The ten panels on the northeast slope have 87% solar access and should generate an estimated 2,464 kwh per year.
The three design options for the southwest slope result in the following:
- With the four panels strung out along the south side, the solar access is 80% and the annual power generation from the southwest slope is 4,245 kwh,
- With the four panels stacked as 2 rows of 2 along the south side, the solar access is 80% and the annual power generation from the southwest slope is 4,290 kwh,
- With the four panels on the top of the dormer, the solar access is 84% and the annual power generation from the southwest slope is 4,560 kwh.
The inital estimate based simply on the panel size and number of panels was 8,722 kwh. That means we will have 1,698 kwh/year less than originally planned, around 20%, primarily due to shading. There is nothing we can do about this (except maybe regularly trim the Polycarpus trees in our neighbor's yard that shade the southwest slope panels). I don't know if REC is including the effect of the Tigo MPP balancing system in the calculation, that could possibly increase the power production somewhat.
My original calculation for how much power we would need including the new electric car, the new hot water heater, the new HRV, and 2000 kwh/year we are currently drawing from the grid came out to around 5,000 kwh/year. The old 2.5 kw PV system was supplying around 3,831 kwh/year estimated. The total that I was trying to design for was 8,831 kwh/year. So we will be around 20% short of carbon neutral, but for that, we will be covering a lot of additional energy uses. That's better than the old system, where we were around
With this system we will have essentially maxed out our easily harvested solar resource. If we want to put any more solar PV up, we will likely get marginal return (both in terms of carbon reduction and financial) because the panels will be shaded much more, maybe even more than 50%. We will have to see how the system performs. Stay tuned.