Ever wonder how many solar panels you’d need to put on your existing home to make it zero net energy?
Like many on the Central Coast, our home uses a mix of fuels to heat and cool the air, heat water and cook food. The energy efficiency “bones” of the building are sound - having completed a gut remodel of the 50-yr old structure a decade ago, so our electricity costs have generally been low. Too low to make installing solar PV cost effective.
However, as some of our gas appliances are approaching end of life and inspired by recent all-electric and ZNE consulting here at In Balance, I decided now seemed as good a time as any to find out what it would take for my own home. I worked with a local provider to consider three options:
o Option A: offset current annual electricity use
o Option B: offset additional electricity use if we converted our gas appliances to electric
o Option C: cover use of an electric vehicle for 5000 miles per year
For Option A, we aggregated our electricity bill (4800 kWh/yr), which was the equivalent to roughly a 3 kW system or nine panels on a south facing roof in Santa Barbara. Pretty straight forward.
For Option B, calculating how to offset expected use once we shifted out of three gas appliances (furnace, tankless water heater, cooktop) was a bit trickier. Our first pass involved aggregating our annual gas use (kBtu) and converting it to kWh, generating a rough estimate of 8800 kWh/year. A straight conversion would land me with another 15 or so panels, but because efficiencies generally improve moving from gas to electric appliances (especially heat pumps), we knew there would be savings and we could probably go a bit smaller.
A second issue was how much usage to assign to each appliance. Our installer provided some estimates based on our average usage patterns as a starting point:
o Induction cooktop = 150kWh/month (1800/yr), ~ 3.5 panels
o Heat pump water heater = 120kWh/month (1440/yr) ~ 2.5 panels
o 3-ton heat pump = 225kWh/month (2700/yr) ~ 5 panels
This is equivalent to 6000 kWh per year, or another 10-11 panels.
For Option C, it turns out there’s a simple multiplier to determine offset – every kWh can fuel approximately 3 miles. For 5000 miles, we’d need 1500 kWh or three panels.
A quick summary:
Ultimately, we decided to install ten panels, which would more than cover our current usage and allow us to switch either our cooktop or water heater in the next year and still offset most of the energy. (Funny side note – exactly 3 days after our solar went live, the water heater let us know it was time to move on. A new heat pump water heater is on the way. We’ll now be able to cover about 50% of our water heating with power from the sun.)
As a footnote, we also considered whether a backup battery would make sense given the challenges of relying on the grid in case of weather or fire emergencies. While the price put it out of our budget, I was interested to learn that technology to use an EV battery as a home backup battery will be widespread in the U.S. market in the near future (Nissan is leading the way), a prospect that makes purchasing that electric vehicle all the more enticing!
P.S. A big thanks to Cash Upton and the Brighten Solar Co. team for help providing estimates and expertise for this project!