20% reduced solar energy production over three months

[UltimateBid]

Hello everyone, I got a 6.7kw system installed in December and all was going well until I noticed that my solar production has already taken a dive. in April on a full sunny day (very smooth curve on the daily graph, no spikes or jumps from clouds) I made up to 43 kwh. In May we only had one day that had a smooth curve and it created 37.3 kwh. Now in June we have finally had a few good sunny days in a row and the highest production has only been 34.5 kwh. The peak production in the middle of the day in April on the graph was 5.8 kw (though instantaneous readings showed just over 6 kw a few times), while peak production in June is 4.9 kw. Can anyone think of why this is happening? I know the angle of the sun and panels can make a difference, so if the sun was perpendicular to the panels in April, as it moves higher in the sky into summer there will be some loss of production, but from what I've read it shouldn't be anywhere near 20%. Thank you in advance for any thoughts or suggestions.

[SunEagle]

There could be a few reasons for the lower production.

Higher temperatures during peak times can limit output
Something on the panels (dirt, grim, bird poop, etc.) will reduce output
Poor Sun angle (as you pointed out) can reduce output
High resistance in the DC wiring (bad design or poor connections) can reduce output

It is hard to say without more data.

In the mean time check to see if the panels are dirty and find out what the temperature is at the panel during peak production.
You could always contact your installer to see if they can find the problem.

[jflorey2]

Can anyone think of why this is happening?

As a simple test, hose down your panels while watching output. If it jumps up back close to what you were generating in April, then it is probably temperature or dirt on the panels.

[J.P.M.]

Hello everyone, I got a 6.7kw system installed in December and all was going well until I noticed that my solar production has already taken a dive. in April on a full sunny day (very smooth curve on the daily graph, no spikes or jumps from clouds) I made up to 43 kwh. In May we only had one day that had a smooth curve and it created 37.3 kwh. Now in June we have finally had a few good sunny days in a row and the highest production has only been 34.5 kwh. The peak production in the middle of the day in April on the graph was 5.8 kw (though instantaneous readings showed just over 6 kw a few times), while peak production in June is 4.9 kw. Can anyone think of why this is happening? I know the angle of the sun and panels can make a difference, so if the sun was perpendicular to the panels in April, as it moves higher in the sky into summer there will be some loss of production, but from what I've read it shouldn't be anywhere near 20%. Thank you in advance for any thoughts or suggestions.

Angles of incidence will have some influence, but array temps. and possible shading will also matter. A 10 deg. C. increase in array temp. will cause something like a 5 % decrease in array instantaneous output. Run PVWatts using about a 10 % system loss parameter, and get the hourly output for a clear day on/around an actual clear day for a dart throw comparison, and know that all clear days are not equal output days.

[UltimateBid]

Thank you for all of the suggestions! The panels should be pretty clean since we have had a lot of rain recently and shadows should also not be a problem. Unfortunately, the panels are 2.5 stories up so I can't easily spray them down with a hose, but I did look into temperature and the panels have a Pmax of -0.420 so with a current panel temperature of 117 degrees f that alone will reduce the output by 10%. And I didn't see the hourly output option for PVWatts before, but it shows a maximum hourly output in all of June as about 4.5 kw and maximum daily output in June of about 32 kwh, so I guess I'm doing OK. Thank you again.

[DanKegel]

As a simple test, hose down your panels while watching output. If it jumps up back close to what you were generating in April, then it is probably temperature or dirt on the panels.

Wait, could that crack the glass?

Glad to hear the output decrease is as expected. Have you set up your monitoring to upload to pvoutput.org? We might have more to say once we see the data.

[azdave]

It is not very scientific, but as an example, I took a look at my best production day of the year, April 24th (low 61F, high 84F). Comparing it to the hottest day so far this season, June 4th (low 82F, high 115F) I see a 14% reduction in output. These are both typical full-sun days in Phoenix AZ with no shading issues. I'm sure panel heating accounts for nearly all of this output drop.

[inetdog]

If you look at corresponding one hour periods in morning and afternoon when the sun angle on the panels is equally poor, you can hope to see the effect of morning versus afternoon temperature. Also recognize that for a fixed amount of solar heating and the same ambient temp it will take awhile for the panels to reach maximum temperature.

[J.P.M.]

If you look at corresponding one hour periods in morning and afternoon when the sun angle on the panels is equally poor, you can hope to see the effect of morning versus afternoon temperature. Also recognize that for a fixed amount of solar heating and the same ambient temp it will take awhile for the panels to reach maximum temperature.

Not a bad 1at approx. Just use sunny days and don't do it too early or too late. Irradiance changes as a % of POA are more pronounced as f(time away from solar noon). 9 A.M. and 3 P.M. might be good choices. FWIW, the thermal time constant on my array is ~ 3 to 6 min. or so, depending on wind.

[bcroe]

Hello everyone, I got a 6.7kw system installed in December and all was going well until I noticed that my solar production has already taken a dive. in April on a full sunny day (very smooth curve on the daily graph, no spikes or jumps from clouds) I made up to 43 kwh. In May we only had one day that had a smooth curve and it created 37.3 kwh. Now in June we have finally had a few good sunny days in a row and the highest production has only been 34.5 kwh. The peak production in the middle of the day in April on the graph was 5.8 kw (though instantaneous readings showed just over 6 kw a few times), while peak production in June is 4.9 kw. Can anyone think of why this is happening? I know the angle of the sun and panels can make a difference, so if the sun was perpendicular to the panels in April, as it moves higher in the sky into summer there will be some loss of production, but from what I've read it shouldn't be anywhere near 20%.

I think 20% difference is a bit much. I would be looking at the day peaks; length of day and weather can vary too much.
You haven't said much here about how your system is configured. If you can check DC voltage & current you might see
something going on. See if performance of similar components is the same. Bruce Roe