"overcast" is pretty vague, but woobat has put together a nice web tool to look at the output of one of the panel array at Williams, and you can overlay various other data like irradiance: http://www.williams.edu/resources/sustainability/solar/solar_data.php. The Morley Science Center panels (the default on the graphs page) are a 7.2 kw array.
If the sky is truly overcast, then all direct beam radiation has become diffuse. Wikipedia suggests that this reduces the total solar flux to about 1/6 (for light overcast). I would have guesstimated a reduction to 1/5, so that's the same ballpark.
I can look this up in a solar data manual, if you really care about actual numbers, but I can't do that right now. Email me if you want the info.
60% of max output sounds WAY too high for a true overcast sky. Although if you're going to talk about "efficiency", the first thing I want to know is if you are comparing the output to the nameplate rating, or to actual measured max output (usually but not always lower than nameplate).
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I can look this up in a solar data manual, if you really care about actual numbers, but I can't do that right now. Email me if you want the info.
60% of max output sounds WAY too high for a true overcast sky. Although if you're going to talk about "efficiency", the first thing I want to know is if you are comparing the output to the nameplate rating, or to actual measured max output (usually but not always lower than nameplate).
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