Enter the battery capacity, inverter efficiency, and load power into the calculator to determine the usage time of an inverter. This calculator helps to estimate how long an inverter can run a particular load with a given battery capacity and efficiency.

Inverter Usage Calculator

Enter any 3 values to calculate the missing variable

Inverter Usage Formula

The inverter usage calculator estimates how long a battery can power an AC load through an inverter. For this calculator, the runtime is based on a 12V battery system, the battery capacity in amp-hours, the inverter efficiency, and the load power in watts.

UT = \frac{BC \cdot 12 \cdot (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 \cdot V

Because this calculator assumes a 12V battery, 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 \cdot 12 \cdot (IE/100)

Finally, dividing usable energy by the load power gives the estimated operating time.

General Runtime Equation

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

UT = \frac{BC \cdot V \cdot (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:

UT_{actual} \approx \frac{BC \cdot V \cdot (IE/100) \cdot DoD}{LP}

Here, DoD is the usable depth-of-discharge expressed as a decimal.

How to Use the Calculator

  1. Enter the battery capacity in Ah or mAh.
  2. Enter the inverter efficiency as a percentage.
  3. Enter the load power in watts or kilowatts.
  4. The calculator returns the estimated usage time in hours or minutes.

Example

If a 100 Ah battery powers a 500 W load through an inverter that is 85% efficient, the estimated runtime is:

UT = \frac{100 \cdot 12 \cdot 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

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.