What about can I expect from panels on a cloudy day?

[oregon_phil]

I live in Oregon and have a 10 kw GT system. Two days ago it rained 2 inches and I made 2 kwH. I calculated an 8kw system would be needed for 1.5 kwH in my rainy, overcast situation.

[chrisski]

I am in AZ and with 1 kw of panels and in winter on our shortest days of the year, I can expect to have that 1.5 kw by noon with a sunrise at 7:30 and sunset 5:30.

I had a cloudy morning where it was overcast and at 10 am I received 30% of the normal amps out. I’ll call that a light overcast and not the dark storm clouds overhead.

When I’ve had an overcast day and needed to charge the batteries, I turn on the generator for 2 to 4 hours a day. Half to that in the morning if I know it will be overcast all day; the rest in the evening.

So without a ridiculously large array, like 10 kw, I doubt you’ll get the power you ask for on a cloudy day. The charge controllers are designed cut off at a max amp output, but not sure how it would do with all that energy on a sunny day.

[bcroe]

So is it a consensus that Poly panels work better in clouds?

One type does not have any large efficiency advantage over the other. As for cloudy operation,
there might be a measurable difference (or not). I have never seen anyone put numbers on
any difference, and I think you will be disappointed to expect any. I run Mono panels under
clouds much of the time, and collect a lot of KwHours, usually below full sun. Bruce Roe

[Mike90250]

My 8 years of experience and daily logging

Shade - expect nothing
Bright haze - most power of all, you have high output in morning and afternoon, overwhelming the reduced noon peak

Heavy rain clouds - I get 200w from 5kw of panels
Cloudy & fog ggy, I see about 20-30% of nameplate
Bright sun, warm day 80% of nameplate
Bright sun freezing day 100% of nameplate

[SunEagle]

Based on my experience the MONO cell panels seem to produce very well in full sunlight it seems the POLY cell panels work better in low or diffused sunlight.

[J.P.M.]

Depending on how overcast the conditions are (like clear days not being equally clear, cloudy days or overcast skies are not eqully overcast in terms of irradiance), that 10 - 15% cloudy output of the max. value of the same day's PV output if that day was completely clear is probably a decent 1st approx.

A rule of thumb, and anecdotally FWIW, I've observed over 40+ years of watching such things that the weakest shadow I can see is at ~ 250 to 350 W/m^2 direct irradiance.

Common values of ratios of measured overcast global diffuse irradiance to theoretical clear sky global horizontal irradiance for identical days and times vary from ~ 5 % to ~ 25 %, commonly 10% or maybe a bit better. Reduce that by the usual reduced PV cell efficiency at reduced irradiance values for a SWAG on cloudy output.

More FWIW, the lowest daily output my array has ever done is 3.68 kWh/day out of a theoretical max. output of 30.16 kWh/day (for date of March 02) using a completely clear sky model and TMY3 weather otherwise. 3.68/30.16 = 0.122.

That day was completely overcast with measured daily total GHI = 0.82 kWh/m^2 out of a theoretical daily max. GHI of 5.53 kWh/m^2 for 03/02.

The calc'd P.O.A. for that day was 0.830 kWh/m^2 out of a theoretical max. max. P.O.A. under a clear sky of 6.65 kWh/m^2 for that date. The GHI and POA are close because the entire day was overcast with the max. 1 minute GHI = 213 W/m^2, and > 200 W/m^2 happened for only 5, one minute increments total all day.

0.83/6.65 = 0.125, giving what looks like reasonable agreement with the P.O.A irradiance ratios of actual vs. theoretical max. for that date.

[NewBostonConst]

My max output was on a slightly over cast day.....The clouds scatter the light and sometimes help.

You can get direct light and added to it scattered light from the light clouds.

I have seen half production on rainy days.

[checkthisout]

I calculated I need 250 watts output for 4 hours to charge my battery bank. This takes into account all the losses through the charge controller, etc.

Located in washington state, partial shade, sometimes fog.

I have concluded that 8 300 watt panels will get me where I need to be.

My ultimate question would be what sort of output could I expect on a cloudy, rainy and maybe even foggy day from 2400 watts worth of panels. I leaned towards looking at only 10-15% rated output. Based on your experience, is that reasonable, optimistic or conservative? Thanks.

I would be using these panels. https://www.ebay.com/itm/Solar-Panel...22h1tezIZyehqQ