Home Battery Backup Calculator
How long a battery powers your home during an outage — and how many you need
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Quick summary
Enter your battery's usable capacity, how many you have, your backup load in watts, and inverter efficiency to see backup runtime, then set a target duration to size the number of batteries needed. A single 13.5-kWh battery at 90% efficiency running a 750-watt essential load lasts about 16 hours; covering a full day at that load would take two. Real outages vary as appliances cycle, so size for essentials — fridge, lights, internet, medical devices — not your whole home.
Reviewed by Simple Calculators TeamLast reviewed
How to use this calculator
- Enter the inputs requested by the calculator.
- Choose the units or options that match your situation and review the assumptions.
- Run or update the calculation to see the result.
- Review the result, breakdown, and formulas shown on the page.
- Check the inputs, units, dates, and limitations before saving or sharing the result.
Frequently asked questions
How long will a home battery last during an outage?
Divide usable capacity (times inverter efficiency) by your load. A 13.5-kWh battery at 90% efficiency delivers about 12.15 kWh; running a 750-watt load, that's roughly 16 hours. Higher loads drain it faster — running central AC or electric heat can cut runtime to a couple of hours.
How many batteries do I need for backup?
It depends on the load you want to cover and for how long. To power a 750-watt essential load for a full 24 hours you'd need about 18 kWh delivered — around two 13.5-kWh batteries. This calculator sizes the count for your target duration. Whole-home backup with heavy appliances needs considerably more.
What is usable vs total battery capacity?
Batteries reserve a little charge to protect their lifespan, so usable capacity is slightly less than the nameplate total (a 13.5-kWh Powerwall is fully usable; some others reserve 5–10%). Enter the usable figure for accurate runtime. Inverter/conversion losses (~5–10%) reduce deliverable energy further.
Can I run my whole house on a home battery?
Briefly, maybe — but high-draw appliances (central AC, electric heat, dryers, EV charging) drain batteries fast. Most backups are configured for essential circuits so a battery lasts many hours instead of one or two. Pairing with solar can recharge the battery during a daytime outage, extending backup indefinitely in good weather.
Key terms
- Usable Capacity
- The energy a battery can actually deliver, in kWh, after its reserve.
- Backup Load
- The continuous power draw (watts) you want to sustain during an outage.
- Inverter Efficiency
- Conversion losses turning stored DC into household AC, ~90–95%.
Methodology, sources & limitations
This page shows the formula, assumptions, and limitations used for this estimate. Rates, thresholds, and other time-sensitive inputs can change, so verify them against the relevant official source before relying on a result.
For our review approach and the primary sources we consult, see the methodology and source policy. If you spot an outdated figure or formula, please contact us with a correction.