Solar Panel Energy on Overcast Days

MarkFromCornwall

Very southerly location, 8 panels facing roughly east, 8 panels facing west, currently (11:30) quite heavily overcast, producing 380W.  Friday was good weather (sunny intervals) and produced 5.3 kWh.  Yesterday was very wet, produced only 0.3 kWh!

nxdmsandkaskdjaqd

[Deleted User] said:

So in summary, a 4kw array, on a wet cloudy day, could just keep the household ticking over with produced energy

Err - no. You cannot take the total kWh generated in a day and divide by 24. If you look at my graph above, the TOTAL output from my array on the 21st November was just 280Wh or c. 1/4 of 1kWh. My inverter normally kicks in when the solar output exceeds about 70W.

As I post, my array is outputting 500W.

I understand.  As you posted you were outputting 500w, therefore it would be keeping most of your low energy products alive.

matelodave

Have a shufti here to give you some idea of the projected solar output where you live - https://re.jrc.ec.europa.eu/pvg_tools/en/

You can see how the performance varies when you may not have optimum slope or direction (azimuth)

[Deleted User]

keeping most of your low energy products alive.

But not all the time. This was yesterday’s graph:

QrizB

My array is smaller than most that you'd get installed today (2.72kWp) but generally performs fairly well.

[Deleted User] said:

Err - no. You cannot take the total kWh generated in a day and divide by 24. If you look at my graph above, the TOTAL output from my array on the 21st November was just 280Wh or c. 1/4 of 1kWh.

The 21st of November was exactly what the OP describes:

nxdmsandkaskdjaqd said:

a cloudy and wet day in say November (such as to today in south England). 

My generation on the 21st was 200 watt-hours, vs. my consumption that day of ~5kWh.

Ectophile

My very rough rule-of-thumb is:-

• Sunny day = up to 100% of rated output (depending on where the sun is)
• Light cloud = about a tenth of the rated output.
• Heavy cloud = about a hundredth of the rated output.

Today, on a foggy wet afternoon at 2pm, my 3.4kW of panels are giving me 85W.  Not even enough to run the computer I am typing this on.

nxdmsandkaskdjaqd

Ectophile said:

My very rough rule-of-thumb is:-

• Sunny day = up to 100% of rated output (depending on where the sun is)
• Light cloud = about a tenth of the rated output.
• Heavy cloud = about a hundredth of the rated output.

Today, on a foggy wet afternoon at 2pm, my 3.4kW of panels are giving me 85W.  Not even enough to run the computer I am typing this on.

That's not a lot.  I would hope for more.

nxdmsandkaskdjaqd

[Deleted User] said:

keeping most of your low energy products alive.

But not all the time. This was yesterday’s graph:

That would keep most basic elec items alive from the panels during daylight hours.  Where are you?

[Deleted User]

The Midlands. I don’t quite get your focus on the word ‘alive’ unless you are trying to go off grid. I prefer to see my 100kWh output in December as a 25% contribution to my monthly usage.

I would hope for more.

It comes down to basic physics. People tend not to get sunburnt on a foggy day in the UK.

mmmmikey

nxdmsandkaskdjaqd said:

Ectophile said:

My very rough rule-of-thumb is:-

• Sunny day = up to 100% of rated output (depending on where the sun is)
• Light cloud = about a tenth of the rated output.
• Heavy cloud = about a hundredth of the rated output.

Today, on a foggy wet afternoon at 2pm, my 3.4kW of panels are giving me 85W.  Not even enough to run the computer I am typing this on.

That's not a lot.  I would hope for more.

I have to say as a rule of thumb that sounds about right to me, although because I have a mix of south and east facing panels I'd probably peak at 75% of rated output - i.e. on the brightest, sunniest day I might get 3kW out of my 4kW array. Other figures are certainly in the ball park - the output power drops significantly when the sun goes behind a cloud (say 300W) and on a dank November day like yesterday 30W is not far from what I'd expect. Having said that, annual generation is about 3000kWh which is a significant amount in financial terms at current rates.