WattThe / Homeowner / Battery sizing
Work out how much usable storage your backup plan needs — and how many typical 13.5 kWh battery units that translates to.
Battery sizing starts with an honest critical-load list, not with whole-home consumption. Refrigeration, some lighting, communications, and a furnace fan might total 8–15 kWh per day; adding central air conditioning can triple that. Multiply the daily figure by how many days of autonomy you want, then gross it up for two losses: depth of discharge (most lithium home batteries make 90–100% of nameplate usable) and round-trip efficiency (roughly 90% of stored energy comes back out).
Energy is only half the sizing problem — power is the other. A battery must also deliver enough instantaneous kW to start motors and carry simultaneous loads; a single 13.5 kWh unit typically supplies 5 kW continuous, which handles essentials but not a 4-ton AC compressor starting under load. If your backup plan includes heavy motors, unit count is often set by power, not energy.
Pairing with solar changes the math favorably: the battery only needs to bridge overnight rather than the whole outage, and it earns daily value through self-consumption in net-billing states rather than sitting idle waiting for a storm.
Essentials-only backup usually needs 10–15 kWh per day; adding air conditioning or electric heat can push daily needs to 30 kWh or more. Size from a written list of the circuits you actually want powered.
It is the share of a battery's nameplate capacity you can actually use. Most modern lithium home batteries allow 90–100% usable capacity, but sizing with a margin extends battery life.
No, but it helps. Solar recharges the battery during multi-day outages and, in net-billing states, lets the battery earn value every day by storing midday production for evening use.
Estimates are for planning and education, not engineering design or financial advice. Verify rates with your utility and confirm electrical work with a licensed engineer or electrician.
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