over built solar PV on roof tops can also pay back with very real results.

    Take for an example a hypothetical -- very efficient certified passivehaus residence in Germany, which has good solar insolation AND built when the FiT was at 0.54EU. While they would not need the energy from the roof to meet the requirements of passivehaus standards -- the power from the array 10kW-15kW would have resulted in perhaps an income of 8000-9000 EU/yr for 20 years. Because the house uses so little power - most of the power generated is income. The array paid back their solar pv investment in five or six years. After that the income would help pay back any additional premium that might have been incurred by adopting passivehaus building methods. In this case the solarPV compliments the passivehaus design - and the two combined together would be more affordable than either alone. The house described in the blog is an example in the US.

    The large array doesn't automatically result in increased power use by the owner - remember owners are likely to be very aware of the generation and the income stream.

  20. User avater GBA Editor
    Martin Holladay | | #20

    Response to Derek Roff (Comment #13)
    Derek,
    I have to replace my home's batteries every 8 years. Ten years ago, I spent about $2,000 for batteries. When these batteries were replaced 2 years ago, I spent about $4,000 for a larger battery bank.

    So I'm spending about $41 a month on batteries. My 0.9 kW PV array produces about 900 kWh per year. So I'm averaging about 75 kWh per month. That means that my batteries cost me about $0.54 per kWh. Living off grid is expensive.

  21. Derek Roff | | #21

    Mismanaged metaphor (and thanks, Martin)
    Thank you, Martin, for posting the cost per kWh for your batteries.

    Peter Yost is quoted as saying, "but for my money, let’s have sun harvesting more like what plants do: large fields that feed the grid more mightily than many tiny green PV roofs!"

    I agree with his idea of comparing solar electricity production to growing plants, but I draw the inverse conclusion. For both PV and green plants, there are many advantages to distributed, small-scale, locally owned production, as well as larger scale production. The worst parts of every city are those where nothing grows. It's nicer and healthier to live in a city area with rooftop gardens, and trees and plants in the medians, sidewalks, and private front, back, and side yards. I favor living near as many growing plants as possible. I have similar views on the advantages of maximizing distributed, small-scale PV.

    Certainly we have need for farms of both food crops and PV panels, but local production is valuable as well.

  22. Dennis Heidner | | #22

    Perhaps an alternative to batteries in five years
    This fall I attended a series of conferences that -- if you were to connect the dots -- it would be possible to see an alternative to batteries in five or seven years.... with a solution for off grid that I think Martin might even approve of.

    There has been continuing progress on fuel cell development; small home units are being test deployed in Germany now. These do not use VW engines! They are either sterling or proton exchange membranes. By product is electricity and hot water. The versions I saw last fall were designed first for natural gas --- BUT -- hydrogen prototypes were also displayed. The hydrogen versions are simpler, and are nearly identical to the NG powered ones -- but without the portion to extract the hydrogen from the natural gas.

    At another show I was shown their hydrolizer that was already shipping. It takes water, provides oxygen and hydrogen.

    Hydrogen storage tanks (bosch) are about the size of a larger acetylene tank. A couple of tanks could hold enough hydrogen for a week without sunlight.

    What I heard was proposals to do just that -- hydrogen powered homes. What I didn't see was any installations that had been built out and were actually being tested. When it does happen - the price will be higher than replacing the battery banks of today, but without the nasty batteries to deal with. Emissions would be nearly zero, hot water for the house, baseline electricity for at night. AND you add more solar panels and storage tanks to match your energy needs for shorter days.

    This is a two year old youtube of the baxi fuel cell -- the stuff I saw last October was much improved.

    https://www.youtube.com/watch?v=JpV1w65GSdo

    Hexis's product.... http://www.hexis.com/en/galileo-1000-n

    Again these are too early for fully off grid houses, they are mostly designed to work where natural gas OR biogass is available, but the speakers were showing a roadmap to use hydrogen as workig fuel also. ...

    Perhaps in ten or fifteen years the need for the traditional batteries used off grid will be eliminated.

  23. User avater GBA Editor
    Martin Holladay | | #23

    Response to Dennis Heidner
    Dennis,
    Perhaps.

    However, I remain a hydrogen skeptic and a fuel-cell skeptic. The hydrogen revolution and the fuel-cell revolution have been on the horizon for a long time, and that's where they remain today: on the horizon.

  24. Jin Kazama | | #24

    MArtin: look for HONDA
    MArtin: look for HONDA hydropgen home stations in the near future ...
    some major car manufacturer want to drop fuel cell because battery powered EV is "catching on a bit"
    but i have had "insider" info that Honda is on the verge ( 1-2 years ) of reveling a home station that will be safe and will use water + eletricity to product hydro/o2 to fuel cell fuel cars.
    Would make a super nice combo with PV and hydrogen electricity generator.

    http://www.solarblvd.com/Solar-Panels-&-Systems-Solar-Panels-By-The-Pallet/c1_250/index.html

    meanwhile, am i seeing wrong or do they sell 0.63$/W panels with 10/25 years on this website ?
    Also look at the SHARP at 0.8$/w ( not sure where they are manuf, though ..doesn't say i usa )
    @ 0.8$ i'll be very tempted in the near future to fill up my parent's , brother's and my house of PV tie grid systems :p

  25. Dennis Heidner | | #25

    Just how big show the photovoltaics be?
    The link below is for a TEDx talk by Matt Grocoff talk, it highlights his 112 year old - but now net positive home.

    http://homeenergypros.lbl.gov/video/tedx-matt-grocoff-shelter-how-to-destroy-the-planet-from-the

  26. Dennis Heidner | | #26

    Fuel cells
    Martin, I agree they are not quite there yet.... distribution and refill stations are still the largest problem. But it is getting much closer. The buses at the 2010 Winter Olympics in Whistler BC were hydrogen powered. Hydrogen powered busses are used in Vancouver BC. I had chance to ride/drive hydrogen powered cars and look at buses while in Stuttgart last October.

    But yes, I think it will be five to ten years before we see a suitable hydrogen solution that would begin displacing any batteries in the US. The EV people are going to like it -- but I think long term, EV's will be a transition technology. Too much weight in the batteries.

  27. User avater GBA Editor
    Martin Holladay | | #27

    Hydrogen and the laws of physics
    No one will ever fuel vehicles with hydrogen produced from water by hydrolysis. The efficiency losses in the hydrolysis process and hydrogen-storage process make no sense when compared to using the electricity directly in an electric car.

  28. Jin Kazama | | #28

    MArtin: that is also what i
    MArtin: that is also what i thought, but i do not believe that Honda as an engine company would research and develop something that wouldn't be interesting, they have all the numbers and then some ...

    Either they are using another form of hydrogen production or they have developped a pretty efficient hydrolysis ... but anyhow they are coming out with something that will be used at home to product hydrogen.

  29. Lucas Durand - 7A | | #29

    At the risk of flogging a dead horse...

    let’s have sun harvesting more like what plants do: large fields that feed the grid more mightily than many tiny green PV roofs!

    Peter,
    If you want to advocate that our energy systems emulate nature, then you should advocate for decentralization and not the other way around.

  30. Lucas Durand - 7A | | #30

    Response to Jin Kazama.
    Jin,
    I agree with Martin.
    The hydrogen dream lives on for some reason, but I'm not sure why.
    A "hydrogen economy" might work if nature had left us vast resources of hydrogen to tap into but considering hydrogen has to be manufactured - implying efficiency losses and energy/resource inputs - I have a hard time seeing it returning enough of an "energy profit" for it to be useful.

    An analogous example is corn ethanol production.
    The net energy return is terrible and this fact is well known, but it hasn't stopped governments from pouring subsidies into it's development - it's about politics and PR (the appearance of a developing a replacement for liquid fossil fuels).

  31. Jin Kazama | | #31

    Lucas : i unfortunately have
    Lucas : i unfortunately have to agree with you ..
    but just as PV are getting cheap enough to be worth it,
    it is only a question of finding a suitable process to split water ( or another ressource )

    Then, i believe that fuel cell technology is focusing on long distance efficiency,
    more than battery run EV ... unless we find a completely new type of battery, EV aren't going to get much more lighter and cheaper ( they will be because of the mass production costs .. but not that much so because of technology )
    our current motor technology is already very efficient

    There is still some space in between combustion engine efficiency and battery EV ,
    and that will probably reside in long distance efficiency, be it in the form of a combutstion/ev hybrid,
    or fuel cell type power.

    Hydrogen as combustion for engine is another example,
    clean process, could be very efficient ... but current tech and storage is a problem
    and they are still trying to use hydro on a regular otto type engine, which is the main problem.
    ( i invented a new type of pistonic combustion engine that i never got the chance to test and or patent ..might do it within the next 10 years if i get enough time/money ... but that is another story )

  32. User avater GBA Editor
    Peter Yost | | #32

    MW vs KW
    Yikes, those bloody decimal places!

    I most definitely meant KW and not MW. Large here in southern VT in a town of 13,000 means a couple of acres or so of PV panels located on ample open space. We evaluated every building our school system owns for roof top PV and not one came even close to what we could accomplish with one or more optimally located ground-mount.

    Sorry for the error but glad in a way that it drove such a meaningful exchange.

    And when I make comparisons of small rooftop installations to large ground-mount I am focusing on all the inefficiencies of way-less-than-optimal angle and aspect of too many of the former. If Nature is honored on homes and other buildings designed to harvest the sun that is quite different than PV panels force-fit on nearly marginal existing buildings.

  33. User avater GBA Editor
    Martin Holladay | | #33

    Response to Peter Yost
    Peter,
    Thanks for your comments. I have edited the article, correcting "MW" to "kW."

  34. Tim Rowledge | | #34

    Generating hydrogen
    There has been an interesting breakthrough in the efficiency and cost of generating hydrogen from water, just in the last couple of weeks. Oddly enough it comes from Calgary University, right in the middle of 'Texas North'. Since the scientific paper was published in Nature I suggest that we can accept that the basic idea works and now it moves to whether the engineering for a practical version can be done. I'd post the URL but that seems to trigger a spam filter; google 'calgary catalyst hydrogen'.

    It might become practical to use a hydrogen tank as a battery; spare PV/wind/hydro/compost-pile-thermal/child-labour energy goes to the cracker, when more power is needed you run the stored hydrogen through a fuel cell. You get some hot water out of it, too, so saving a bit of power for that need. A unitised device would avoid any worries about piping hydrogen around.

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