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

[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

[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.

[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.

[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.

[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

[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

[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.

[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]

My Victron controller can power a relay with a low voltage cutoff and energize the relay based off battery voltages. You could build for a day or two of power and that relay would to run it back on once the battery is charged.

[SunEagle]

Ouch. My plan was to have this keep a battery bank live at a cabin that in turn would power some security cameras. I calculated a constant 50 watt draw 24/7. Amazing that 10 270 watt panels probably won't meet this need but live and learn I guess.

I did get a POE switch and 12-48 converter so I guess I will go ahead and get that up and running just to see what the actual real-world draw will be.

I'll report back. This is in Washington state and we are nearly solstice.

I was a bit shocked to see how much just a bit of smoke from the chimney drifting over the panels diminished their output by about 80%.

How about any smoke from the wild fires up there in WA. They have hurt production at most northern CA sites.

[oregon_phil]

Our fires were so bad in Oregon, we went from 50 kwh/day to 4 kwh/day for about a week. It was surreal.

[chrisski]

Is there lower wattage equip? Perhaps wired cameras IPOe vs WiFi or Bluetooth? Even the night time LEDs add up.

Now that my system is up and functional, I am seeing what I can do to reduce power.

Last discharge was 101 ah. Turns out I left a few things on overnight I didn’t need. The inverter at 1 amp per hour, 10 amp total; a set of outside LEDs at 1.2 AHs, for 12 amp hour total. I needed the fridge, but found out my gas fridge pulls .5 amp in propane mode or 5 amp hours total. That .5 amps is better than the 18 amps on electric mode.

Those 27 ah added up to about a quarter of my usage that day.

[Mike90250]

DIY Auto Start generator ?
I'd like you to read these 2 links and the comments below the articles
Just some points you should be aware of. Be safe - don't burn the genny shed down

https://www.utterpower.com/zachs-remote-start-generator/

&

https://www.utterpower.com/zach-asks-is-your-generator-ready-for-winter/

excerpt:
"It’s often a learning experience what can go wrong and Murphy always finds something that can be wrong! Example: Some operators might purposely close the vent for the fuel tank to keep moisture out and forget.

Murphy takes full advantage of this situation and the power outage arrives right in the middle of family night. You’re all watching a rented movie, eating popcorn, and out goes the lights! You jump to your feet, it’s your first opportunity to be the family Hero, and you strut into the garage, make a flawless transfer to generator power, and return to the family room with a big applause.

The wife hugs you, the kids hug you, the dog even drops by to give you a lick on the face, the kids have the movie back up, and there’s more pop corn going in the microwave and…. and…..the generator dies! You’re back in the dark, you left the flash light somewhere, you were just too excited, and don’t remember. ...."

[Ampster]

....

In other words my question is, in regards to maximizing power output, especially in shaded conditions, is it better to have each individual panel on its controller.

It is better from a power standpoint in shaded situations to have MPPT at the panel level. It may NOT be cost effective unless you are comparing micro inverters to a string inverter. Even then it may depend the type of shade and the effectiveness of the diodes. It is not a question answered easily on the internet without being there.

[Mike90250]

Shaded panels in parallel (or differently oriented arrays in parallel) work with MPPT controllers. You take a small hit in wattage tracking, but the panels in full sun, will still produce close to their normal power
You do NOT want to series a shaded panel with a lit panel, the shaded panel will throttle the output of the lit panel. Bypass diodes in the panel may alleviate some of the issue, but then continued operation cooks the bypass diodes and you have to buy a new panel (most modern panels use chip diodes embedded under the glass) Old school panels use leaded diodes in the J-box. Your single panel has to produce enough voltage for the MPPT to work (usually 1.5x max battery charging voltage of 15V per 12v battery) read the charger data sheet to be sure