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Calculating Solar Panel Size


Take a look at this ....

I installed a 10.85 kW system this past June and fully turned it on at the end of July. 4.65kW are southern facing and 6.2 kW are eastern facing. Obviously this is my first winter with solar and I'm wondering if my production is normal. I live in Northern New Jersey (Bergen County) so we do get real winter weather here.

I track my production daily both with PVOutput (from my Solaredge inverter) and by reading my production meter (revenue grade meter for SREC purposes). I also have my PVWatts projections by month (and day but I know those aren't very helpful) using a 10% loss. My system has very little shading, probably a little in the early morning and possibly a drop at the end of the day.

While I've never really studied the number of cloudy/sunny days in previous winters, the overall weather here this winter has been fairly normal, if not a little better than normal. Also, we haven't had very much snow yet, just one storm of about 5 inches. With that said I'm trying to figure out if the PVWatts numbers for the winter are just crazy high. I just don't see how I could possibly average 27 kWh per day in January. I had a few near perfect sunny days a few days ago and produced 33 and then 34 kWh on those days. That means on perfect days I'm only out producing the average by 6-7 kWh. Yet on a cloudy/rainy day I'm only going to produce 0-4 kWh. For example, yesterday with rain was 2 kWh and today with just clouds and no rain is looking like it will be about 3-4 kWh. So for every cloudy day, I need about 4 nearly perfectly sunny days. That's just not realistic. Could the PVWatts numbers be wrong? That doesn't seem likely either. I put in my measurements based on what my installer had calculated. The Azimuth appears correct and the tilt is probably not off by more than 5 degrees based on my visual estimate.

Also, regarding snow, I had read that snow typically melts/slides off of a tilted array the next day if there's sunshine. I haven't found that to be true. My panels are at a 25-30 degree tilt and it took over 3 days to get melting of a 5 inch snow storm. The first day after the snow had plenty of sun but temperatures in the 20s. The panels remained nearly fully covered. Over 4 days (including the day of the snow) I got a combined 2 kWh of production because of the snow.

I'm not really trying to just complain about my performance. I just want to see if others have experienced this as well and if what I'm seeing is relatively normal. I was within +/-12% of my PVWatts numbers for each month in August - November. It's really just been December and January that my numbers seem significantly low relative to PVWatts. I have my system on PVOutput and can post a link if that would be helpful.

Calculating Solar Panel Size

by on ‎03-31-2017 12:54 PM - edited on ‎06-04-2017 06:46 PM by Community Manager (765 Views)