# Solar: KWH per KW of Panel Rating



## jebatty (Sep 23, 2014)

One measure of panel performance is AC kwh of production per DC kwh rating of the array. For example, my array is rated at 6.89 DC kwh, and yesterday my array production was 41 AC kwh on the production meter.

Production AC kwh / Rating DC kw = 41 / 6.89 = 5.95 kwh/kw for the day

The best day I have ever had was 48 AC kwh (6.97 kwh/kw).


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## Vic99 (Sep 23, 2014)

My understanding is that panel rating in Watts is based on lab tests at 25 C.  It can be up to 4 W higher than that in that testing scenario.  So a 250 W panel can actually be 250-254 W panel.

For every degree C less than 25C, increase efficiency by ~2%.  Do the reverse for warmer temps.

My panels usually max out during peak sun in Jan in MA . . . Because my microinverters clip at 225 ish.  Obviously, day length is substantially less.


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## jebatty (Sep 24, 2014)

Good point on temperature impact. I know too that on a cold, clear day in Feb-Mar, my peak output is higher than on a clear day during summer. But as you say, day length in winter is short compared to summer, so summer per day output is considerably higher than winter per day output. Perhaps a good idea would be for anyone interested in reporting kwh / kw to also report daytime temperature. The air temperature high on the 41 kwh day was 71F (21.7C), and on the 48 kwh day (Feb 23) the high temp was 14F (-10C).

I've been fortunate to have very little clipping, with a good match between the micros and the panels. I have 26 panels. The panels are rated at 265w (6.89 kw), micros at 250w (6.5 kw) with maximum output at 260w (6.76 kw), and maximum usable DC input at 265w (6.89 kw) I have seen actual array maximum output as high as 6.8 kw (261.6w per panel). Clipping has been a rare event, right around the 261w per panel point.


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## Where2 (Sep 24, 2014)

We're also assuming any panels face due south. If you turn them slightly, to the east or west, then you don't reach the same output peak, or daily output.

The effect is similar to having the wrong tilt on the panels.


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## jebatty (Sep 25, 2014)

I think I inadvertently landed on a sensitive spot for PV owners regarding performance. There are many things that impact performance, and the idea was not to be critical of anyone's PV, but rather to obtain information to better inform PV owners, present and prospective, on what they might be able to expect regarding performance. I have been doing this with another local PV owner whose PV is not optimally sloped and who has substantial morning and evening shading, both things that could not be avoided. We have used the kwh/kw measure to better understand our systems and not to attempt to show one system is better than another. At the same time, the other PV owner has several different arrays with different mfr PV, different slopes, and also both microinverters and DC string inverter arrays. The kwh/kw measure has been very useful to that owner in evaluating his systems.

If the kwh/kw measure of performance is not useful information to share, then no need to do so. It may be time to move on to a better topic.


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