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Battery Runtime Calculator: LiFePO4, AGM & Inverter Tare Losses

Accurately calculate battery bank backup hours under real-world continuous electrical loads. Accounts for inverter conversion efficiency, no-load tare overhead, and chemistry discharge curves.

Core relationship Runtime (hrs) = [Nominal Wh × DoD%] ÷ [(AC Watts ÷ Efficiency) + Inverter Idle Watts] Planning estimate. Verify equipment limits and site-specific requirements before final design.
Enter your assumptions
DC battery bank nominal voltage
Rated capacity per battery or bank
Active continuous equipment power
Determines maximum safe discharge depth
80% for LiFePO4, 50% for Lead-Acid
Typically 88% - 93% for pure sine wave
Tare load consumed simply by being ON (10-35W typical)
Check if running 12V/24V native DC devices
Estimated Battery Runtime 7 hrs 37 mins Operating down to 80% Depth of Discharge
Nominal Bank Energy 1200 Wh
Usable Energy 960 Wh
Total Load + Losses 126 Watts
Battery DC Draw 10.5 Amps
How to use the result

Start with the estimate, then compare it with actual equipment specifications, environmental conditions and applicable electrical requirements.

Mathematical Model & Engineering Assumptions

Standard theoretical runtime calculators divide battery watt-hours by load wattage, yielding wildly optimistic runtime estimates that can leave off-grid systems stranded in darkness. This engineering calculator incorporates the two critical real-world drain factors: inverter power conversion efficiency and inverter tare (idle) consumption.

1. Usable Energy Calculation

Nominal energy in Watt-hours (Wh) is calculated from nominal pack voltage (V) and rated Amp-hour capacity (Ah): 

Nominal Wh = V_nominal × Capacity_Ah
Usable Wh = Nominal Wh × (Depth_of_Discharge ÷ 100)
  • Lithium Iron Phosphate (LiFePO4): Sized for 80% to 90% Depth of Discharge (DoD). Because LiFePO4 exhibits a flat discharge curve and minimal rate-dependent capacity loss, linear watt-hour modeling provides an accurate practical runtime estimate.
  • Absorbent Glass Mat (AGM) / Flooded Lead-Acid: Sized for 50% DoD to preserve cycle life. Note: Lead-acid effective capacity varies significantly with high discharge rates (Peukert's Law). This calculator provides baseline runtime modeling based on rated capacity; heavy loads on lead-acid banks may result in shorter real-world runtimes than linear calculations suggest.

2. Total Battery Draw Modeling

An inverter converting DC battery voltage to 120V or 240V AC power is not 100% efficient, and it consumes internal quiescent power just staying powered on: 

Total Load Watts = (AC Load Watts ÷ Inverter Efficiency) + Inverter Idle Watts

Worked Real-World Example

Scenario: Powering a 100W Starlink dish + networking switch from a 12V 100Ah LiFePO4 battery via a 1000W pure sine wave inverter (90% efficiency, 15W tare loss) down to 80% DoD:

  1. Nominal Wh: 12V × 100Ah = 1,200 Wh
  2. Usable Wh: 1,200 Wh × 0.80 = 960 Wh
  3. Effective Load: (100W ÷ 0.90) + 15W = 111.1W + 15W = 126.1 Watts
  4. Delivered Runtime: 960 Wh ÷ 126.1W = 7.61 Hours (7 hrs 37 mins)

Note: A naive calculator would estimate 1,200 ÷ 100 = 12.0 hours, an error of over 4.3 hours (+57% overestimate)!

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