what wattage to expect on cloudy day?

al128 Solar Expert Posts: 65 ✭✭✭✭
hi there,

lets assume you have a solar system that harvests 1000Watt per day on a sunny day ...

how much would you expect to pull on a:

- partly cloudy day
- overcast day (where you can't see the sun)

just wondering ...



  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: what wattage to expect on cloudy day?

    that's difficult to say. think of the modules as eyes. the more light there is the better they see (yes more eyes will see more) and that alone varies the net production of light seen depending on the degrees of cloudyness, rains, solar eclipses, etc., but over time it can change between those conditions at a variable rate from what is available at that time of year. this makes the exacting total of what will be seen unpredictable and as such it can't be said for sure what the pv production will be as it depends on what it actually sees.
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: what wattage to expect on cloudy day?

    Partly cloudy day, can have a pretty good harvest. I don't think there can be any general rule, but edge of cloud events can give huge spikes in output for a few minutes. If you have more sun than cloud, you have have a day that rivals a clear day. Add in reflection off of snow and winter cold and you can have a day that exceeds a clear summer day.

  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    Re: what wattage to expect on cloudy day?

    Partly cloudy, figure 20% reduction
    Overcast/Rain Figure 90%+ reduction
  • BB.
    BB. Super Moderators, Administrators Posts: 33,402 admin
    Re: what wattage to expect on cloudy day?

    Just looking through the last ~2.5 years of my system...

    Summer I get around 17-19 kWhrs per day on sunny and warm days. With a few 7-14kWhrs per day when the coastal fog rolls in (remembering that hot sunny days can reduce my array output by ~2kWhrs per day vs cold sunny days).

    During the winter, 10-11 kWhrs per day when sunny, a few 1kWhr or less thrown in when heavy dark storms (typically winter cloudy weather is around 3-7kWhr per day).

    Year over year, my winter weather output is much more variable and therefore frequently lower than predicted output.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SolarJohn
    SolarJohn Solar Expert Posts: 202 ✭✭
    Re: what wattage to expect on cloudy day?

    I have 425 watts of PV. During peak sunlight hours, in full sun, current from the panels might peak at 29 amps, with 23 amps being about normal (at 12 volts). On a cloudy day, with no shadows, it's not unusual to see 8 amps of current from the panels at mid-day. Dark, heavy clouds can reduce the panel output to zero.