Calculate inverter battery backup time from battery Ah and AC load in watts, or the battery size you need, using typical efficiency and battery type.

12 V is the most common inverter battery; change it for a 24 V or 48 V bank.

90% is typical for a modern inverter near its rated load. If it already includes idle loss, leave idle draw blank.

Optional settings
Blank means 0 W; add only losses excluded from the efficiency figure.

Inverter Usage Formula

The inverter usage calculator estimates how long a battery can power an AC load through an inverter. Choose amp-hours with the nominal bank voltage, or rated watt-hours. Enter usable depth of discharge and efficiency at the load; optional additional DC idle draw is blank for zero. The simplified equations and table below assume zero additional idle draw.

UT = (BC × 12 × (IE / 100)) / (LP)

Where:

  • UT = usage time in hours
  • BC = battery capacity in amp-hours (Ah)
  • IE = inverter efficiency as a percent
  • LP = load power in watts (W)

Why the Formula Works

Battery capacity is usually rated in amp-hours, but runtime is easier to estimate using energy in watt-hours. The conversion is:

Wh = Ah × V

For the 12V example, the available battery energy starts with BC × 12. That energy is then reduced by inverter losses, which is why inverter efficiency is included in the equation.

Usable Energy = BC × 12 × (IE / 100)

Finally, dividing usable energy by the load power gives the estimated operating time when all nominal capacity is usable and additional idle draw is zero.

General Runtime Equation

If you are working with a 24V or 48V battery bank, the more general form is:

UT = (BC × V × (IE / 100)) / (LP)

In real systems, actual runtime is often lower because not all battery capacity is fully usable. A more practical planning equation is:

UTactual ≈ (BC × V × (IE / 100) × DoD) / (LP)

Here, DoD is the usable depth-of-discharge expressed as a decimal. With separately modeled DC idle draw I, the calculator uses UT = (BC × V × DoD) / (LP / efficiency + I). Do not add idle loss twice if it is already included in the efficiency figure.

How to Use the Calculator

  1. Select the solve mode. For runtime, enter nominal battery capacity in Ah and bank voltage, or energy in Wh or kWh.
  2. Choose a battery type (or a custom depth of discharge) and keep the 90% inverter efficiency or enter your own.
  3. Enter the load power in watts or kilowatts.
  4. Select Calculate for the estimated runtime in hours and minutes. Other modes solve required nominal capacity or supported average AC load.

Example

If a 100 Ah, 12 V battery powers a 500 W load through an inverter that is 85% efficient, with 100% usable DoD and zero additional idle draw, the estimated runtime is:

UT = (100 × 12 × 0.85) / (500) = 2.04

That is about 2.04 hours, or roughly 2 hours 2 minutes.

Runtime Planning Tips

  • Higher load power reduces runtime: doubling the wattage roughly cuts runtime in half.
  • Better inverter efficiency improves runtime: less energy is lost as heat.
  • Larger batteries increase runtime: more stored energy means longer backup time.
  • Battery voltage matters: the same amp-hour rating stores more energy at higher voltage.
  • Continuous load is what matters most: startup surges affect sizing, but average watt draw affects runtime.

Common Reasons Actual Runtime Is Lower

  • Battery age and reduced effective capacity
  • Cold temperatures
  • Inverter idle power draw
  • Cable losses and poor connections
  • Lead-acid batteries delivering less usable energy at high discharge rates
  • Avoiding full discharge to protect battery life

Quick Reference Table

Illustrative 12 V bank, 100% usable DoD and zero additional idle draw. Use battery-specific limits for real planning.

Battery Capacity Efficiency Load Power Estimated Runtime
50 Ah 85% 100 W 5.10 hours
100 Ah 90% 300 W 3.60 hours
100 Ah 85% 500 W 2.04 hours
200 Ah 90% 150 W 14.40 hours
400 Ah 92% 1000 W 4.42 hours

Practical Interpretation

This calculator is most useful for backup power planning, RV electrical systems, solar battery banks, marine power systems, and small off-grid setups. It helps answer questions such as how long lights, electronics, networking equipment, fans, or appliances can run before the battery needs recharging.

For best results, use the average running wattage of the connected equipment, not just the maximum rating shown on the inverter. If you are estimating runtime for essential loads during an outage, it is also wise to leave a safety margin so the battery is not completely drained.