How the backup time is calculated
Each appliance uses watts × hours per day of energy. Their total, spread over 24 hours, is your average draw. Backup time is usable kWh × 90% ÷ average draw — the 90% covers inverter and wiring losses. With solar, each day's average generation (kW × sun-hours × 0.8) is subtracted from the daily use first; if solar covers it all, the battery refills every day and backup is indefinite on an average day.
Capacity versus output
Two numbers matter. Capacity in kWh sets how long the battery lasts. Output in kW sets how much can run at once — and motors need extra for the instant they start. This calculator checks your selected loads against the output you enter, including the start-up of the largest motor, using a conservative 1.5× short surge allowance. Check your battery's spec sheet for its exact surge rating.
Stretching your backup
The biggest gains come from the biggest loads: central air conditioning, electric heat and well pumps. Running the AC a few hours less, or keeping only one fridge on, often doubles the time you can ride through. The result shows exactly how much each switch-off would add.
How long will a home battery last during a power outage?
A 13.5 kWh battery running only essentials — a refrigerator, lights, Wi-Fi and phone charging (about 2 kWh a day) — lasts roughly five to six days. Add central air conditioning and the same battery lasts well under a day. The answer depends almost entirely on what you keep running.
How much battery backup do I need?
Add up the daily kWh of the loads you must keep on and multiply by the number of days you want to ride through. For essentials only, 10–15 kWh usually covers one to several days; whole-home backup with air conditioning typically needs 30 kWh or more, or solar recharging.
Will my solar panels keep the battery charged in a blackout?
Only if the system is designed for backup — a battery with a hybrid or backup-capable inverter that isolates your home from the grid. A standard grid-tied system without a battery shuts down during an outage for line-worker safety.
Why does my battery need a big enough output, not just capacity?
Capacity (kWh) decides how long it lasts; output (kW) decides what can run at the same time. A well pump or AC compressor briefly draws about three times its running watts on start-up, so the battery's inverter must handle that surge too.