Compare Panel kWh Output (Sunpower vs others)

[erock]

Hi, I'm looking to put solar on a roof in Colorado, and have gotten quotes from several vendors, with different panel types, including a few with Sunpower. My question is around the projected outputs of the different panels/systems.

Does a Sunpower system with microinverters produce significantly more kWh production than others? Without getting into too much detail (though I can provide if needed...), the one quote for a sunpower system (using 14 - 250 Watt X20 panels with microinverters) gives approx. 1,715 kWh production per 1 kW of size (3.5 kW system size, 6,003 kWh production). The other quotes, including one other for a sunpower system (without microinverters) has productions of around 1,500 kWh for 1 kW of size. For example, one quote for 11 - 335 Watt sunpower panels (3.685 kW size) says it will produce 5,698 kWh, or 1,546 kWh per 1 kW size.

Does this make sense, and who is telling the "truth" about the sunpower panels production? I asked the vendor with the high production to check his numbers, and he says he double-checked and this is what he gets using sunpower's website and PV Watts. The agreement for this company clearly states that higher production amount on the paperwork to be signed, so I suppose I am "covered" in a sense if the system under-produces to that number, right?

Thank you for your help/feed-back.

[J.P.M.]

Hi, I'm looking to put solar on a roof in Colorado, and have gotten quotes from several vendors, with different panel types, including a few with Sunpower. My question is around the projected outputs of the different panels/systems.

Does a Sunpower system with microinverters produce significantly more kWh production than others? Without getting into too much detail (though I can provide if needed...), the one quote for a sunpower system (using 14 - 250 Watt X20 panels with microinverters) gives approx. 1,715 kWh production per 1 kW of size (3.5 kW system size, 6,003 kWh production). The other quotes, including one other for a sunpower system (without microinverters) has productions of around 1,500 kWh for 1 kW of size. For example, one quote for 11 - 335 Watt sunpower panels (3.685 kW size) says it will produce 5,698 kWh, or 1,546 kWh per 1 kW size.

Does this make sense, and who is telling the "truth" about the sunpower panels production? I asked the vendor with the high production to check his numbers, and he says he double-checked and this is what he gets using sunpower's website and PV Watts. The agreement for this company clearly states that higher production amount on the paperwork to be signed, so I suppose I am "covered" in a sense if the system under-produces to that number, right?

Thank you for your help/feed-back.

Run PVWatts and see for yourself.

A properly paired inverter/panel combo, Sunpower's or anyone else's panel will produce about equal output per installed kW per year. Sunpower's advantage is a smaller footprint. That's about it. Sunpower's big disadvantage is about a 20 - 25% upfront price premium for little if any, measureable increase in annual output for probably longer than you may own them, vs. less expensive but still quality equipment.

So, if your roof real estate is worth more than the average, and bragging rights are what you fancy, Sunpower may be for you. Otherwise, let common sense be your guide.

[erock]

Run PVWatts and see for yourself.

A properly paired inverter/panel combo, Sunpower's or anyone else's panel will produce about equal output per installed kW per year. Sunpower's advantage is a smaller footprint. That's about it. Sunpower's big disadvantage is about a 20 - 25% upfront price premium for little if any, measureable increase in annual output for probably longer than you may own them, vs. less expensive but still quality equipment.

So, if your roof real estate is worth more than the average, and bragging rights are what you fancy, Sunpower may be for you. Otherwise, let common sense be your guide.

Thanks for the input, I've tried running PVWatts myself, and can get to various production numbers depending on the system losses & other variables that I plug in, is there a way to "know" what the correct values to put in for system losses & etc. are?

[J.P.M.]

Thanks for the input, I've tried running PVWatts myself, and can get to various production numbers depending on the system losses & other variables that I plug in, is there a way to "know" what the correct values to put in for system losses & etc. are?

You're welcome.

The short answer is yes. The rest of the answer is something like: but it may not matter a great deal. The default for system losses is 14%. that's adjustable and maybe a bit high, depending on a lot of stuff. Probably closer to 10% may be a bit more realistic for most well designed systems, perhaps less. A lot of system estimates using PVWatts seem to get closer to a lot of systems outputs when the default is set closer to 6-8 %, but that's a hard thing to est. or rely on given the limiting nature of the tool's long term estimates. PVWatts estimates and those of another tool called SAM match when the PVWatts default for my system is set to 6%.

There are finer adjustments avail. If you read the help info screens you'll get a better feel for what to use.

In the end, it's an estimate of long term system output over many years with individual years varying as much as the weather.

It will show that system output, one panel mfg. to the next is not that great.

BTW, the diff. in output between premium panels ( Sunpower) and std. panels (everyone else) is about 2% according to PVWatts. I think it's closer and in a few cases greater for the std., plain vanilla panels.

[sensij]

BTW, the diff. in output between premium panels ( Sunpower) and std. panels (everyone else) is about 2% according to PVWatts. I think it's closer and in a few cases greater for the std., plain vanilla panels.

I'm not sure that "Premium" really just applies to Sunpower any more. Per the PVWatts documentation, the following guidelines are offered for choosing the panel type:

Panel types.JPG

Most of the panels that are frequently discussed now have an AR coating, and have a thermal coefficient somewhere in between that of standard and premium.

I'd agree that using a loss factor of 6-10% should get you a decent range to plan around, and just keep in mind that production in any given year may vary by 10% or more from that, since weather is so unpredictable.

[Mb190e]

This may not be the case here, but I also had people giving me estimates for sunpower panels that I believe were overestimating production to make the numbers look better.

[J.P.M.]

I'm not sure that "Premium" really just applies to Sunpower any more. Per the PVWatts documentation, the following guidelines are offered for choosing the panel type:

[ATTACH]6764[/ATTACH]

Most of the panels that are frequently discussed now have an AR coating, and have a thermal coefficient somewhere in between that of standard and premium.

I'd agree that using a loss factor of 6-10% should get you a decent range to plan around, and just keep in mind that production in any given year may vary by 10% or more from that, since weather is so unpredictable.

I've been clamoring about the yr/yr variance and the problems of using design tools such as PVWatts as predictors of performance since I got here.

On the diff. between std. and premium as PVWatts defines it:

I understand your point. I tend toward a similar but not identical opinion.

The LG 305's, for example, come in with an STC eff. of about 19%, and claim anti reflective coating. So, by that stated criteria, they do qualify as "premium". I'm sure there are other quality panels that fit those criteria.

Lots of others do not (yet) make the cut.

As a sort of aside, and not my call, but I'm not sure I agree with using an antireflection coating as a criteria to differentiate standard from premium in the context PVWatts uses those terms. First of all, as you note, many mfg. now claim AR coating. While it MAY have some measureable benefit at large angles of incidence, the benefit gained in terms of actual output is less, due to that high angle of incidence - a 20 % decrease in reflectivity (of one surface) of a 40% reflection loss, which loss is about 30% of the P.O.A. irradiance, which is itself a small # at those angles doesn't add too much to a day long total. Also, I'm not completely convinced of the durability of such coatings, or their efficacy under fouled conditions. I think some of the AR hoopla is advertising hype. Perhaps AR is not harmful, depending on method used (I believe the jury's still out on that), but I'm not convinced it produces long term benefits to the degree implied. Maybe I'll be proven wrong by the test of time.

On the PVWatts cut criteria, and perhaps slightly less conservative than you note, my observation is that most panels do not have the lower temp. coeff. of performance, with most being in the 0.5%/deg.C. range.

Additionally, for my purposes anyway, and to differentiate in a conservative way that Sunpower panels are less cost effective than other panels for most applications, Giving Sunpower ~ a 2% boost in estimated long term output and still be able to call it less cost effective seems wise.

Given the somewhat approx. nature of the definitions, I'd put the S.P. stuff in the premium category and a few others, on a case/case basis in the top shelf "premium" category as well. I'd put most of the rest on the standard category unless the situation dictated otherwise.

[J.P.M.]

This may not be the case here, but I also had people giving me estimates for sunpower panels that I believe were overestimating production to make the numbers look better.

Or perhaps another way to see it, seeing as how most panels have very similar output/nameplate Watt, they were underestimating another panel's output.