# Battery Backup Runtime Calculator

> Estimate how long a battery, UPS or inverter backup will run a load from its Ah and voltage or kWh, usable depth of discharge and inverter efficiency.

Category: Energy & environment · Applies to: Worldwide · Last updated: 2026-10-29
Interactive calculator: https://calculators.geekmidst.com/calculators/battery-backup-runtime-calculator

Runtime is the battery’s usable energy divided by the power the load draws. Usable energy is the rated capacity times the depth of discharge you allow, less inverter losses.

Also called: UPS runtime calculator, Inverter backup time calculator, Battery life calculator.

## Inputs
- Battery capacity (default 200)
- Capacity in (options: Amp-hours, kWh; default ah)
- Battery bank voltage (unit V; default 12)
- Usable depth of discharge (unit %; default 50)
- Inverter efficiency (unit %; default 90)
- Load (unit W; default 300)

## Results
- Backup time
- Backup time in hours
- Rated energy
- Usable energy delivered
- Current drawn from the battery

## Formula

```
Wh = Ah × V (or kWh × 1000);  Runtime (h) = Wh × DoD × Inverter efficiency ÷ Load (W);  Battery current = Load ÷ Efficiency ÷ V
```

Where:
- Ah: rated amp-hour capacity
- V: battery bank voltage
- DoD: share of capacity you allow to be used
- Load: power drawn by the connected devices

## Worked example
1. Rated energy = 200 Ah × 12 V = 2,400 Wh.
2. Usable = 2,400 × 0.50 × 0.90 = 1,080 Wh.
3. Runtime = 1,080 ÷ 300 W = 3.6 h = 3 h 36 min.

**Result:** A 12 V 200 Ah battery used to 50% through a 90%-efficient inverter runs a 300 W load for about 3 hours 36 minutes.

## What the result means

Treat the result as a best case. At high currents, in the cold, or with an older battery, real runtime is shorter, so allow a margin for anything critical.

## Assumptions and limits
- The load is constant for the whole runtime.
- The battery delivers its rated capacity at this discharge rate.
- Ignores the Peukert effect (lead-acid capacity drops at high currents), temperature and battery age.
- Inverter standby consumption is not included.

## Questions

### Why not use 100% of the battery?

Deep discharges shorten battery life, especially for lead-acid. Most systems cut off before the battery is fully empty.

### How do I find my load in watts?

Add up the rated watts of the devices on backup, or read the output power from the inverter display.

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Source: GM¹X CalcVerse — https://calculators.geekmidst.com/calculators/battery-backup-runtime-calculator