Battery Runtime Calculator

Enter your battery capacity, voltage, load and system parameters to estimate how long your battery will last. Accounts for depth of discharge and system efficiency.

DoD: 0.8 = 80% discharged. Efficiency: 0.95 = 95%.

How It Works

The formula

The calculator uses a two-step formula:

  1. Usable energy (Wh) = Capacity (Ah) × Voltage (V) × DoD × Efficiency
  2. Runtime (h) = Usable energy (Wh) ÷ Load (W)

Depth of discharge (DoD) limits how far you draw down the battery before recharging, to prolong cycle life. The efficiency factor accounts for inverter losses, wiring resistance and heat.

Worked example

A 100 Ah, 12 V battery with DoD = 0.8 and efficiency = 0.95 powering a 120 W load:

  • Usable energy = 100 × 12 × 0.8 × 0.95 = 912 Wh
  • Runtime = 912 ÷ 120 = 7.6 hours

With DoD = 1 and efficiency = 1 (theoretical maximum), the same battery would deliver 1 200 Wh and run for 10 hours.

Battery Type Reference

Battery type Typical DoD Notes
Lead-acid (flooded) 0.5 Deep discharge shortens life significantly
AGM / Gel 0.5 - 0.7 Sealed, lower maintenance
Lithium (LFP) 0.8 - 0.95 Tolerates deep discharge, long cycle life
Lithium NMC / NCR 0.8 Higher energy density, more temperature-sensitive

Frequently Asked Questions

How long will my battery last?

Divide the usable watt-hours by your load in watts. Usable watt-hours equals capacity (Ah) times voltage (V) times depth of discharge times efficiency. For a 100 Ah 12 V battery at 80% DoD and 95% efficiency powering a 120 W load, that is 912 Wh divided by 120 W, giving 7.6 hours.

What is depth of discharge?

Depth of discharge (DoD) is the fraction of a battery's capacity you draw down before recharging. A DoD of 0.5 means you use half the rated capacity, then recharge. Lower DoD values extend battery cycle life, particularly for lead-acid types. Lithium iron phosphate (LFP) batteries can be discharged to 0.8 or higher without significant life reduction.

Why does efficiency matter?

Real systems lose energy to heat in wiring, inverter switching losses, and battery internal resistance. A typical off-grid inverter runs at 90 to 95% efficiency. Multiplying by the efficiency factor accounts for these losses so your runtime estimate is realistic rather than optimistic.

What is the difference between amp-hours and watt-hours?

Amp-hours (Ah) measure charge: how many amps can flow for how many hours. Watt-hours (Wh) measure energy: power multiplied by time. To convert, multiply amp-hours by the battery voltage. A 100 Ah battery at 12 V holds 1 200 Wh. Watt-hours are more useful for comparing batteries at different voltages and for calculating runtime against a wattage load.

What depth of discharge should I use for lead-acid versus lithium?

For flooded lead-acid and AGM batteries, limit DoD to 0.5 (50%) to preserve cycle life. Gel batteries can go to about 0.7. Lithium iron phosphate (LFP) batteries are designed for 0.8 to 0.95 DoD without significant degradation. Check your battery manufacturer's specification for the recommended maximum DoD and cycle life at that depth.

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