Solar Output - theoretical or realistic?

[Crashmaster]

We just had solar installed and been using it only a few weeks. Here is the system we have.


42 JA Solar panels 355 Watts per panel

42 SolarEdge P370 Optimizers

2 SolarEdge SE6000H-US Inverters

Unirac Ground Fixed Tilt Racking


If you multiply 355 watts x 42 panels you get 14.9kW. We thought we were purchasing a 14.9kW system. So far, our system has put out only 12kW maximum. We have had some very clear and very sunny days, but no more than 12kW. I spoke to my solar installer and he told me that the 14.9kW is only theoretical, sort of like 35 mpg. You might get 35 mph on your car under perfect conditions, but in day to day life, you won't. Should I be getting 14.9kW?

Who is correct here?

[azdave]

The panel output rating only applies when tested under STC (standard test conditions in a lab) so that all panels can be compared fairly between manufacturers. I have a 6.63kW system and see at most 5.5kW on the best cool days around noon in the spring and fall. That is 83% of their rated STC output. Just spit-balling the same 83% of your system STC numbers gets you around around 12kW. Since you are in the midwest, I would guess that 12kW is in the ballpark for you but that a rough guess.

This is stuff you should have understood before you purchased. Have you looked at the free site PVWatts to get output estimates for your system and location?

[J.P.M.]

We just had solar installed and been using it only a few weeks. Here is the system we have.


42 JA Solar panels 355 Watts per panel

42 SolarEdge P370 Optimizers

2 SolarEdge SE6000H-US Inverters

Unirac Ground Fixed Tilt Racking


If you multiply 355 watts x 42 panels you get 14.9kW. We thought we were purchasing a 14.9kW system. So far, our system has put out only 12kW maximum. We have had some very clear and very sunny days, but no more than 12kW. I spoke to my solar installer and he told me that the 14.9kW is only theoretical, sort of like 35 mpg. You might get 35 mph on your car under perfect conditions, but in day to day life, you won't. Should I be getting 14.9kW?

Who is correct here?

You did purchase a 14.9 kw system. Your problem is your lack of understanding and education about what you were buying and how it works is misleading you into unrealistic expectations of system performance.

As Azdave notes, you will rarely, if ever, see a system producing its STC output.

[peakbagger]

Time for some education. The panels are rated at kilowatts (KW) which if they are illuminated with a standard light source at a standard temperature they will put out the rated KW rating for as long as the standard light source it turned on. If the standard lights are left on for exactly one hour, your panels would put out 14.9kilowatt hours (KWh) DC at the leads. Think of KW as hose connected to pump. The KW is the gallons per minute rating of the pump and the KWH is how many buckets of water you fill.

The useful output of the system is kilowatt hours (KWh). Its rare that a PV system would even be illuminated with standard light source and in the real world even if they did there are conversion losses between the DC output of the panel leads and the typical location for a meter which is measuring KWh sent to the grid. These losses vary but figure 15%(it could vary).

Of course the standard lights at the factory are directly in line with the panels. Its pretty rare for the sun to be directly in line with the panels so you need some trigonometry to calculate the loss from the angle being different over the course of the day. And now add in loss for clouds and shading. Temperature changes the efficiency of the panels so that varies over the course of the day. So in order to account for variables the installer uses software that references nearby locations where the suns irradiance has been measured for long periods of time over many years. The assumption is that that site experiences similar temperatures, clouds and hours of daylight as your installation. The installer also used a device to see if there is any shading at your location that wouldn't be at the reference site and also factors in other local factors. From there they can make a informed guess on how many KWhs per year you generate and see how it lines up with your prior yearly purchase of KWhs from the power company. The sweet spot can be to offset your PV production versus your prior power production but depending on where they are installed and incentives in place the system may be optimized for other sweet spots.

If you want to play around, a simple version of the sizing program is PVWATTS which is free on the internet. Plug in your basics and it will estimate your monthly production. One thing to keep in mind depending on where you are in US, summer production is generally much higher than winter production so you are probably seeing the low end of the curve and missed out on the best part of the year. There are also seasonal variations, my spot in the northeast tends to have a lot of weather systems come through in Ocober and November so my production is quite low due to clouds

[J.P.M.]

Over the years, I've thought that it might be useful to create a sticky or other way to inform folks of common yet most likely incorrect or at least non productive or uneconomical thinking about what works, doesn't work and why, and/or pros/cons of an issue such as string vs. micro inverters for example, or the benefits/drawbacks of conservation before solar generation, etc., lots of other common questions with common answers.

Doing so might save a lot of time if only to keep from answering the same questions or re-disabusing a new set of people of the same incorrect notions every few weeks or so.

The subject of this thread is one such item.

Maybe such a thread's title might be "Read this before you ask". For those too lazy or ignorant to read that first, answers to such repeated questions could be in the form of : " See item 4 of the 'Read this before you ask sticky'."

[Crashmaster]

Many thanks for the responses. I now understand. It is such a learning curve, but really enjoying the solar.

[SunEagle]

I am curious what the rated output of those SolarEdge SE6000H inverters are.

To me they equal 12000 watts total unless they can output more then their model number suggested so even with 14.9kw of panels you won't get more then 12kw because of clipping.

[JSchnee21]

Yup, unlike the A-series, the rated output of the HD series is right on the money. Only 6kW @240V from the 6000H-US (only 5kW @ 208V). That's your primary issue.

Now, that said, under most real world conditions, it's pretty rare you will ever see the STC rated kW output and if you do, it usually won't be for more than a hour or two per day. And then only when its cool, clear, and the panels are clean, and optimally aligned (pitch and orientation). Unless you live somewhere where the solar insolation is higher than average (aka parts of the South West United States)

[J.P.M.]

Yup, unlike the A-series, the rated output of the HD series is right on the money. Only 6kW @240V from the 6000H-US (only 5kW @ 208V). That's your primary issue.

Now, that said, under most real world conditions, it's pretty rare you will ever see the STC rated kW output and if you do, it usually won't be for more than a hour or two per day. And then only when its cool, clear, and the panels are clean, and optimally aligned (pitch and orientation). Unless you live somewhere where the solar insolation is higher than average (aka parts of the South West United States)

FWIW, the highest instantaneous output I've measured from my array (5.232 kW) after measuring that sort of thing for 4+ years was 4,883 W into the inverter on 04/20/18, 1307 hrs. P.D.T.
Several other days have been close to that, +/- a few W over the years, usually in April.
I measure a bunch of stuff on clear days only - and I mean cloudless - (when I'm not otherwise occupied) at the minute of the daily min. incidence angle which for that day and time was 3.32 deg.
The G.H.I. at that instant was 968 W/m^2.
The calc'ed P.O.A. using a modified HDKR model was 1,036 W/m^2.
The ave. cell temp. at that instant was ~ 46.2 C.
The amb. air temp. at the array at that instant was 21.8 C. (a cool day for April).
The ave. (7 minute) wind vector was 3.9 m/sec. from the WNW/NNW. That wind mag. is rather high for this location. (ave. ~~ 1.8 m/sec. +/- a bit.) ,

The lowest clear sky instantaneous output I've measured using the same methods was on 11/23/2015 at 1155 hrs. P.S.T.
Instantaneous array output at that time day was 3,724 W into the inverter.
G.H.I. was 589 W/m^2.
P.O.A. was 792 W/m^2.
Ave. cell temp. was 50.9 C.
Amb. air temp. at the array was 28.6 C. (a warm day for Nov.).
Wind was 0.8 m/sec. from the SW and lower than average.

All of which seems to point to numbers that support the idea that STC rating won't happen too much - even with an array semi optimized for annual production, although I'm not sure what location has to do with it. Clear skies happen on an instantaneous basis in other places besides the SW U.S.

[solarix]

I think the word your installer should have used is "ideal" rather than "theoretical". Under ideal circumstances, your system will produce 14.9kW. But, ideal conditions are quite rare. Ideally - they are full sun with cool, room temperatures. When your panels have full, they are not cool (except when tested by the manufacturer for a one second flash test). PV panels lose efficiency the hotter they get. Label rating or "STC" is to the benefit of the manufacturers as it is standardized and gives a high number. Everyone in the industry knows this and uses this rating as just an apples to apples comparison. Then we use the less well known "PTC" rating as to what the panels will do in the real world.

[Crashmaster]

Would you say that the 6000 watt inverters were incorrect for my system? Should it have been the next size up being the 7600 watt inverters? Would I get any better return with the 7600 watt inverter?

[J.P.M.]

Would you say that the 6000 watt inverters were incorrect for my system? Should it have been the next size up being the 7600 watt inverters? Would I get any better return with the 7600 watt inverter?

Well, in your case if the inverter data sheets say the max. output is less than 1/2 your array STC output, I'd expect some clipping. The farther away the combined inverter rating is from the STC rating of the array, the more clipping will occur. Make sense ?

Now, whether or not the amount of production lost due to inverter undersizing is significant is a somewhat unanswerable question, but some perhaps ignorant but common sense would tell me that undersizing an inverter is the same as making an array with an output larger than the inverter's ability to turn that output into about 95+ % or so of that array's input. Sort of like designing a 600 HP ICE vehicle and then intentionally throttling the output to 500 HP. Why would I do that ?

Maybe I'm looking at the wrong data sheets, but the data sheets I've seen for the SE 6 kW inverter say its max. output is 6 kW. If that's true, then every time the array's output is greater than (2 * 6000 W)0.95 clipping will occur. How often and how much ? Unknown. Educated guess ? Run PVWatts, get the hourly output on a spreadsheet and see how many hrs. the output exceeds 12 kW, and/or by how much. That's probably a reasonable guess.

So, Looks to me like your question is how much clipping associated with a 15 STC kW array feeding into 2 ea. 6 kW inverters.
and what's the long term NPV ? of those losses vs. the cost differential between an SE 6000 and an SE 7600 ?