Solar & battery
Solar Backup Calculator
What it takes to back up a home on solar and storage, and what it costs.
Result
Outages rarely happen in good weather. Sizing on a sunny-day solar figure is optimistic, because the storm that took the grid down is the same storm shading the array. Run this with the solar figure set low as well.
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About this calculator
Battery or generator?
A generator is far cheaper per kWh for a long outage and needs fuel, maintenance and noise. A battery is silent, instant and useful every day through rate arbitrage. Many homes end up with both.
Can I back up the whole house?
Technically yes, practically it is expensive. Air conditioning, electric heat and well pumps push the required bank into territory where a generator makes more sense for the rare long outage.
The formula
battery = (daily backup load - daily solar) x days / usable fraction
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