Ian,
What was the actual annual production for each array?
I ran a simulation, using latitude 47, and weather data for Olympia, and
using a module efficiency of 16%, which corresponds to Sanyo 190 watt
modules, and used 95% for inverter efficiency.
simulation at 47 lat, 36 tilt, south predicts 2224 kwh/yr for 10 mods
at 30 tilt, west predicts 1401 kwh/yr for 8 mods
2224 @ 10 -> 1779 @ 8
1401/1779 = .7874 (21% difference)
I reran the simulation but using SF weather, but still at latitude 47
3306 for 10 mods facing south
2207 for 8 mods facing west
2207/(.8*3306) = .834 (17% difference)
One not-surprising conclusion is that Washington weather has a huge effect
on predicted production. Another is that the details of local weather time
asymmetries are going to affect relative performance between different
facing arrays (what is relative cloud cover through year morning compared to
mid-day compared to afternoon?). Is PVWatts accounting for that, or is it
just using sun-hours in the different locales?
Antony Tersol
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