Energy Products

How much solar energy on a cloudy day?

Even after my installation, I often wondered if I can expect SOME solar production on a dark & miserable Seattle day. I think I found the "edge of the envelope" answer a few days ago...

Last Friday, Dec 20, Seattle had the darkest day on record, since the University of Washington started recording total solar radiation 20 years ago. They recorded only 0.37 megaJoules/square meter that day, beating the Dec 14, 2014 record of 0.39. On Dec 20 my system recorded 0.93 KWh of production.

For comparison, the July daily peaks are around 27 megaJoules/square meter. I didn't get my system until late August, but my peak production so far was 52.8 KWh on Aug 28. Average for the first 4 months was 20.4 KWh/day, which is very close to my average consumption.


  • bpbp
    edited November -1
    How many KW for your panels?
  • edited December 2019
    52 kWh peak production per day... that’s probably around 10kW system...
  • edited December 2019
    To make such a guess requires a *lot* of assumptions. I have a near ideally sited 5kW array in So. CA , and my best production is about 32kWh per day in April. Being much further North than me, I'd guess his array is closer to 15kW. It will be interesting to find out...
  • edited December 2019
    FWIW I’ll share two monthly generation reports from our 14 kWh rooftop solar generator in FL. March thru May have consistently been our top production months with roughly 2 MWh of monthly generation and 70-75 kWh per day.

    Aug shows roughly 1.7 MWh of monthly generation and 60 kWh per day.

    Note that we see some level of generation every day - rain or shine.
  • edited November -1
    Here in Florida, near Orlando, we have had some really cloudy weather during the week of December 21st and the worst point of the year.

    Even with pure grey cloudy weather, no breaks, it seems like the system still produced about 20% to 25% of nameplate capacity during the middle of the day.

    Our nameplate capacity is 16.38 kW.
    I saw the system producing between 3.0 to 4.0 kW around noon that day.
    Even when raining and pure grey above, I would see 1.0 to 2.0 kW being produced.

    Normally sunny during December our system will produce 60 kWh for the day.
    Rainy and heavy clouds all day in December, system will produce maybe 15 kWh for the day.

    Summer weather with longer days I expect our system will do 110-120 kWh.
    We will see. Just installed in November 2019.
  • edited December 2019
    bp: My system is 9 KW.
  • edited December 2019
    DING DING DING! Passion2Fly WINS!
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