Battery Runtime Calculator
Estimate how long a battery can power a load after accounting for inverter/system efficiency.
Interactive Electrical Calculator
Battery Runtime Calculator
Estimate how long a battery can power a load after accounting for inverter/system efficiency.
Result
Enter values and calculate.
About the Battery Runtime Calculator
The Battery Runtime Calculator handles battery capacity, runtime, charging time and series/parallel bank sizing without needing a spreadsheet, app download or manual lookup table. Estimate how long a battery can power a load after accounting for inverter/system efficiency. Battery capacity is usually rated in Ah or Wh, while the load is usually rated in watts or amps, so converting between them correctly is the difference between a battery bank that lasts and one that runs flat early.
Solar and off-grid system builders, RV and marine owners, backup-power installers, and hobbyists working with lithium or lead-acid packs typically reach for a tool like this to double-check a number before it goes into a real installation.
How the calculation works
The calculator is built around this relationship:
Runtime = (V × Ah × efficiency) / load.
Where each input maps to a field in the calculator above:
- Battery Voltage in V.
- Battery Capacity in Ah.
- Load in W.
- Efficiency in %.
You can adjust any of these values as many times as you like; the result updates on every change with nothing sent off your device.
A worked example
For example, start with 12 V for battery voltage, 100 Ah for battery capacity, 300 W for load and 90 % for efficiency. Those defaults are only a starting point — replace them with your own measured or nameplate values, then press Calculate to get a result specific to your situation.
When to use the Battery Runtime Calculator
Typical reasons people reach for the Battery Runtime Calculator:
- Sizing a battery bank for a specific backup runtime target.
- Working out how long a charger will take to top up a given battery.
- Converting a battery spec sheet between Ah, Wh and mAh so it can be compared with another product.
- Planning a series/parallel wiring configuration to hit a target system voltage and capacity.
Good practice: Depth of discharge matters: a lead-acid battery rated for 100 Ah cannot safely deliver anywhere near 100 Ah of usable capacity, unlike most lithium chemistries.
Also worth knowing: Wiring batteries in series adds their voltages while capacity (Ah) stays the same as a single cell; wiring in parallel adds capacity while voltage stays the same.
Frequently asked questions
What does the Battery Runtime Calculator calculate?
Estimate how long a battery can power a load after accounting for inverter/system efficiency.
How accurate is this calculator?
The formula above is applied exactly to the values you enter, so the math itself is precise. Real-world results can still differ due to component tolerances, temperature, installation method, equipment efficiency and local code requirements, so treat the output as a strong planning estimate and verify final designs against manufacturer data and the electrical code that applies to your installation.
Do I need to sign up or install anything to use it?
No sign-up, download or installation is needed. The Battery Runtime Calculator runs entirely in your browser and is free to use.
Why not just estimate this instead of calculating it?
Series and parallel wiring change voltage and capacity in very different ways, and getting it backwards can damage cells or leave a system under-powered.
Can I use this on a phone, or does it need a desktop?
The Battery Runtime Calculator works on any modern browser, phone or desktop, and recalculates as soon as you press Calculate. If a result looks off, double-check that every field is filled in with the unit shown next to it (mixing up units, such as W with kW, is the most common cause of an unexpected answer) and re-enter the values.