What Size Battery Do I Need for a 24-Hour Outage?
Calculate one-day home battery requirements from refrigeration, internet, lighting, pumps and other critical loads.
A 24-hour battery target should be based on a daily energy budget, not home square footage or a generic battery recommendation.
For a 24-hour outage, add the watt-hours used by each essential load over one day, then divide by expected usable battery fraction. A household that consumes 3kWh of essentials may need roughly 3.5–4kWh of nameplate battery after inverter losses and reserve.
What matters
- List only loads that need to survive the outage.
- Use equivalent run hours for cycling appliances.
- Keep watts and watt-hours separate.
- Add reserve and conversion losses after calculating the load budget.
Decision factors that change the answer
Duty cycle
Refrigerators, furnaces and pumps cycle; multiplying nameplate watts by 24 hours usually overstates usage.
Reserve
Do not design to hit 0% state of charge at hour 24.
Simultaneous power
Verify inverter output separately from daily energy capacity.
Recharge opportunity
If grid, solar or generator recharge is realistic during the day, required storage can be lower.
How to size it
- List each load and watts.
- Estimate hours or duty cycle over 24 hours.
- Sum daily Wh.
- Divide by your usable-energy factor.
A 3kWh delivered critical-load budget may justify roughly 3.5–4kWh of nameplate storage depending on reserve and conversion assumptions.
Worked example
Refrigerator 1.2kWh/day + networking/lights 0.6kWh + furnace blower 1.2kWh = 3kWh delivered. At 85% usable AC energy, 3kWh ÷ 0.85 = 3.53kWh nominal before extra reserve.
Common buying mistakes
- Running every appliance as usual.
- Treating peak watts as daily energy.
- Leaving no reserve.
Use the home-backup calculator to build the one-day load budget before shopping.
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 →.