PowerLab — Energy Planning Tools

Battery planning

Battery Runtime Calculator

Estimate how long your battery can power a device or group of appliances. Enter battery capacity and load, then adjust reserve, battery health and conversion losses if needed.

Calculate estimated runtime

Quick Scenarios:
Battery

Common starting values — adjust if you know your battery specification.

LoadHow do you want to enter your load?

How to use this battery backup time calculator

  1. Enter the battery's Wh capacity, or choose Ah and enter its nominal voltage.
  2. Enter the average load in watts—not the appliance's brief startup surge.
  3. Keep the editable reserve, health and conversion settings accurate to your setup.
  4. Use the result as a planning estimate, then compare it with measured device consumption where possible.

Battery runtime formula

The calculator converts battery capacity to usable battery energy, then divides it by the battery-side average load. It calculates at full precision and rounds only for display.

runtime hours = (battery Wh × usable state of charge × health) ÷ battery-side load W

Ah vs Wh: why voltage matters for a 12V battery

Ah alone does not describe battery energy: Wh = volts × Ah. A 12V 100Ah battery is approximately 1,200 Wh before reserve and conversion losses. That is why a 12V battery run time calculator needs both Ah and voltage.

How watts and inverter efficiency change runtime

Watts describe the appliance load. For AC appliances, the inverter draws more from the battery than the device receives, so the estimate uses battery-side watts. A 100W AC device with 90% inverter efficiency draws about 111W from the battery; a direct DC load uses the separate DC efficiency setting instead.

Battery reserve and usable capacity

The estimate uses one charge window: starting charge minus minimum remaining charge, then battery health. It does not apply a second usable-capacity percentage, which avoids counting the same reserve twice. Chemistry presets are editable planning starting points, not device-specific claims.

12V battery run time example

For a 12V 100Ah battery at 100% charge, a 20% reserve and 90% inverter efficiency, the engine calculates 960 Wh of usable battery energy. A 100W AC appliance draws about 111W from the battery, giving an estimated runtime of 8 h 38 min.

Battery runtime by watts: 12V 100Ah illustrative scenario
Average loadEstimated runtime
50 W17 h 17 min
100 W8 h 38 min
300 W2 h 53 min
500 W1 h 44 min

Common battery runtime examples (100Ah & 200Ah setups)

How long does a 12V 100Ah or 200Ah battery actually last with real-world appliances? The table below models realistic runtime with 10% reserve (LiFePO4) and 88% AC inverter efficiency:

Real-world appliance runtime for 12V 100Ah & 200Ah batteriesEstimated runtime across 100Ah and 200Ah 12V LiFePO4 batteries
Appliance / DeviceAverage Power100Ah 12V Battery (~864 Usable Wh)200Ah 12V Battery (~1,728 Usable Wh)
Wi-Fi Router & Modem15 W~57.6 hours (2.4 days)~115.2 hours (4.8 days)
CPAP Machine (no humidifier)35 W~24.7 hours (3+ nights)~49.4 hours (6+ nights)
Starlink Satellite Terminal50 W~17.3 hours~34.6 hours
12V Portable Camping Fridge30 W avg (cycling)~28.8 hours (1.2 days)~57.6 hours (2.4 days)
Desktop PC + Monitor200 W~4.3 hours~8.6 hours
Full-Size Refrigerator (cycling)150 W avg~6.3 hours~12.7 hours

Calculation Formula & Mathematical Methodology

Calculates exact continuous running duration by determining net usable stored energy after Depth-of-Discharge (DOD) limits, battery health degradation, and inverter conversion losses.

FormulaRuntime (hours) = (Capacity_Wh × Usable_SOC × Battery_Health × Efficiency) / Load_Watts

Variable Definitions

Capacity_WhNominal Battery Energy(Wh)
Rated battery watt-hours (or Volts × Amp-Hours).
Usable_SOCUsable State of Charge Window(fraction)
Fraction of capacity available above minimum reserve (e.g., 80% for LiFePO4, 50% for Lead-Acid).
Battery_HealthState of Health (SOH)(fraction)
Available capacity relative to original factory rating (default 100%).
EfficiencyConversion Efficiency (η)(fraction)
Inverter efficiency for AC loads (85%–93%) or DC-DC step efficiency.
Load_WattsContinuous Power Demand(W)
Average real-time appliance consumption (Running Watts × Duty Cycle).

Engineering Notes & Standards

  • For intermittent loads like refrigerators and AC compressors, average load = running watts × duty cycle (typically 30%–45%).
  • Lead-acid and AGM batteries experience Peukert capacity loss under heavy discharge rates (>0.2C).

Frequently Asked Questions (FAQ)

How long will a 100Ah 12V battery run a refrigerator?
A standard household refrigerator averaging 150W (cycling with a ~35% compressor duty cycle) will run for approximately 6.3 hours on a 12V 100Ah LiFePO4 battery (assuming 80% usable capacity and 90% inverter efficiency). On a 200Ah battery, it will run for about 12.7 hours.
How long will a 100Ah battery run a CPAP machine?
A CPAP machine consuming 35W without a heated humidifier will run for approximately 24.7 hours on a 12V 100Ah LiFePO4 battery, or around 3 full 8-hour nights of sleep before needing recharge.
Why does a 12V 100Ah battery not provide the full 1,200 watt-hours?
Nominal energy is 12V × 100Ah = 1,200Wh. However, usable capacity is reduced by minimum state-of-charge reserve limits (typically 20% for LiFePO4 or 50% for Lead-Acid) and AC inverter conversion losses (typically 85%–92% efficiency).
How do I calculate battery runtime for AC appliances?
Divide usable battery watt-hours by the battery-side load. For AC equipment: Usable Wh = Rated Wh × Usable Fraction. Battery-Side Load = Appliance Watts ÷ Inverter Efficiency. Runtime Hours = Usable Wh ÷ Battery-Side Load.

Related battery planning

Need to size a battery for a target runtime? Use our Battery Size Calculator. For whole-home backup, use the Home Battery Size Calculator.

Methodology and sources

Generic battery defaults are editable planning estimates. See the methodology and sources used to maintain them.