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Free home battery backup sizing calculator. Estimate minimum and recommended nominal battery capacity from average backup load, target hours, discharge efficiency, usable DoD, and a planning margin. See more tools in our energy and sustainability calculator category.
Last updated: April 18, 2026
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Continuous power for critical circuits you plan to keep online (not whole-home peak unless that is your target).
For inverter sizing: well pumps, HVAC, or motor start surges. Leave 0 if unknown.
Extra nameplate capacity for cell aging, cold weather, and uncertainty.
AC energy needed
42 kWh
Usable battery DC (min)
45.65 kWh
Minimum nominal battery (nameplate)
50.72 kWh
Recommended nominal (with margin)
58.33 kWh
Inverter: size continuous output for at least your average load (3.5 kW), and ensure surge capability covers peak 7.0 kW if accurate.
Planning note
Typical focus
Lights, fridge, comms
Smaller average load stretches the same battery hours further.
Hours target
8–16 hr
Pair with PV recharge planning for multi-day outages.
Planning
Safety margin
Usable capacity can drop in low temperatures; increase margin or consult manufacturer curves.
Peak kW field
Inverter kW
Energy (kWh) and power (kW) constraints are both binding in real designs.
Example
90% usable
Enter the fraction of nameplate you intend to use regularly for warranty alignment.
Next step
Production tools
Use solar sizing calculators to estimate how quickly batteries refill on sunny days.
Default scenario: 3.5 kW average load, 12 hours, 92% discharge efficiency, 90% usable DoD, 15% margin:
Min nominal kWh
50.72 kWh
Recommended nominal kWh
58.33 kWh
The calculator starts from the AC energy you must deliver during an outage: average kilowatts multiplied by hours. It then increases DC energy required from the battery by dividing by inverter discharge efficiency. Nominal battery capacity is computed by dividing usable DC energy by your usable depth-of-discharge fraction. A safety margin scales nameplate capacity upward for real-world uncertainty.
AC energy (kWh) = average load (kW) × hoursUsable DC energy ≈ AC energy ÷ (inverter efficiency)Nominal kWh ≈ usable DC energy ÷ (usable DoD)Recommended nominal ≈ nominal × (1 + margin)Efficiency and DoD are expressed as percentages in the form and converted to fractions internally.
Pair sizing with solar battery storage and solar panel sizing tools in this category.
Get a custom calculator for your platformUse manufacturer-approved stacking, BMS limits, and installer load calculations before purchasing equipment.
Share it with homeowners and installers planning backup power
Suggested hashtags: #BatteryBackup #HomeBattery #SolarStorage #Resilience #Calculator