Our system has a total of 30 panels, and we need to optimize the amount of power the system will generate, given the constraints of our roof. The directional and large scale shading constraints above are one set. Another set is stuff already on the roof. This includes:
- A solar powered attic fan to keep the attic cool,
- Various vents from the kitchen and laundry room fans on the southwest slope,
- The 2 panel solar thermal hot water system on the northeast slope we had installed last year,
- Shading from the two dormers, one on either slope of the roof,
- The skylights, which are actually along the spine running from the north side to central roof but do project slightly down the two slopes.
On the northeast facing slope, the solar thermal panels take up prime real estate on the top of the slope near the ridge. Having the solar thermal panels there is better than having PV, since this is the area that only gets 4 hours of sun in winter and the solar thermal panels won't heat up much if they don't get any sun. REC doesn't want to put any panels on top of the feed lines leading from the solar thermal panels to the house, so that leaves just the space on the bottom of the slope below the solar thermal panels. Space for 2 rows of 5 is available below the solar thermal panels, 10 panels in all.
That leaves 4 panels that still need placement. Since PG&E pays the best rates for power during summer afternoons, it makes the most sense to place the panels on the southwest slope. Given that, though, we want to optimize the amount of electricity generated regardless of the tariff, since we are constructing the system primarily to offset our carbon footprint.
So the first design REC proposed was with the 4 panels strung out along the south side of the southwest slope (the panels are outlined in magenta):
According to their calculations, the array on the southwest side has 80% annual sun and 20% shade while the array on the northeast side has 87% annual sun and 13% shade.
I asked them if they could do better, so they proposed two additional configurations. One has the 4 panels on the south side of the southwest slope, but stacked instead of strung out:
Tomorrow REC will remove the old solar panels and at that time they'll take a shade measurement on the dormer to get an exact value, so we should know shortly. Once the final design has been decided upon, REC will send it to the city and they'll take 2-3 weeks to consider sign off, provided there are no problems. Then comes the installation. After the installation, PG&E must certify the system (they don't have to install a dual ported meter in our case, we already have one) which should take a day or two (or three, PG&E has already taken 2 months with our request for a 200 amp service upgrade) and then we can turn on our new system.