The final step in the design was to achieve net-zero by powering our energy-efficient house using a rooftop solar electric system.
This was a two-step process:
- Determining the yearly energy requirements, and
- Estimating what size solar system would be required to generate that much energy.
Following those calculations and estimates, we estimated the annual energy requirements for just the house to be approximately 7704 kWh.
Based on the actual performance of a nearby solar system for which we had three years of measured generation data, we estimated that each 1 kW of nominal system would generate 1197 kWh/year of energy, and so a system of 6.3 kW capacity would be required to achieve net-zero.
We elected to initially install the 6 kW system shown above, while pre-wiring for expansion later due to:
- The approximate nature of some of our annual power estimates.
- Having long-range plans to add an electric car which would inevitably require adding more solar generation.
- The unknown energy requirements for the well pump, garage, and woodshop.
As it turned out we added the electric car (a Chevrolet Volt) earlier than planned — at the end of 2017 – to take advantage of the Federal tax incentive that was expected to expire at the end of the year (fortunately it didn’t).
We also added the extra solar capacity earlier than expected – March 2018 – to avoid the effects of a billing change planned by the power company (which was later reversed by the Washington State legislature).
Our very limited usage data for the electric car indicated that an additional 1.5 kW of solar capacity would be required to fully power it. Since our original estimate was that the 6 kW system was possibly slightly undersized for the house, and no allowance had been made for any of the outbuildings, we elected to install the 3 kW system expansion shown to the left of the original system.
During 2019, our first full year of after both these changes, the combined solar system generated 11,516 kWh, slightly more than the 10,773 kWh we estimated, while our combined usage was 9173 kWh, so we produced 26% more power than we used.
In summary, the entire property, including the car and outbuildings, is net-zero with a reserve for future usage increases and future degradation of the solar system.