How big a battery does your home need?
Sizing a home battery is a balance between resilience and cost. Too small and it cannot carry your home through an outage or soak up your midday solar; too large and you pay for capacity you rarely use. The right size starts with a clear answer to two questions: how much of your home do you want to keep running, and for how long?
From daily usage to usable kilowatt-hours
Begin with your average daily consumption — your annual usage divided by 365. Most US homes use 25–40 kWh per day. You rarely need to back up all of it; many homeowners pick a critical-load fraction that covers the refrigerator, lights, internet, and a few outlets, which might be half of total usage. Multiply that by your desired days of autonomy to get the usable energy your battery must deliver.
Why nameplate capacity is larger than usable
Manufacturers rate batteries by nameplate capacity, but you can’t use every kilowatt-hour. Lithium-ion home batteries permit roughly 90% depth of discharge to protect cell life, and a small amount of energy is lost to round-trip charging inefficiency. We account for both, so the nameplate figure we report is the capacity you actually need to purchase — then we translate it into a count of real-world units like a 13.5 kWh Powerwall.
Backup versus self-consumption
Batteries serve two distinct goals. For backup, size around the loads and outage duration you care about. For self-consumption — storing cheap midday solar to use in the expensive evening — size around how much excess your panels produce on a typical day, which matters most in states that have moved away from full-retail net metering. Either way, this tool gives you a defensible starting number to take into installer quotes.