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Our All-Renewable Energy Future

Here’s why the overhaul of the Passivhaus standard resulted in two new certification levels, Plus and Premium

Why site “zero” is not source “net zero.” Transmission and generation losses are accounted for in source “zero.”
Image Credit: Images #1, #2, and #3: Bronwyn Barry
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Why site “zero” is not source “net zero.” Transmission and generation losses are accounted for in source “zero.”
Image Credit: Images #1, #2, and #3: Bronwyn Barry
Renewable energy sources are incentivized with PER factors. [PHI logo used with permission. Original Illustration by the author.] Tall and shaded buildings are not penalized by the PER calculation. PER demand and generation results table showing Certification Classifications. [Taken from the PHPPv.9 — Copyright Passive House Institute.]
Image Credit: Images #4, #5, and #6: Passive House Institute
PER factors vary by city. Calculations on PER factors are based on fuel source plus site-specific load profiles. As a result, PER factors are not the same in every city. [Taken from PHPPv.9 — Copyright Passive House Institute] Energy storage options are growing. Developing technologies will make it easier to store renewable energy on both a short-term and long-term basis. [Credit: Passive House Institute]

If you’ve been puzzled by the proliferation of “net,” “nearly” and “almost ready” zero-energy definitions and standards and have wondered just how net or nearly they truly are, take heart. The Passivhaus Institut (PHI) has introduced an equitable assessment of energy use to help guide us toward the 100% renewable energy future we must rapidly achieve.

Inspired in part by the impressive leaps in the efficiencies of renewable energy generation, coupled with the urgency of meeting global climate change goals, PHI initiated a review of non-renewable energy use in buildings in 2013. It recognized their previous calculations for primary energy needed updating, especially as they favored the use of natural gas over electricity. (Primary energy accounts for all the source energy used by a building, including the amount of energy it takes to generate and transmit power to the building site.)

PHI recognized that non-renewable forms of energy use by buildings needed to be rapidly phased out, so it devised a method to incentivize the use of renewable forms of energy in buildings. The research resulted in the overhaul of the existing Passive House “Classic” standard and the introduction of two new standards: Passive House Plus and Passive House Premium.

Primary energy renewable factors and how they work

All of the new Passive House standards now calculate primary energy using Primary Energy Renewable (PER) factors. These are designed to encourage the use of renewable energy sources and create either incentives, or disincentives, for installing various types of mechanical equipment in Passive House buildings. For example, in San Francisco, using a heat pump water heater to produce hot water will result in lower primary energy requirement numbers than using a gas-fired tank water heater would, making it easier to meet the certification target. (A heat pump water heater has a PER factor of 1.25 versus the 1.75 factor for a gas-fired water heater.)

PER factor calculations are based not only on fuel source, but also on site-specific load profiles calculated on an hourly basis. In this way, variations in regional utility grid source energy and typical time-of-day use profiles (which impact the availability of renewable energy to meet a utility’s load) for the local climate and region are factored into these calculations.

As a result, the PER factors can vary from city to city in California (see image #4 below). For example, the electricity PER factor for heating demand via heat pumps is 1.80 in Sacramento. This relatively high PER factor incentivizes reducing heating demand in winter, when renewable energy supplies are low. In San Diego the comparable PER factor is set at 1.30, where the climate is milder and cooling is typically a greater peak load issue.

Crediting renewable energy equitably

Conventionally, calculations of net zero depend on the difference between a building’s annual energy demand and annual on-site renewable energy production. These calculations penalize tall buildings with small roof areas, buildings with no solar access, or buildings that opt to use their roof area for green space or as active living spaces.

PHI took a major deviation from such traditional methods for crediting renewable energy supply to buildings, recognizing that all sites are not created equal in this regard (see image #2 below). PHI’s approach uses the following principles:

1) Renewable offsets are calculated as a function of Projected Building Footprint (PBF) rather than total floor area. PBF is more proportional to available roof area than total floor area, which means multi-story buildings may achieve the Plus and Premium standards.

2) Buildings with no solar access on site may purchase off-site renewable energy facilities to achieve Plus or Premium certification.

3) PH “Classic” buildings with no on-site or off-site renewable energy supply are still optimized for efficiency first and a future grid supply of all renewable energy.

Biofuels, micro-grids, and battery storage

While biofuels are considered a renewable energy source, they carry a penalty for replacing food production. Burning biofuels also generates particulate matter that is both unhealthy and emits carbon. For these reasons, the use of biofuels is allowed, but has been capped.

The most intriguing areas of innovation with regard to manifesting the 100% renewable energy future currently look to be in developing our capacity to store renewable energy (see image #5 below). We’re excited by the contributions being made right here in California to develop technologies that are contributing to our new energy future. Existing storage capacity from hydroelectric schemes is now being joined by a growing array of affordable short- and long-term battery storage options. Converting renewable energy into methane gas is another rapidly developing technology that could increase the viability of renewable energy by allowing us to store it for longer.

Remarkably, these options are all currently supported by the Primary Energy Renewable calculations embedded in the Plus, Premium, and Passive House Classic standards. Indeed, the Classic standard at the heart of all of them remains the foundation that most equitably supports an all-renewable energy future. The Classic standard ensures that these buildings are optimized to become batteries themselves: They’ve been proven to retain an unprecedented level of thermal comfort while eliminating peak loads.

This optimization ensures that even without the addition of active power, their passive capacity is what is literally doing the heavy lifting. These buildings enable occupants to survive in adequate comfort for very lengthy periods of time without any active energy inputs. This quality offers economic benefits to both the utilities and micro-grid designs of renewable energy storage systems that extend well beyond comfort. Just imagine what we could do with renewable energy if we didn’t need so much of it to simply operate buildings? The possibilities are boundless.

Bronwyn Barry is a Certified Passive House Designer and the co-president of the North American Passive House Network. This post was first published in Passive House Buildings: California’s Energy Future. Additional articles and California project examples are available in the free e-book and PDF here.


  1. Expert Member
    Dana Dorsett | | #1

    It's about time!
    The prior completely un-nuanced not very localized crude estimates of primary energy use were an abomination! Now they seem to be taking it to the opposite extreme, with both city-scale local-grid and time of use reflected in the method(?)

    While comparing the PER factors from city to city or the factors of a heat pump water heater relative to a gas burner, I'd like to read more about the methods by which the factors are calculated, walking through a specific example.

    Also, the period of time between updates needs to be fairly short- annual or biannual at worst, given the high rate of (and accelerating) evolution of the grid sources, but also the way grids and loads are controlled. A grid-aware electric hot water heater bid into an aggregated demand response market can have more net benefit to grid-efficiency and reduce curtailment of "excess" renewables. In many situations that will be lower-carb than a heat pump water heater operating at typical time-of-day use profiles.

    "The Classic standard ensures that these buildings are optimized to become batteries themselves: They’ve been proven to retain an unprecedented level of thermal comfort while eliminating peak loads."

    Really? Show the math! Any high-mass house can eliminate the peak heating & cooling loads, but the peak loads isn't the peak heating or cooling load for most houses in CA.

    The peak load of a PassiveHouse is usually in hot water heating, but that's also true of most code min houses in CA, and unless PassiveHouse dwellers inherently use power at something other than typcial time-of-day use profiles, the magnitude of the peak load isn't very different from a code min house. If one leverages the water heating peak with a heat pump water heater might have a peak load comparable to the heating/cooling peaks in a typical CA house, but one can also use heat pumps to leverage the space heating & cooling loads.

    Moving the TIMING of the peak draws from the grid to support those loads to correlated better with the availability of intermittent renewables competes with importance with the raw magnitude of the load. That is do-able (and being done) with smarter loads and better electricity markets & rate design, even without going high-R.

    Managing the load to better match the timing of renewables output is much cheaper than any energy storage scheme. But when heading down the storage path, a smart hot water heater is a lot cheaper storage than any battery technology or high-R house, and can buffer a lot of renewable output no matter what time of day or night the surplus is available.

  2. Jon_R | | #2

    I predict that there will be
    I predict that there will be rapid progress on peak reduction/load shifting as soon as significant numbers of people have mandatory time-of-use pricing.

  3. Bronwyn Barry | | #3

    Response to Dana
    Thanks for your comments Dana.

    I've asked Scott Gibson to add a few additional images and tables that were included in my original post. They may help clarify the calculus for PER factors as applied to six Californian cities. In the meanwhile, you can read them here, and also find my original source references:

    I thank you too for the correction. You're right that Passive House projects don't eliminate peak loads. I should amend my article to read 'reduce' in lieu of 'eliminate.' Passive House buildings do virtually eliminate peak heating and cooling loads though, hence my reference to them as thermal storage batteries. From the monitored data we've captured on 3 of my One Sky Homes projects, I can confirm the internal temperatures remain virtually constant. This summer I've enjoyed taking visitors into the attic of our latest project to enjoy the cool temperature there, despite record summer heat outside and despite the mechanical system not being installed yet. (We have subsequently installed a ducted heat pump now that the project is completed.)

    Lastly, you're also correct about the time management of renewables and load. Unfortunately we haven't found an affordable storage system plus micro-grid controller to manage the flow directions just yet. I have no doubt these are coming. In the interim, our focus should be on maximum passive comfort delivery (because that's what will sell) plus maximum carbon emissions reduction (because that's what will save us.) Luckily both coincide pretty well with Passive House and the new PER calculus.

  4. Dana1 | | #4

    Better load control at the house level is happening in Hawaii
    Under the recently revised solar net-metering/compensation scheme new solar customers on substation feeders that are backfeeding in the middle of the day won't be allowed to hook up to the grid unless it's set up for self consumption. In response to the change in the regulations Solar City modified a system they had developed for the Australian market, where compensation for solar exports are abyssmal (outside of earlier feed in tariff compensation.) I consists of a grid-aware hot water heater, a Tesla battery, and a smart inverter to manage the interface between PV, grid, battery & hot water heater, and a Wi-Fi thermostat that can be tweaked remotely, or under the control of the inverter's software. Until regulations/markets change to where the homeowner would be compensated for grid services it's all under the control of the homeowner, but there would be more value to the grid (and cheaper for all ratepayers) if the utility could directly use the battery & water heater for grid stabilization. It's moving pretty fast.

    At Hawaiian residential retail rates they can still beat grid retail if those systems are financed for 25 years at 4%, and it's only a matter of time before costs drop to make it competitive with grid-power at CA electricity rates.

    In Australia there are several players(including Solar City & Tesla) marketing home-scale storage. But as in Hawaii, electricity markets crafted for a 20th century grid model haven't been sufficiently restructured to get the full value out of those distributed behind-the-meter assets.

    The tables of PERs for different loads & locations don't really illuminate the model from which they were derived. It's a tabulation of the answers derived from a grid model, but the math & date behind the model are still obscure.

    I'm still not convinced that the battery analogy really works for a PassiveHouse. At such low HVAC load levels from a storage capacity point of view it's more like a AAA battery next to the Prius battery represented by a hot water heater. It's the water heater, not the HVAC that's driving both peak & average loads, which is also true for a code-minimum house.

  5. Jon_R | | #5

    Break-downs of the usage at
    Break-downs of the usage at peak times show that it's more complex than "it's the water heater". But that's mostly irrelevant compared to "what can be easily shifted". HVAC and water heating match that well.

  6. Bronwyn Barry | | #6

    Source info
    Dana - you may want to read further directly from the source info of my blog topic. Here's the link: Be sure to click through and read the additional posts.

  7. exeric | | #7

    Very interesting
    I have to say that after glancing at the link Ms. Barry provided I'm thinking that using water heaters to cure the duck curve is a very wasteful and short term solution. Perhaps it would be better, as that link suggests, to move to either electrolysis or methane production for storage of energy. The drawback is that the infrastructure required for creating it and storing it would be expensive. Also the conversion efficiency does not equal batteries. But the advantage would be unlimited durability, unlike batteries that need to be replaced, and long term storage ability that would last the entire winter if you have enough renewables to supply the energy in spring, summer, and fall.

    I really like that idea. I'm thinking that because the infrastructure for the conversion and storage capacity would be expensive that the natural place to fund it would be the electrical utilities. The physical facilities for doing the storage and conversion could be located right next to the actual electrical generation plants . They would own it and run it and use their economy of scale to make it efficient. And they would derive a justified income from the customers for providing it. It seems to me it would work great. It goes without saying that there would be no need to alter the current plan of separate pv panels at private homes and commercial sites and using grid tie agreements with those entities. It might even provide the lifeline utilities need to justify their existence.

  8. Bronwyn Barry | | #8

    Eric nails it!
    Eric - that is exactly right. This calculus deftly provides the opportunity for each participant in our energy infrastructure to play their appropriate role in a manner that is both efficient and equitable. Out utilities do need to revise their 'raison d'etre.' This PER structure allows them to serve as the renewable energy generator, aggregator and storage facility which is a much more efficient use of resources than a system that requires everyone to purchase a second water heater. (That works for rural, single famlly homes, but the rest of us in small, urban dwellings will be quite literally squeezed out of the closet!)

  9. Jon_R | | #9

    when matters
    I agree with Dana. A building that concentrates electricity use during non-peak and sunlight hours is greener than the typical use profile. But apparently little is being done to encourage such designs. Even to the extent that they are simple to implement (eg, timers on various devices).

  10. Bronwyn Barry | | #10

    Fantasy vs Reality
    I think we can all agree that a building that shifts use to off-peak hours is a greener building. However, making this happen requires either:
    1. well-trained, highly dedicated occupants
    2. sophisticated control systems that monitor and shift use to off-peak hours

    Both options have been shown to be highly problematic. From various studies I've seen (and one was done on a house I designed), occupants are not easily trained to use even simple tools like an 'all off' switch.' Highly automated systems have a habit of either failing, or being interfered with by building occupants. This means that these two options are likely to fail for broader implementation.

    We need to focus on the reality of what works. Maximizing the passive systems to the lowest efficient level possible, then supplementing the remaining energy demand with small, simple mechanical systems is a much more realistic path and has repeatedly shown to work well.

    PER encourages optimized efficiency first, and then expands the opportunity for energy generation beyond the individual building. Building energy use becomes integrated into the larger infrastructure context. In California, with 60% of our residential buildings being single-family detached homes, and 12% of our carbon emissions coming from single occupant vehicles, for an honest emissions accounting, we need to include vehicle energy use with the house. This is entirely possible for many homes in California, but not all, which again points to the need for an integrated energy network. As Eric has pointed out, this is the role of the utilities and it is encouraged by the PER calculus.

    What I really love about this system is that it quite literally manifests a built environment where we share things, like free energy from the sun and the wind. Apparently there's an endless supply of both and plenty to go around. All we need to pay for is the storage and distribution costs.

  11. exeric | | #11

    Bronwyn, apparently we're on the same page with a lot of this stuff.

  12. Expert Member
    Dana Dorsett | | #12

    Don't conflate long & short term issues.
    The duck curve is very short term. The seasonal storage problem has no bearing on the issue.

    The "well-trained, highly dedicated occupants" and "sophisticated control systems that monitor and shift use to off-peak hours" aren't really a problem- it's an electricity market design problem. Where aggregators are allowed to bid into ancillary services & capacity markets both Wi-Fi thermostats & Wi-Fi water heater controllers are being put to good use now, today, with effectively zero training of the occupants beyond convincing them to take some cash in exchange for the utility or aggegator to tweak the setpoints (within specified bounds) to manage the grid load.

    In the PJM region companies like Mosaic Power ( ) will pay homeowners $100/year to be able to ramp their water heater from 120F (or whatever the homeowner prefers) and the ~200F high temp limit (or whatever the manufacturer specifies) to be able to provide those services. Utilities and aggegators in CA and elsewhere are already using Wi-Fi thermostat and "rush hour rewards" type pitches.

    And it's all cheap stuff: A Wi-Fi thermostat retails for a few hundred USD quantity 1, a retrofit Wi-Fi controller for a water heater is less than $200, quantity-1, and are being built-in to new water heaters as a feature (with the upcharge often subsidized by the utilities.)

    Occupant education & dedication required: Sign up online, let the tech install the controls, take the cash kick-back. Once.

    Sophistication of controls: Low- all the smarts are on the utility or aggegators side (where it belongs, if the goal is to manage the grid), very little sophistication in the house.

    This works, has the ability to manage the duck, and it's reality right now. Implementation was delayed by the D.C. District Court's ruling on FERC Order 745, but that has since been overturned by the Supremes. Now it's a matter of adjusting the electricity market rules to accomodate the roll-out.

    Coming soon: Smart EV charging controllers, both one way & two way power flows. Electric vehicles are primed for exponential growth, since battery costs have already crashed through barriers not anticipated by most industry analysts to happen before 2030, in projections made as recently as 2 years ago. Internal combustion engine cars & light trucks will not be legal to even sell in two European countries beginning in 2025 , and India beginning in 2030. The Los Angeles Air Force Base has had a fleet of EVs and 2-way flow EV chargers bid into the ancillary services market for a few years now (and is making money on every car every year!) This is coming sooner than you think.

    By 2025 there will be a large quantity of used EV batteries no longer suitable for use in a car, but with plenty of capacity left for "second life" use as grid batteries (on either side of the meter). Fantasy? Not really. Nissan is already selling packaged scalable storage systems based on used Leaf batteries in Europe, as well as 2-way flow smart EV chargers suitable for utility or microgrid control. It's not super-cheap now, but it will be by 2025:

    Between Wi-Fi thermostats & water heaters (and soon smart EV chargers) the duck is really going to be a dead-duck before it really becomes a problem in CA. With smart EV charging it addresses the transportation carbon emissions head on too (killing two ducks with one stone? :-) )

    Putting all the sophistication inside the house and trying to control EVERY load when just controlling and time shifting the two (or with EV, three) largest loads is just a brain-dead approach, which is why people aren't really doing it. It's hard to do well, and most of the benefits can be had from managing the few largest. And the magnitude of the heating /cooling load of a PassiveHouse it so small it hardly counts when looking at peak loads.

    In most CA climates even the magnitude of the SEASONAL load of a Y2020 code-min house (= Net Zero Energy) is small enough to hardly matter too, since it's so much smaller than the hot water heating load. The seasonal energy use of most recent CA home builds are already quite a bit smaller than the hot water load, and that will be even more so for homes built in 2020.

  13. exeric | | #13

    Reply to Dana
    I think you still may be missing the point of several recent posts here. I'm not sure why. There is no battery made that can store energy for months at a time without replenishment. There is just less radiant renewable energy in most places in the USA in winter, leaving aside CA, to provide sufficient stored renewable energy throughout winter in those places. It's basically has to be like a squirrel storing nuts for the winter. Batteries or water heaters just won't do it.

    But once you put in a system that can store that energy long term, however you go about it, it completely eliminates most of the reasons for all the finicky automated systems to cut energy use during times of low renewable energy supply. It also makes sense when you have such a system to have it function centrally on a shared basis since the basic infrastructure of utility electrical generation is already in place and that storage system could use that.

    Sure, it's not here today or right now. But how could one argue that water heaters or used car batteries would be an improvement on such a system when it is eventually put in place?

  14. Jon_Lawrence | | #14

    I agree that EV


    I agree that EV batteries will play a roll in load shifting, but I think people are underestimating the longevity of EV batteries. There was a story recently about a company that provided transportation between LA and Vegas in a Tesla. Over the course of 2 years, they put 200k miles on the battery pack and always Supercharged and did plenty of full capacity charging, which according to Tesla, are the worst things you can do to a battery. The total capacity loss of the battery pack after 200k miles was a measly 6%. This does not take into account age beyond 2 years, but even if age has a large impact on battery life, then repurposing them as grid storage makes sense. As I recall in one of the PJM articles you linked to recently, they only needed around 3% of total capacity to be backed up by storage to be either 90% or 100% carbon free using solar and hydro as the power generators. Tesla currently has about 150k vehicles with let's say an average of 85KWH pack. The Bolt I believe is going to be equipped with a 60KWH pack as will the Model 3. So by 2025 there will a large quantity of either used or still operational large capacity battery packs out there. In additional to that, there will plenty of Commercial installation with battery storage and Utility combined solar/battery farms.

    The problem wth these big vehicle battery packs is they are designed to enable long-distance travel. I use maybe 25% of my battery capacity on a daily basis. Why shouldn't the pack be used to allow me to help with the load shifting. I see a future where people drive their EV's to work, plug-in and fill up with solar powered electricity and then plug in when they get home and feed the house until the sun shines again the next day. It would make a lot more sense for me to use my car's excess capacity, than to add a bunch of Powerwalls. And Tesla can make a business model of it by charging owners a tap fee to allow them to use their car's battery in bi-directional manner.

    A big issue with load shifting as it pertains to my situation is that I am incentivized by my local utility to shift my load to "off-peak" hours which is currently defined as 8pm - 8am. Seems to me that off-peak should be changed to 9am - 6pm in states where the duck is present. You could let the grid manage the load with smart appliances that they can control or alternatively you could incentive folks to to it themselves. Personally, I am not a fan of having a 3rd party manage my usage. I would rather do it myself and I save a lot more than $100/year by switching to TOU. I think anyone who has a Nest would be willing to invest a few minutes to program it to take advantage of lower daytime rates if that were to happen.

    It really comes down to supply and demand. Too much supply during the day, lower the rates and demand will shift. People will buy wemo's and Nest's and shift their loads. People drive and extra 30 miles do save a few cents on gasoline. They can save a lot more by shifting load and the technology exits for them to do it. The problem is they (me) save by shifting load to the overnight hours.

  15. Expert Member
    Dana Dorsett | | #15

    The winter vs. summer availabilty of renewables.
    The difference in radiant energy between summer & winter doesn't mean as much as one might think. Despite having only bout 40% of the insolation at the winter solstice than at the summer solstice, in New England the average weekly availability of PV kwh varies by about 30% between summer and winter, and the peak weekly output is typically in late winter (not the summer), when there is still a heating load. The reason for that un-intuitive result is that cooler temperatures yield a higher conversion efficiency than at the standardized panel test conditions, and snow on the ground adds a measurable kicker to the insolation at the panel, whereas in summer higher panel temps deliver lower than spec efficiency. The only time solar output goes completely off the rails is when it's covered by snow, but that is a manageable (and managed) problem. Snow falls that can cover panels typically occur during the warmer winter weeks, whereas the peak heating loads occur during clear weather, when the daytime insolation is higher. It's not nearly as bad a problem as it appears at first blush.

    At some price point over-building PV and curtailment of the excess is cheaper than storage, and it's really nearing that price point for battery storage (though the cost of both is racing to the bottom) but not yet for thermal storage in existing hot water heaters.

    Wind power in New England is considerably more available in winter than in summer. This is true generally across the region but to use one example: At sea level in Boston the average wind speed on the ground in winter is about 12.6 mph, but its only 10.33 mph. At 50 or 80 meters (hub height of typical turbines) the wind speeds are higher, but proportional. The power output is a function of the velocity cubed, so the average power in winter is about (12.6^3 )/(10.33^3), or about 2000 /1100, which is nearly 2x the total amount of power. This seasonal difference in availability also plays a factor in how much storage is needed to manage the seasonal heat loads, and whether or how much seasonal storage (if any) make sense compared to managing how much of what type of renewables gets built (and where.)

    The notion that you must squirrel away enough summertime nut-fall to cover the wintertime heating load isn't really true unless you're looking at a very narrow range of renewables options, and a dumbed-down model of renewables output.

    The offshore wind carve out proposed by MA governor Baker (R) was very cognizant of the seasonal output differences, and the MA Energy Secretary Beaton ( ) knows both the cost and value of a PassiveHouse- he lives in one, built by his own company. But managing & the peak load rather than the seasonal load was still the primary focus of the storage mandate & solar carve out in the most recent energy bill. The storage will enable better management of all grid assets, not just the solar & wind, and it will come at a cost savings to the ratepayers.

    But also note: A Net Zero house in New England uses significantly more total energy for hot water than for space conditioning, and taking it all the way to PassiveHouse on the building envelope addresses only the lesser space heating load. In short:

    1> If trying to constrain the peak load, it's about the hot water.

    2> If you're trying to constrain the average load, it's STILL about the hot water, once you are at Net Zero for the house.

    3> At the residential level, it's really the hot water, and that thermal mass can be put to further good use for managing and lower the carbon footprint of the collective grid assets than simply for bathing, etc.

  16. exeric | | #16

    Also another important point if it wasn't already obvious. If you have a system that can store renewable energy long term it is no longer a problem if you have more pv energy generation than one can possibly use. You just store it for winter. There would be no reason to need a water heater to dissipate that oversupply. I'm not saying water heater power dissipation will not be important in the interim. There is a good possibility that their use will be required during a period where there is an oversupply of pv energy generation and the new systems of centrally located hydrogen or methane production is not yet in place.

  17. exeric | | #17

    I guess we'll have to agree to disagree
    Dana, I think you are still ignoring that millions and millions of homes are nowhere close to passivhaus or net zero ready. You and me are green building nerds. Let's face it. The vast, vast majority of homes in this country do not use more energy in water heating than in other uses. My home is included in that. But I'm working on that not being the case.

    So I'll ask one more time in a different way than before. What do you think would be easier:

    1.Get all the homes in the USA super insulated and air sealed so that the water heaters are the main source of energy usage. OR

    2. Do a Manhattan type of project nationwide involving all the regional utilities. It would be something similar to Tennesee Valley project in the 30s where a electrolysis plants are created regionally and gifted to those utilities.

    One would involve millions and millions of individuals and convincing them one at a time to upgrade their homes at extreme expense. The other would involve perhaps a hundred or two utilities who already see the writing on the wall. Why do you think so many utilities are being driven kicking and screaming to avoid having grid tie agreements. They don't see a viable financial future for themselves in that direction. Having a plan in which they can participate in a green electrical generation grid, and that also is the most efficient use of the existing PV and other renewables is a very attractive option. The difference with having their own central long term storage system is that they are much better equipped to do that than the individual consumer.

    Basically it comes down to herding millions of cats that don't want to be herded versus herding cattle that are already inclined to go in a safe direction for them. Which do you think would be easiest to do in real life?

  18. exeric | | #18

    Oops, mixing metaphors here
    Should have said "what do you want to do, herd cats or herd squirrels with nuts?" Squirrels with nuts would definitely be more entertaining. Just make sure the nuts are already in the place you want the squirrels to move to. Making a consistent metaphor is definitely hard work. :-)

  19. AndyKosick | | #19

    First, my thoughts are that the future of the grid is most likely a complex, computer controlled, balancing act that will include everything imaginable rather than a question of long term storage. My thoughts are that we will end up with an excess of renewable generation to be sure and meet demand, and that industries will lined up out the door to soak up any excess generation at rock bottom prices when ever it's available.

  20. exeric | | #20

    A question
    I guess my basic premise has to do with what Martin says about his own experience living off the grid. In winter it can be overcast for long periods of time and PV won't recharge electrolytic batteries. Batteries will completely discharge and won't keep his home warm after 3 days of that. That's not to mention that most electrolytic batteries to have long life they shouldn't be drained below 50%. Basically the same thing also goes for any thermal device no matter how well it's insulated, including a very well insulated home.

    This article by Ms. Barry was about an entirely renewable powered grid. Often in winter a whole region can be overcast in winter for up to a week. All you need to understand is that an entire regional utility in which that overcast is happening will have a PV energy deficit if it is relying primarily only on that for its renewables. This even happens in California where I live at least once or twice a winter. If you have wind energy making up for it then you might break even but that is a hit or miss thing that Is not entirely predicable.

    I have tremendous respect for Dana because he knows a lot more than me about a lot of things. I think he is incorrect on this. To have a fail safe grid in the future that relies only on renewable energy, as this article was purportedly talking about, then some form of long term energy storage is required in winter in most states. What that proportion would be is where the regional computer algorithm might help. But it seems like long term storage is still required. The alternative which many people are advocating is a highly overbuilt infrastructure where absolutely huge renewable energy surpluses exist in summer and then an associated increase in infrastructure to sink that excess energy. That is not the smart way to go about things. You often do not have a choice to wait for summer to use cheap energy.

  21. exeric | | #21

    NREL insolation maps
    I think the contrast here should be illuminating.



  22. Bronwyn Barry | | #22

    Squirrels with nuts vs water heaters
    Eric, I'm still chuckling over your nutty metaphor and will be calling my own clients 'Squirrels with nuts' from now on. Thanks!

    The storage concept conveyed by your metaphor is highly appropriate, and is unfortunately lost in many 'net zero' calculations where the energy boundary condition is drawn at the property line (site net zero.) This conveniently ignores seasonal use/generation disparities, plus generation and transmission losses. Utilities must magnanimously provide the deficit. At minimum, we need a more honest accounting of energy use to include source energy use.

    However, I'm hoping that ideas such as PER will provoke/inspire our industry to expand their vision beyond short-term solutions and quick mech-based fixes. PER offers a visionary long-term solution. It places buildings within their appropriate context of neighborhood and region (the utility scale) and blurs the borders between them to encourage an equitable distribution of a free resource. (The sun and wind are gratis to all - we only pay for the cost of conversion, distribution & storage. Why should this not be shared?) PER also allows for buildings to become part of a much larger infrastructure solution that must encompass transit and manufacturing energy production. As I mentioned in my article, the opportunities are quite literally boundless. (Perhaps I should have said 'boundary-less?')

    Dana - I'm intrigued by your water heater as battery concept and don't doubt that it may be useful. It doesn't conflict with the PER vision and PER could actually enhance it because it incentivizes heat-pump water heaters over gas-fired water-heaters or boilers. Heat pumps are more readily powered directly by renewable energy sources and would optimize your solution for many smaller projects that could utilize that option.

    I imagine we'll need to use multiple pathways to reach an all-renewable energy future. But, as Eric and the Passive House Institute have recognized, the framework for doing so will require some form of long-term storage of renewable energy. Squirrels have already figured this out. 'Nut' storage - in whatever form - appears to be the solution. (Can you hear the bio-mimicry folks squealing with delight? :) The perfect design and flavor of the 'nuts' remains to be innovated and this is the place for the larger scale tech/mech solution, not at the level of each individual building.

    BTW, we'll be covering all this and more at the NAPHN17 Conference & Expo in Oakland next year: October 4th-8th.

  23. girwin | | #23

    Yes Thermal Storage

    You make some interesting points, but I have a different take on this.

    First of all the PER calculations are based on the PHI's assumptions about what the eventual 100% renewable energy grid mix will look like in a particular location (a bit like this prediction, and how various building loads impact that mix accordingly. As such, an area with wider seasonal swings in insolation (and a heavy reliance on solar) would have a stiffer "cost" for heating energy than a location where it didn't vary so much. Hot water, lighting, and other things that are flatter seasonally, would see less of a variation throughout the year. These calculations are likely in need of refinement over time, but there aren't intended to update on a yearly basis with the current grid mix, they're long reaching, attempting to guide design of long-lived buildings for the future.

    As to the statement that "Any high-mass house can eliminate the peak heating & cooling loads..." This is not correct. A building with a long thermal time constant can mitigate peak heating and cooling loads, and the thermal time constant is the thermal mass divided by the conductance of the building. As an example, an English castle with the wind howling through it cannot mitigate heating loads because the conductance (due to infiltration) is too high, despite its large mass. Mass can be a helpful strategy, to a point, but conductance is often a more feasible path, as shown in the 1970s "passive solar vs. super insulation" debate.

    Long building time constants are only useful in the short term, as you suggest, but they can be very helpful now (mostly for shifting cooling peak loads) and in the future (for cooling and heating), as they would allow the occupant to operate their heat pump at times of maximal efficiency and/or maximal renewable energy availability. In California, the current peak electrical grid load is unquestionably driven by cooling (late summer afternoons), and there are those in the future who will attempt to address this with solely with batteries vs. efficiency, I expect at higher expense than with Passive House.

    When heating and water heating are added to the electrical grid, even via good heat pumps, I expect that the California peak load may shift to winter, since we use large amounts of natural gas for those loads now. Further, even small heating loads have an outsized overall impact because they occur at times of low solar availability. This makes heating a building with solar energy difficult, and PHI's current expectation is that power to gas, pumped hydro, etc., will be required in many locations to deliver this with renewable energy. Power to gas has an energy yield of about 30%, so space heating has a high PER if it uses this fuel to a significant degree in a particular location.

    Water heating is certainly the largest single load once the shell is brought under control, but that is easier to deal with than heating, and it is more uniform throughout the year, so it tends toward baseload, not peak, at least on a seasonal basis. Because water has such good thermal mass, it is relatively easy to shift daily water heating to whatever time is optimal. Efficiency improvements, drainwater heat recovery, etc., plus some sort of solar system can take a huge chunk of the water heating away, though the depths of winter are still a problem.

    Your mention of similar summer and winter solar availability in New England suggests to me not that sun in winter is abundant (otherwise, you'd have palm trees and mangoes growing there) but rather that summer is not all that abundant either, as this map suggests ( My recollection is that New England has very cold winters, which furthers my conclusion, since that is heavily correlated with insolation. I am skeptical of your hypothesis that additional PV panels would accomplish much heating in an effective manner. Were that so, it would suggest that additional South glazing could do it even better, would it not, since passive solar heating is on the order of 4x more efficient than PV production? Wind is a harder source to generalize about, as it determined by so many factors, but it is also unlikely to be a building-by-building solution.

    When the grid shifts to all-renewables, the current peak loads (which are demand driven) will interact with “supply-driven” peaks, and I expect we’ll find space heating to be the greatest challenge.

    I've been lecturing lately about the untapped value potential of thermal storage in Passive House, which I expect to come to be realized (along with the value of other energy storage systems) once the grid switches to a time-of-use pricing system (the writing is on the wall and in policy, at least in California, for 2019.) I call Passive Houses "thermal batteries" by way of an analogy, for both short term (daily time constant effects) and what is more properly described as "demand reduction" on a seasonal basis for heating. Passive House insulation, air tightness and heat recovery of internal gains are superb methods of reducing heating demand and far more cost effective than meeting that demand with storage and/or renewables once the true cost of “power,” not “energy,” is passed along to the masses and we’ve eliminated the grid “storage” we currently implement via piles of coal, volumes of natural gas and nuclear fuel. Here’s the most recent online version of my lecture, I’d welcome your thoughts.


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