Home Battery Sizing Calculator
Work out how much usable storage your backup plan needs — and how many typical 13.5 kWh battery units that translates to.
load × days
after DoD & efficiency
e.g., Powerwall-class
avg kW of your critical load
Results update live as you type. For planning and field-check estimates — always verify against applicable standards and equipment ratings.
Sizing storage without overbuying
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.