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Musings of an Energy Nerd

An Introduction to Photovoltaic Systems

Solar electric panels are getting cheaper every year

A roof-mounted photovoltaic array. [Photo credit: Mike Spasoff]
Image Credit: Mike Spasoff

By now, photovoltaic (PV) panels are familiar to most Americans. You’ve seen them on your hand-held calculator, on top of illuminated highway signs, and maybe even on your neighbors’ roofs. With PV systems becoming more common, perhaps you’ve been dreaming of making some homemade electricity. The dream is achievable, as long as you own a sunny patch of lawn or an unshaded south-facing rooftop, and as long as you have a bank balance of several thousand dollars.

A PV array is made up of rectangular modules (or panels) that measure between 2 and 5 feet on a side. The most common type of PV module has an aluminum frame and a glass cover protecting a collection polycrystalline PV cells. When exposed to light, each PV cell produces 0.5 volt DC — so if you add up the number of cells and divide by 2, you know the voltage of the module. The best performing commercially available PV cells are roughly 20% efficient at converting solar energy into electricity.

Unlike polycrystalline PV cells, thin-film (amorphous) PV products are manufactured on a flexible sheet. These thin-film PV products have many applications; for example, they are used to make PV roof shingles and peel-and-stick membranes designed for use on metal roofing. Thin-film PV products have relatively low efficiencies — usually in the range of 10% to 12% — so they require almost twice the area required for a polycrystalline PV array with the same electrical output.

Both polycrystalline and thin-film PV arrays produce DC power. This DC electricity can be used directly to charge a battery; in most homes, however, the solar electricity is sent to an inverter that converts the DC power to AC. The inverter’s AC output can then be used directly by the homeowners or fed into the power grid.

Most grid-connected PV systems won’t provide any…

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  1. user-946029 | | #1

    Good summation
    Nice job, Martin.

    You're right; people should take a closer look at PV.

    I'd like to add a couple of points:

    Here is a good article on the durability of solar panels:

    One thing not often mentioned is overcoming HOA rules against panels on your roof. I have heard that federal legislation has been passed that overrules any inflexible HOA rules, though I couldn't cite which bill the language is in. I've also heard some states passed their own rules allowing homeowners to bypass any HOA restrictions, with IL being one of them.

  2. GBA Editor
    Martin Holladay | | #2

    Response to Mike Collignon
    I think the article that you linked to is too conservative when it reports that photovoltaic modules "can last up to 20 years or longer."

    My oldest PV module has been up on the roof for 32 years, and is still operating flawlessly. Here's a link to an article I wrote on PV module longevity: Testing a Thirty-Year-Old Photovoltaic Module.

  3. Expert Member
    Michael Maines | | #3

    Neighborhood scale
    For more information on the idea of neighborhood-scale energy production, I'm reading an interesting book right now, "The Third Industrial Revolution," by Jeremy Rifkin ( The idea is gaining traction in Europe, and makes sense in the US as well. The idea is analogous to mainframe computers vs. PC's and the internet.

    I still want my off-grid cabin in the woods, though.

  4. user-965582 | | #4

    Best thing on a roof

    I've always believed that the best thing on a roof is shade. With the development of community solar, we can have our cake and eat it too: shade on the roof and part ownership of a larger, cheaper (economies of scale) community solar arrangement that may also be providing shade for cars at, say, a school parking lot. See:

  5. 5C8rvfuWev | | #5

    re: durability of PV panels
    Martin, et al

    A lot of neighborhoods around her have large vacant lots that (because of terrain) would be very difficult to build on. I'm thinking they may be available for very little money and could be used by the neighborhood to develop a PV resource for residents.If so:

    are PV panels as fragile as they look? I'm wondering about other features of modern urban life ... like vandalism -- rocks, graffiti, etc.

  6. GBA Editor
    Martin Holladay | | #6

    Response to Joe W
    Standard polycrystalline PV modules have glass covers. It wouldn't take much effort for the average teenage vandal equipped with a rock to bust one. Once the glass is busted, it's generally cheaper to throw the PV module away and install a new one than to try to repair it.

    Thin-film PV arrays are evidently more durable. You can damage sections of them, and they'll still work. However, if you can keep the vandals away, I'm pretty sure that the glass-covered polycrystalline arrays will last longer than the thin-film arrays.

  7. user-984364 | | #7

    A real-world datapoint
    For anyone who is curious, you can see just what my array in the upper midwest produces here:

    It's 2.53kW, with 11 230W panels, all that my roof could hold. With conservation efforts, it has provided about 70% of my home's electricity since the installation. Gross cost was around $16k, if I recall, and out of pocket was about $6k - that was a year and a half ago.

  8. user-984364 | | #8

    That natgeo article...
    ... is a mess. It presumes PV, for starters, without explicitly saying that and comparing to, say, solar thermal systems.

    But then it goes on to say "The DOE states that most solar systems have controls that keep the panels at an even temperature" - which sounded awfully odd to me, until I realized that the reference she cites is for solar thermal, not PV. Ugh.

  9. thoughtful | | #9

    $10.000 for Comfort, Health, Quality & Aesthetics
    I agree that we need to provide homeowners with an answer of how to spend "the last $10,000." In this discussion we should not quickly dismiss better windows, especially in a cold climate, without discussing issues such as comfort, health, quality (longer lasting) and aesthetics.

    A better window, if chosen correctly, will reduce convection currents and condensation. A better window, if chosen correctly, will last longer and have aesthetic qualities.

    Like anything we need to discuss all the advantages and disadvantages to any given choice and not simply say, "A more expensive window is not cost effective."

    A BMW, Mercedes or even a "loaded" pickup truck is not cost effective but people still buy them (and buy them and buy them...). If there is any place to argue for spending money on comfort, health, quality and aesthetics it is in a home. To ignore these qualitative is to ignore the reasons we build homes (or should build homes) in the first place.

  10. GBA Editor
    Martin Holladay | | #10

    Response to Garth Hood
    I agree with your points, none of which contradict my advice.

    A good designer does the math to calculate the energy savings attributable to all of the measures under consideration. Then the designer (in consultation with the homeowner and builder) decides where to allocate available resources in order to achieve the desired result. Of course, comfort and durability are factors to consider.

    But this analysis can't happen unless you first do the math.

  11. MPFadR5c6L | | #11

    Solar PV is great. Like
    Solar PV is great. Like anything else, it needs to advance to the next generation. Higher efficiencies need to be achieved before PV really becomes practical. If people could spend $10K or less to provide over 100% of all electrical demand, we'd really be onto something. ...sort of like the laptop. I remember seeing price tags for a slow, useless thing for $3500. Today, you can get something with 10X the speed and capacity for $500.

    The aerospace industry claims over 40% efficiencies. In part, of course, are the advantages of being well outside the earth's atmosphere---no atmospheric distortion/filtering, temperatures of -200 degrees (the colder, the higher the voltage)--. However, those modules are made using Gallium Arsenide, which is superior to silicon electrically --for starters, less electrical resistance within the crystal, less affected by heat--. While this couldn't work here on earth (the oxygen in our atmosphere breaks it down for one, and if that weren't enough, it is REALLY expensive).

    To stay on track, more R & D is needed in the industry so that it is not so cost prohibitive.
    I won't even begin to mention the political nonsense involved with it. Here in NJ, it is almost a joke, with literally over half an inch of paperwork to fill out, both before and after the install.

    1. bje11 | | #43

      Deniz, it sounds like you're not wanting higher efficiency, but lower cost. Those aren't the same thing. Rooftop solar systems in the US are, for some reason, considerably more expensive than they are here in Australia. I am looking at a quote right now for a friend of around USD 3430 for a 5kW system including installation. No permitting is required here, many electricians hold solar tickets, and the whole thing can be installed in around four hours. Some crews have been known to install three systems in a day.

      When I run out the levelised costs of a PV system, it is producing electricity at around one third the retail price of electricity. It is a no brainer.

  12. MPFadR5c6L | | #12

    Martin, I do agree with your comment regarding the lifecycle of a battery bank being 8-9 yrs. I would like to further point out that the figure is based on the proper maintenance of the bank. This includes a constant 77 degree Fahrenheit environment; no discharging past 80 percent--and not usually more than 40-50%; proper equalization--once every few months under normal use, more often if deeper discharges are more frequent; proper charging---making sure the bulk and float charging rates are set properly. If not, you can overcharge (or undercharge) your battery bank, in which both cases will significantly reduce the life of the bank.

    The bottom line is that unless you need a battery bank and plan to maintain it properly, it is probably more money and work than it will be worth. I have seen $20,000 battery banks go bad after only 2 years due to improper maintenance.

  13. GBA Editor
    Martin Holladay | | #13

    Response to Deniz Bilge
    You wrote, "Higher efficiencies need to be achieved before PV really becomes practical." Believe it or not, PV is already is already "really practical." I bought my all of my PV equipment without any government subsidies, and I'm happy with it. PV equipment has already reached "grid parity" in areas of the U.S. with high electricity prices.

    PV efficiencies may or may not improve in the future; but remember, a PV array is not a smart phone. Much of the cost of a PV module is the cost of the glass and the aluminum frame. The efficiency of the cells may matter on a satellite, but the efficiency doesn't matter so much for a ground-mounted array, especially in rural areas or at utility-scale plants.

    I'm glad you agree with me on my estimate of battery longevity; my estimate is based on experience as well as reading about the experiences of others. Many of the difficulties you mention -- the need to avoid deep discharging and the need for regular equalization charges -- would, of course, be handled automatically by a good charge controller if you have a grid-connected PV system. Off-grid users usually find it harder to schedule their equalization charges, and some are tempted to discharge their batteries too deeply.

  14. markgimmeshelter | | #14

    As usual you have

    As usual you have done a good job of outlining the issues. Here are some things that occurred to me while reading. An off grid home that is focused on electrical efficiency can exist pretty comfortably on about 2K of PV which many roofs can accommodate. It is true that batteries are an ongoing cost and liability although the cost of grid access (service fee) is also an ongoing cost which may have more weight in the world of the future . Although many of my 20 to 30 year old panels were still generating electricity when I upgraded my system over the past years I am generating almost twice the electricity on the same roof by moving to a new generation of panels. The effect has been to almost completely retire my back up generator. It is true that off grid homes have to oversize their systems to avoid dependence on generators which is why grid tie systems are always the first choice but winter lighting loads are partially replaced by harder working summer loads for refrigerators and freezers.

  15. user-1089563 | | #15

    Other costs

    Your introduction to PV systems speaks of a 3-4 inch space between the shingles and the underside of the modules on a roof-mounted system. You might want to consider that squirrels just love to nest in such spaces and, while under there, enjoy chewing all your wires. Ask me how I know. Shielding that cavity from nesting squirrels by the installation of hardware cloth or equivalent can save you days of chasing squirrels with sticks while perched on a slippery roof, and the inevitable rewiring that is necessary after several years of squirrel habitation.


  16. GBA Editor
    Martin Holladay | | #16

    Sorry to hear about your squirrels, Don
    That doesn't sound like much fun.

    My PV modules are on an adjustable aluminum rack on my roof, accessible (for snow-clearing duties) by a kind of widow's walk plank. For most of the year (that would be for our 7-month winter) they are almost vertical, and so I haven't suffered from the squirrel problem.

  17. DFMallinson | | #17

    PV System
    I'm about to find out if there is gold in sunshine. Here on Cape Cod, a solar panel array company convinced me to spend $28,000 on a roof mounted 5kW system of 24 panels each equipped with a micro inverter. Payback will come from 30% contribution in the form of tax credits from the Feds, an $8,500 grant from the state of Mass. Regardless of my own useage, I will be selling my generated electrical power to an aggregator for a currently contracted price of around $3,00 annually for 5kW. This price is market driven and will vary in the years to come. If it all works according to the pro forma, my PV system will be paying me in a few years. BTW, NStar will not credit me for any electricity I produce above an beyond what I use. They require that I identify another account to receive such credit. The PV array was just installed this month. It will be an interesting year.

  18. xHkZBSHpJL | | #18

    interest in interest? seems to be usual, the example of a typical system, in Chicago, fails to account for the cost of borrow that $17,500.00 at 4% and you will pay $700 a year in interest...which means the payback is more like 45 years...PV is getting less expensive...but it is still only a feel good or look green thing...until it is really driven by economics it will not take off...PV is at the top of the green pyramid and should be the last thing we do

  19. wjrobinson | | #19

    Joe McNally. Run your cost of
    Joe McNally. Run your cost of money for a granite countertop. Hmmm, I guess my own thoughts prove your statement as to solar being a "feel good" thing. OK, I agree. But some of us definitely feel good thinking solar instead of granite. :)

    I am agreeing with both sides of what Joe is saying... crazy. Just like many hard decisions today. Ends up being a personal decision. Though if we all bought solar in my town and no granite... wow... would love to put it to National Grid and grab every solar photon possible.

  20. GBA Editor
    Martin Holladay | | #20

    Response to Joe McNally
    When I discussed payback in this blog, I included a link to a previous article, Payback Calculations for Energy-Efficiency Improvements. In that article, I include an extensive discussion of the topic you raise -- the cost of money.

    As I wrote in that article, payback calculations are always estimates, because none of us can predict the future. Among the uncertainties in this analysis are the future cost of electricity, the future general inflation rate, future interest rates, the longevity of the PV equipment, and maintenance costs.

    Not all of these uncertain factors trend in the same direction. Some factors, like an increase in the cost of money, tends to lengthen payback periods. Other factors, like an increase in the cost of electricity, shorten payback periods. Historically, an increase in the cost of money -- that is, rising interest rates -- usually means a period of high inflation, when electricity rates also rise. In other words, these two factors historically rise together -- one lengthening payback periods, and the other shortening them. It's hard to predict which factor wins in this push-pull situation.

    People who assume that payback periods can be determined exactly, and who include the cost of money in their calculations, are certainly introducing factors that (according to accounting rules) are necessary to consider, as I explained in my article. However, it behooves us all to maintain a humble attitude when predicting the future.

    Here's what I know from personal experience: I have every reason to believe that polycrystalline PV modules will last longer than 30 years. While inverters often need replacement after 15 or 20 years, PV modules rarely need any kind of maintenance other than snow removal.

  21. user-980774 | | #21

    PV @ Full Moon?
    Does a PV system generate any power at night during a full moon from reflected sunlight?

  22. GBA Editor
    Martin Holladay | | #22

    Response to Richard Patterman
    Sunlight provides between 32,000 and 130,000 lux, while a full moon on a clear night provides 0.27 lux. In other words, sunlight is more than 100,000 more powerful than the full moon.

    So, if your PV array produces 1,000 watts in full sun, it will produce 0.01 watt on a full moon night -- too low to measure.

  23. zWKNrHcG2k | | #23

    Pfffft !!!!
    16 years until ROI ???!!! Try running a business or your home budget with this kind of return..then write your article.

    PV's would either have to become a whole lot cheaper or easier to produce or be more efficient than the measly 20% noted in the article to be economically all.

    Additionally, most manufacturers warranty their product to only 25 years due to the fact that these panels lose about half a percent of their output per anum. Talk about diminishing returns!

    Pure fantasy.

  24. GBA Editor
    Martin Holladay | | #24

    Response to Warren Crossfield
    I provided an example from Chicago to help readers understand how to go about making a simple payback calculation for a PV system. If you read the article I linked to, you'll see that I have also written a long article on payback calculations.

    Once homeowners make a payback calculation, it's obviously up to them to decide whether they would be satisfied with a 16 year payback, followed by 9 to 14 years of free electricity. You've clearly made your own decision -- 16 years is too long a payback period for you. Fair enough. I never advised any readers to rush out and buy a PV system.

    As I wrote in the article, plenty of energy-efficiency measures have a much faster payback than a PV system.

    Don't be too fixated on the 20% efficiency number. A PV array is not a furnace. It is a miraculous crystal that converts a significant portion of the light energy that strikes it into electricity -- with no moving parts. The idea that a crystal or a rock can even do this in the first place is rather mind-blowing. So don't think that you need 95% efficiency to be impressed. What matters isn't the efficiency; it's the price.

    Right now, the price of PV modules has already reached grid parity in many locations. That's why the PV business is booming in many parts of the world.

    My own experience (based on testing my own 30-year-old PV module) is that PV modules don't experience significant decreases in electrical output over time.

    Go ahead and sniff at a 25-year warranty if you want... but you can't get that kind of warranty for a furnace, refrigerator, or car.

  25. chimewind | | #25

    Snow and payback time
    If you live in snow country it is far easier clearing snow off the panels if they are pole mounted on the ground.

    As far as payback with regard to Warren's comment, you also have to remember that in the USA we do not pay the full cost of the electricity we are producing, especially utility power generated from coal burning power plants. They cause immense amount of health and environmental damage that you do not see on your electric bill. In Minnesota for instance women of child bearing age, pregnant women, and children need to limit the amount of fish they eat from lakes here because of the mercury. Most of the mercury contamination in the fish are from coal fired power plants. This also the case with tuna you buy at your grocery store. On top of that you have premature heart attacks, asthma, etc. all directly due to coal fired power plants.

    How do you put a price on that when PV panels do not cause any of those issues?

  26. Zolton | | #26

    Another calculation..

    I think you're missing an opportunity here to make a point about the "investment" potential of PV systems.

    In my case, our utility provider was mandated by the state of Michigan to produce 10% of its power using renewables by the year 2015. To meet that deadline, the utility set up a program that pays incentive rates for homeowners and businesses willing to invest in solar, wind or hydro technology. I was lucky to be admitted at the $0.525 reimbursement rate level. That means that for every KWH I generate, I get back more than half a buck. My contract runs for 12 years.

    So, I had some money in a money market fund making 0.18% a year. My 2.1 KW system cost 13k to install. After the 30% Federal tax credit, I've got just over 9k into it.

    On that 13k, my money market fund was paying $23.40 a year. I just passed the one year anniversary mark of my installation, and I netted $1,118 in payments from my utility provider for that year's worth of solar production (2,265 KWH), after "system access fees" and sales tax were deducted.

    So, on the one hand, with 13k in a money market fund, that money was making $23.40 a year. On the other, with it invested in my solar array, I've got an income stream of over $1,100. Kind of no-brainer to figure out which was the better deal..

    So, although most people talk about the "payback time" on an installation such as mine, I'm not as concerned about that as I am with putting idle money to work and having it produce a better income stream than I can get otherwise. Even without the incentive rate I'm paid, you could still make a case from an economic standpoint, especially in areas of the country where electricity costs are high, given the low interest rates on money market, CD and savings accounts.

    Plus, having the solar array and its immediate production readout available for perusal via the micro-inverters installed under each panel (Enphase) has made my wife and I acutely aware of how much electricity we use and how much we can conserve. I'm proud to say we've cut our consumption down to just over 2,900 KWH this past year. So that was nearly offset by our solar production - and this is with a central air system...

    One final point: The fact that the solar panels on our roof are mounted about eight inches off the shingles provides a tremendous, unintended benefit. As you pointed out, the panels shade the roof. Air also travels under the panels, coming in at the bottom and exiting the top of that "chute" area, keeping the roof (and attic below) cool. Our A/C system ran a lot less last summer (one of the hottest on record) because of that. And that, in turn reduced our electrical consumption.

    I'd be interested in hearing some more about your "widow's walk" arrangement for clearing snow from your panels. So far my best solution has involved climbing to the top of a 28 foot ladder and scraping snow off my array with a roof rake. Not my favorite thing to do...


  27. GBA Editor
    Martin Holladay | | #27

    Feed-in tariffs
    That's a great feed-in tariff that you are being paid. Many homeowners would jump at the chance to sell PV-generated power back to the grid for 52.5 ¢ per kWh.

    Concerning my "widow's walk": My PV modules are installed on adjustable aluminum racks near the peak of my 12-in-12 roof. The aluminum racks are attached to a "saddle" of pressure-treated lumber that straddles the ridge; the PT lumber sits on top of the steel roofing. On both the north and south sides of the racks, I have installed planks as walkways for access. A trap door in the roof (accessed from my attic) allows me to get up to the walkway when necessary to clear snow from the PV modules.

    It looks goofy, frankly -- but this is a hippie house. It works.

  28. Zolton | | #28

    Sounds safer..

    That sounds safer than my procedure for raking snow off the panels. If I had it to do over I might choose a ground-mount system. But neighbors trees grow tall around our small city lot, so it probably wouldn't be all that practical.


  29. user-1033003 | | #29

    There is the theoretical and then there is the actual output of your solar array. We put one on our roof last spring. We spent a lot of time researching and even had a guy go up on our roof with a special computer that was supposed to evaluate the full arc of the sun's travel throughout the year and estimate the output of a solar array on our roof. The one thing we didn't do was to take that estimate and then divide by two.

    So here is my advice. Listen to the experts. Do your research. Get the best possible estimate of your solar production after applying all the inefficiency factors and so on. Then divide by two. That's what you will actually get from your array.

  30. GkrNt5rHLH | | #30

    What about leased systems?
    No mention is given to leased systems. By letting the system supplier write off the cost of the system as a business expense, the cost of a leased system makes the payback time a whole lot shorter. It turns the 30% government help into 100%. It also puts the hassle of maintenance on the supplier. There's even an advertisement at the bottom of the blog page by a lease supplier and no one says a word. Look into "zero down" offers.

  31. GBA Editor
    Martin Holladay | | #31

    Response to Brian Godfrey
    Many factors can affect the output of a PV array, including the following:
    1. Shading from trees or nearby buildings,
    2. The angle of the array, and
    3. The weather during the monitoring period.

    However, if you estimated your PV array output using the PVWatts calculator recommended in this article, there is no need to "divide by two." As long as you provide the correct inputs into the program, the results will correspond fairly well to your actual electrical production.

    Of course, if you didn't enter the right data into the program -- for example, if you supplied the wrong information on the mounting angle, or if your PV array is partially shaded, or if your location experienced an unusually cloudy year -- then you can't blame the PV calculator for the fact that your electrical production differed from the modeled prediction.

  32. zCMxph5kLK | | #32

    This is too much trouble and too expensive. Some day it will work but not yet. Natural gas creating electricty is much more efficient and cheaper.

  33. user-971679 | | #33

    No Payback Period
    When clients ask me what the payback period is for an energy saving feature in their house design I tell them that there is no payback period. Pay back what? The cost of an improvement or an upgrade? If you want granite countertops instead of laminated countertops, what is the payback period for the granite countertops?

    If you and I buy identical new cars at the same dealership on the same day and I pay $5,000 less than you pay--.and you drive your car home but I have to get mine towed home because my car has no engine--what is the payback period on your engine?

    Energy saving features of a house are not extras. They are the new baseline standard. They cost what they cost. They don't have to be looked at as an investment. If you want a PV array and you have the money, go ahead and buy it. If your house costs more than your neighbor's house because yours has a PV array, then that's it. You don't have to show that your house costs no more than the substandard code-built house next door.

  34. MPFadR5c6L | | #34

    Don, about your PV
    Just wondering why you went with the microinverters. Don't get me wrong--they have their place, primarily for systems experiencing shade issues in which it is more beneficial to directly harness the AC power, and possibly for spit pitches and orientations. Other than that, I question the advantages over a traditional string inverter. The maintenance and troubleshooting has both pros and cons---sure, if a string inverter is down, you lose all your revenue grade power, vs losing only the power of one module. The cons are, however, that the microinverters can be really fun to get to (under the modules, on the roof) if need be, and their reliability has no track record...I'm not sure how I feel about having my inverter in the sunny heat, under a solar panel, exposed to the elements, with big clusters of wires, for 25 years, vs a readily accessible string inverter.

    Also, they're expensive--at over $150 each, plus those bulky connectors, gets expensive quickly. Next, I would be very cautious about purchasing an inverter with a 20-25 year warranty. Most, if not all, solar modules guarantee a performance of 90% of their nameplate rating (given -0%tolerance) over the first 10 years of service. The remaining 10-15 years of service are guaranteed at a 80% tolerance. In other words, the inverter must be able to produce power from a solar module performing at only 80% of its nameplate value, which means that it dumps the extra power for the first 10 years. For example, it is often suggested to have a 235W DC input to produce only 190W AC out. If it were 210-215, even 220W, maybe.....Once again, it depends on your circumstances. I'm only offering something to consider. Also generalizing, but $28K for a 5kW system is almost $1/watt more than installed systems cost today (at least here in south NJ), unless you're using Sanyo modules, FatSpaniel monitoring....or microinverters....

  35. MPFadR5c6L | | #35

    Ted, Energy Saving Features
    Solar PV isn't an energy saving feature. It has nothing to do with conserving energy. It is simply a supplemental source of energy, regardless of how much you use or save. If you are tied to the grid, the solar production is the amount of energy you are not purchasing from the local utility. If you are off grid, it is the total available energy for use, whether or not you are frugal or wasteful...

  36. MPFadR5c6L | | #36

    Brian, Response to comment #30
    I am confused about the "divide by 2" ... I believe in accurate site surveys and proper installations. The electronic "computer" on your roof was probably a Solmetric SunEye, although there are others out there. Knowing how to do the math comes in handy when the need to make field adjustments happens. Converting real to solar time, for example, magnetic declanation, determining exact azimuth, etc...The surveyor should know how to do a proper survey without the "shortcut" tools, and use them as a supplementary data collection tool only. They are convenient in terms of having a built in fish eye camera, but the electronic compass on the newer model is very sensitive and not always accurate. Power lines above, even a belt buckle, cause interference with the orientation reading. I always carry a marine grade magnetic compass to compare notes. Normally, however, the differences and inaccuracies aren't a big deal.

    Next, and even more critical, are the points in which the shade analysis data is taken. For example, suppose your roof will have 3 rows of 8 modules each in portrait (approx 16' x 22'). There is some early morning and late afternoon shading for purpose of this example. If you took pictures of each corner and averaged the production estimate of each picture, you will determine an overall system production estimate. Now, if you used the same four corners, but also took pictures of the sunnier middle section of the array and averaged them in with the corner pictures, the result will be greater annual production. This is a big point because the sunny portion of a string does not count if even a small portion of the string is shaded, and I don't think this crosses everybody's mind. I've seen too many systems performing 5-15% below their predicted output.

    I think the problem is that people are trying to sell, and forget the simple rule of thumb--conservative predictions often result in pleasant surprises---. I'd rather expect less and get more than vice versa.

    My next point is about the installation and materials as well. 240W modules are not necessarily the same. You must also consider the power tolerances. Each batch of silicone is basically 240W, but not always EXACTLY 240W. For example, many modules have a -0%+5% rating. This means that the actual 240W module will perform at a minimum of 240W and possibly as much as 252W. Another 240W module may have a power tolerance of -5%+10%. This means that the module may not actually be putting out 240W. It could be generating as little as 228W or as much as 264W---a BIG difference. So just based on this power tolerance, our example system of 24 modules has a large range of power production.

    Also, installation can affect power. For example, if the inverter is installed on the sunny side of the house with no shade protection, the heat will cause greater inefficiency in terms of DC to AC energy conversion, which adds up over the course of the year. Another example is the space between the roof and the module. A 4" minimum airspace should be achieved. Any less and you're compromising the airflow underneath the modules, causing heat build up. The hotter the modules, the more inefficient they are. Voltage drop from long wire runs will lose power, as will wire in conduit attached directly to the roof. I always suggest raising the conduit off the roof, even if it's only by 1/2". It makes a huge difference in terms of the heat potential within that conduit. There are several other things to think about, but these are the biggees...

  37. GBA Editor
    Martin Holladay | | #37

    Response to Deniz Bilge
    Your concern that declining power output by PV arrays as they age might interfere with the operation of the inverter is, as far as I know, groundless. An inverter has a maximum current rating but is able to work efficiently when the DC current it receives is less than the maximum. (After all, that's how the inverter works at 8 a.m., when sunlight is weak, or on a cloudy day, as well as at noon on a sunny day.) It can handle varying inputs without a significant decrease in efficiency, as long as its maximum current rating isn't significantly exceeded.

    Moreover, maximum power point tracking (MPPT) technology goes a long way towards addressing the problems you worry about. This technology optimizes incoming voltage and current for maximum inverter efficiency under a wide variety of conditions.

  38. user-971679 | | #38

    response to Deniz Bilge
    Deniz, thanks for your comment. I agree with you that a PV array is not necessariy an energy saver. I was just using it as an example of an expense you might have in building, say, a net zero energy house. But the PV array is not what I'm writing about. I'm asserting that there is no payback period because there is no extra feature to pay extra for. The feature is a basic piece of the net zero energy design, like the engine of a car. The engine is not an extra, you don't pay extra for it, and you don't expect to get paid back the money it costs.

  39. DFMallinson | | #39

    micro inverters
    deniz bilge, you asked why I added micro inverters in place of one large DC/AC inverter. Alas, a neighbors tree at certain times of the year and day casts a degrading shadow. An expense for sure but the feds would be picking up 30% of the additional cost. Will this be something else to go wrong and necessitating repairs? Perhaps, but from my experience in the computer industry, failure of electronics follow a bathtub curve. Steep at the beginning, followed by years of expected performance, followed by steepening failure rates. This is why most electronics go through a brief burn-in at the factory prior to shipping. As we all know, statistical averages apply to large numbers.
    You also mention I was paying a seemingly high cost. The PV system was going to cost me 11% of total construction cost. With rebates, tax credits, grants, etc my costs were going down to 4% of my budget. In a few years, sale of SRECs credited to me would pay back all costs before generating income. I focused on other budget issues. BTW, I'm an owner worried about green environment unlike most contractors worried about greenbacks.

  40. MPFadR5c6L | | #40

    Responses to Martin, Don
    About microinverter performance: I understand that today's sine wave inverters combined with MPPT tracking are 95% efficient and up...when they are "converting" energy. What I am saying is that given the same amount of DC input power, the AC output power is less with microinverters than is with string inverters. There's also the factors that contribute to the high efficiencies. As far as string inverters go, generally speaking of course, there is a "sweet spot" of highest efficiency close to, but not quite at its maximum input power. I feel that this characteristic allows the system to squeeze out a bit more power than its microinverter counterpart. I am far from an engineer and am only speculating, but the facts are that I have seen a higher DC(STC) to AC conversion ratio than with microinverter systems.

    Don, I couldn't agree with you more about solar PV---it isn't just about the payback. Solar PV was so heavily marketed here in NJ as a 2 year payback, 20 year money making venture, bla bla bla, and sold by people who knew or cared nothing more than signing lease or financing contracts...The real environmental benefits, while somewhat obvious, were treated as politically correct finishing touches. Everybody sure saw green(backs), but nothing more. As a result, the rat race was on, solar contractors (who I wouldn't even trust to caulk around my kitchen sink, let alone install 350V on a roof they just made 48 penetrations through) sprouted everywhere, and I mean everywhere...Now that the financial incentives are winding down, the whole buzz seems to be disappearing with it, except for the not yet emerging solar installations repair industry---you watch, those 5 yr installer warranties should be expiring soon ... Nobody really cares about environmental benefits, or the substantial benefits of solar. ..and it's a shame.

    However, these "green" conscious people still play video games and use their $700 facebook machines all day and night, blow leaves instead of rake, sit down lawn mow instead of blade'n push, drive to the local store (less than 1/4 mile away), etc....It's all about CONSERVATION, not production.... Getting back on track:

    I do think PV is a no brainer for new construction---for $25-40K added to a mortgage ($150-200/month)---I'd rather pay a higher mortgage and own a solar PV system than pay a lower mortgage and higher utility bills.

    I wasn't trying to get you all worked up when I mentioned that I felt over $5/watt was high. I don't know your neck of the woods, nor do I know exactly what system you went with. Sure, a crappy inverter with cheap racking components and inferior grounding lugs, etc, will be cheaper than an above and beyond system. I was only trying to offer outside sources of information, namely that a well installed system of medium grade components is around $4.50/watt installed,period, regardless of incentives, SREC's, and other paybacks.

    Next, I must agree and disagree with you as far as the maintenance issues go. Once again, I don't know your situation--you may live 5 minutes from readily available parts, and have a 1 story rancher with a 6/12 pitch roof. What I do know is that the "years of performance" part of your bathtub curve statement are missing from the live world experience of microinverters, and that it can be quite time consuming to A) troubleshoot & determine the problem, and B) to physically isolate and remedy it...Dealing with a slate 12/12 pitch roof 37 feet up is not easy or fun to deal with. I'd rather there be 1-3 inverters READILY ACCESSIBLE. When comparing micro to string inverters side by side, I was offering my opinion as to why I would choose string.

    Once again, microinverters have their place when used properly, but I would still question the use of one instead of a string inverter in most general situations without shade issues.

  41. Solorea | | #41

    Panneaux Solaires
    This is the good news: In France it is now possible to have a Solar Panel of your roof-top without investing a penny! Find out about Jeremy Rifkins impact on the solar energy sector in this article:
    Greetings from France!

  42. rabakomaba | | #42

    build it cheap
    You can build your own solar panels from home and most of the parts can be found from your local hardware store. If you're interested take a look at these website :

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