Are Panels meeting output rating

[plannersteve]

My system consists of 30 panels with 290 W Nominal output with a Sunny Boy SB 7.7 1 SP US-40 inverter rated at max continuous output of 7680 Wac. Panels are roof mounted 7:12 pitch, oriented to 170 degrees. I'm in North Idaho, so June sun's altitude during mid day is around 60 degrees. I have no shade.

Am I correct in believing that there should be long periods of time (a couple hours) in late June on a sunny day that my inverter should be maxed (clipping) given that my panels rated output would be 8700 W? I never observe this. I attached two output curves. Typical sunny day shows the curve on a blue sky sunny day. this one peaked at around at around 7100 W, but typically I'm seeing 6800 W. Max inverter shows that once in a while, the output of the inverter is at maximum, but this is very rare.

Should I be raising this as an issue, or is this what can be expected for a system with this DC to AC ratio?

Thanks, Steve

[littleharbor]

Have you tried running this info thru PV Watts? STC ratings on panels are rarely achieved. Warm summer days and imperfect orientation will affect your output.

[J.P.M.]

+1 on checking against PVWatts. Use 10 % system losses instead of the default 14 %. Use the hourly output option and look for a clear day's output around the date of your recorded output. Look for agreement or not.

Have you called/asked the installer the same questions you ask here ?

[peakbagger]

Generally the panel put out more power the colder they are. I have seen my panels exceed their rated output on very cold days with snow on the ground and may panels set at winter angle. minus 20 F with sun and light breeze can definitely crank out the watts, the bummer is at my latitude, the days are quite short.

[plannersteve]

I plugged my system info into PV Watts with 10% losses. Using that information, my panels are producing as expected, maybe 1 or 2% more. Thanks for that suggestion!

Yes, I have asked my installer but gotten no response. But even if I had I would consult this site for the collective knowledge here.

Peakbagger, I have noticed that my periods where I came close to exceeding my inverter rating were also on cooler days. Looking at the temperature curve for my panels, it looks like the output at 25 degrees C is between 5 to 10% higher than output at 45 degrees C.

[J.P.M.]

I plugged my system info into PV Watts with 10% losses. Using that information, my panels are producing as expected, maybe 1 or 2% more. Thanks for that suggestion!

Yes, I have asked my installer but gotten no response. But even if I had I would consult this site for the collective knowledge here.

Peakbagger, I have noticed that my periods where I came close to exceeding my inverter rating were also on cooler days. Looking at the temperature curve for my panels, it looks like the output at 25 degrees C is between 5 to 10% higher than output at 45 degrees C.

If that thank you is meant for me, you're welcome.

If you like our service, tell your friends.

[JSchnee21]

Hi plannersteve,

When you say your panels are 290W nominal, is that STC or NOCT? What is the make and model of your panels? If they are 290W STC, you're unlikely to ever see 30*290W. The STC test conditions by the MFG aren't really representative of real life.

80%*290W*30=6960W is likely to be much more representative.

-Jonathan

[plannersteve]

They are 290 W Nominal Output (Pmpp). All data at STC. Your estimate seeems about right. Thanks for the input.

[plannersteve]

Now that fall is here, it is more apparent to me how much impact temperature was having on my results. I'm seeing my inverter clipping some now that the temps here in N Idaho are dropping. That's even with the sun at a lower than optimal angle.

This makes me wonder if ground mount systems might get higher output? It would seem that airflow around the panels would reduce the temperature a bit compared to a roof mount system. I didn't have the real estate for ground mount, but might be something to consider for those that do.

[bcroe]

This makes me wonder if ground mount systems might get higher output? It would seem that airflow around the panels would reduce the temperature a bit compared to a roof mount system. I didn't have the real estate for ground mount, but might be something to consider for those that do.

Some advantage, the difference will not be huge. I have a list of 43 advantages of a ground mount. Bruce Roe

[J.P.M.]

Now that fall is here, it is more apparent to me how much impact temperature was having on my results. I'm seeing my inverter clipping some now that the temps here in N Idaho are dropping. That's even with the sun at a lower than optimal angle.

This makes me wonder if ground mount systems might get higher output? It would seem that airflow around the panels would reduce the temperature a bit compared to a roof mount system. I didn't have the real estate for ground mount, but might be something to consider for those that do.

For the same orientation if you have little space between the roof deck and the underside of the array, you may see a bit of improvement with a ground mount, but as long as you've got a reasonable clearance, say, 6" or more off the roof deck, a ground mounted array may run a couple deg. C. cooler than the roof mount, which might get you a % or so improvement a couple of hrs. on either side of high noon on a sunny, calm day, otherwise, not much improvement from improved circulation behind the array. Bottom line, a slight increase in output by itself probably isn't worth the extra cost of a ground mount.

[plannersteve]

A final post on this topic as the reason I observed lower than expected performance is now clearly temperature related.

During recent cold weather we had a sunny day. My inverter clipped for several hours that day. The sun was only about 32 degrees above the horizon, but the temp was in the low 20's. Contrast that to a good day in June where the temp got to the low 80's and sun was at an optimal angle of 60 degrees. Inverter topped out at 7.1 kW, well below max of 7.8.

Attached is the original summer curve and the cold weather curve for comparison.