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Battery Runtime Calculator: LiFePO4, AGM & Inverter Tare Losses

Estimate battery-bank backup hours for steady loads using nominal energy, selected depth of discharge, inverter conversion efficiency, and inverter tare draw.

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
Selects a planning DoD default; verify the specific battery manufacturer's limits
Planning defaults: 80% LiFePO4, 50% lead-acid; manufacturer guidance takes precedence
Use the inverter datasheet or measured value; 90% is the planning default
Use the inverter datasheet or measured idle draw; 15W is the default
Check if running 12V/24V native DC devices
Estimated Battery Runtime 7 hrs 37 mins Using 80% of nominal capacity (80% depth of discharge).
Nominal Bank Energy 1200 Wh
Usable Energy 960 Wh
Total Load + Losses 126 Watts
Battery DC Draw 10.5 Amps

Planning model assumes a steady load and the entered nominal voltage, DoD, inverter efficiency, and tare draw. Temperature, aging, BMS cutoffs, load variation, and manufacturer-specific usable capacity can change real runtime.

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

A simple watt-hour ÷ load calculation ignores important losses. This planning model includes two common battery-side drains that materially affect steady-load runtime: inverter 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): The calculator uses an 80% planning default for Depth of Discharge (DoD). A linear watt-hour model can provide a useful baseline for a steady load, but actual runtime varies with pack voltage window, temperature, battery age, BMS cutoffs, cell balance, load profile, and the manufacturer's usable-capacity limits.
  • 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. The equation below is a simplified planning model: real efficiency specifications may already include internal overhead, so do not blindly add separately measured idle watts to total measured DC input or double-count losses: 

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. Modeled Runtime: 960 Wh ÷ 126.1W = 7.61 Hours (7 hrs 37 mins)

For comparison, a nominal-only calculation of 1,200 Wh ÷ 100 W gives 12.0 hours because it ignores the selected DoD, inverter loss, and tare draw.

What this calculator does not model

  • Temperature-dependent battery capacity, aging, cell imbalance, BMS behavior, or manufacturer-specific voltage cutoffs.
  • Time-varying loads, inverter efficiency curves at different load levels, surge events, or sleep/search modes.
  • Peukert-rate effects for lead-acid batteries beyond the warning above, cable/terminal losses, or DC-DC converter losses.
  • Equipment-specific safety limits or code-compliance decisions for conductors, overcurrent protection, and installation.

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