Battery Backup Load Sizer

It depends entirely on what loads you run. A Powerwall holds 13.5 kWh of usable energy. Running just essentials — fridge (3.6 kWh/day), lights (1.4 kWh), router (0.36 kWh), phones (0.2 kWh) — totals about 5.5 kWh/day, giving you roughly 2.4 days per Powerwall. Add a window AC unit and you are down to about 14 hours.

DoD is how deeply you can drain a battery without damaging it or reducing its lifespan. Most home batteries operate at 90–100% DoD. A 13.5 kWh battery at 90% DoD delivers 12.15 kWh of usable energy. Round-trip efficiency (typically 90–95%) further reduces this slightly.

A standard 3-ton central AC draws 3,500W. Running it 8 hours/day = 28 kWh/day — more than two Powerwalls per day. A mini-split at 1,500W for 8 hrs = 12 kWh, which is more manageable with a single battery. Solar recharging during the day can offset much of this, but AC is the single biggest challenge for battery backup sizing.

Critical loads backup powers a sub-panel with your most important circuits: fridge, some lights, internet, and medical equipment. This is typically achievable with 1–2 batteries and is what most installers configure. Whole-home backup powers everything simultaneously, which often requires 3+ batteries and a larger inverter.

A battery is silent, requires no fuel, needs zero maintenance, charges from solar, and is instantly available. A generator is cheaper upfront, can run indefinitely with fuel, and handles higher loads. For typical power outages under 24 hours, a battery with solar recharge wins on total cost of ownership. For multi-day outages in disaster-prone areas, a battery plus a small generator for recharge backup is the most resilient setup.