What Size Battery Do I Need for a 72-Hour Power Outage?
Turn a three-day outage plan into a battery target by separating daily critical-load energy from inverter output and recharge options.
Three-day backup is too long for a generic “whole-home” rule. The right answer comes from daily critical-load kWh multiplied by three, then adjusted for reserve and recharge.
A 72-hour plan multiplies daily essential energy by three, then subtracts realistic recharge and adds reserve. Storage-only designs become large quickly, which is why solar, generator or intermittent grid recharge can change the economics dramatically.
What matters
- Build a one-day critical-load budget first.
- Multiply by three only after removing discretionary loads.
- Solar or generator recharge can cut required stored capacity dramatically.
- High-power loads still need sufficient inverter output even if daily energy is low.
Decision factors that change the answer
Daily essentials
Start with measured or realistic daily kWh for refrigeration, heating controls, pumps, communications and lighting.
Three-day multiplication
A modest 4kWh/day essential budget becomes 12kWh delivered over 72 hours.
Recharge credits
Only subtract recharge you can reasonably expect under outage conditions.
Weather sensitivity
Solar-dependent plans need margin for cloudy or stormy periods.
How to size it
- Calculate daily critical-load kWh.
- Multiply by three days.
- Apply reserve and conversion assumptions.
- Subtract realistic recharge only if it is dependable.
A 4kWh/day essentials plan becomes 12kWh delivered energy over 72 hours before reserve; reliable daily recharge can reduce how much must be stored at once.
Worked example
At 4kWh/day, three days needs 12kWh delivered. With 85% usable AC energy and no recharge, that points to about 14.1kWh nominal before additional reserve. Reliable daily recharge can reduce the storage requirement materially.
Common buying mistakes
- Using whole-home utility consumption without prioritizing loads.
- Assuming perfect solar every day.
- Ignoring 240V and motor-start requirements.
For 72-hour resilience, load reduction and recharge strategy are usually more valuable than simply buying three times more battery.
Check the underlying numbers
Use these site resources to replace generic assumptions with your own load or the source-linked data behind this guide.
Use the numbers before choosing a model
Start with your load list. The battery needs enough watt-hours for the desired runtime and enough continuous/surge watts to operate the appliances at the same time. Then check whether your recharge method can replace the energy you expect to consume.
For outage planning, we recommend keeping reserve capacity instead of designing a system that reaches 0% at the end of the forecasted outage.
Next step
Estimate battery runtime → or Build a critical-load energy budget →.