According to the National Renewable Energy Laboratory’s PVWatts calculator, 1 kW of solar in Thurston County will produce an average of 1,059 KWh of power a year. (This estimate is based on its default assumptions – a standard fixed panel, facing due south at an tilt of 20 degrees, and with 14.08% system losses due to a variety of factors including soiling, wiring, interruptions in availability, etc.) The most recent TCAT emissions inventory uses 1.03 lb/kWh as the emissions for PSE’s current power. At that rate (leaving aside the emissions involved in producing and installing the panel) our 1kW avoids 1,091 lbs of CO2e/year.
This will continue for five years, until 2025, when PSE has to eliminate coal power, which produced 73.4% of its electricity emissions in 2017. It’s expected to basically replace that power by running its gas plants more. Calculating from values in Figure 7.5 in the company’s 2017 GHG inventory report, it got a kWh of power from gas with 43% of the emissions that it took to get a kWh from coal, and replacing all the coal power with gas would lower its overall emissions to 58% of the current total. That would continue for five years, until 2030, when its power is supposed to become “carbon-neutral”. If we treat the power after 2030 as emission free, on the somewhat dubious assumption that the 20% of power from natural gas that’s still allowed will actually be effectively offset by the measures in the Clean Electricity Act, our panels will not be reducing emissions after that, because the grid power they are replacing will also be emission free.
On that basis, we have five years in which we reduce emissions by 1,091 lbs of CO2/year, or 5,455 pounds altogether. We have five years in which we reduce emissions by 58% of that, or a total of 3,164 pounds, giving us a total reduction of 8,619 lbs. That’s 3.92 tonnes altogether. According to a local solar installer, residential rooftop solar currently costs about $2.50/Watt. (A small system might cost $3.00/Watt; a larger, easy installation might cost $2.25.) At $2.50, our imaginary 1kW panel costs $2,500, 0r $1,850 if we get the 26% Federal tax credit. (This steps down to 22% next year and then goes away unless it’s renewed.) If power power prices stayed the same, we’d save $0.1035/kWh on the 1,059 kWhs of power they produce in the first year, or $109.61. However, between 2009 and 2019 power prices in the state as a whole went up about 2.5% a year. If that kept happening to PSE’s prices over the life of the system, perhaps twenty-five years, our total savings would be $3,947. At a discount rate of 5%, the net present value of that works out to a gain of $176.01 in current dollars in addition to our emissions reductions.