Battery Capacity Calculator
Estimate required battery capacity in amp-hours for a load and desired runtime.
Interactive Electrical Calculator
Battery Capacity Calculator
Estimate required battery capacity in amp-hours for a load and desired runtime.
Result
Enter values and calculate.
About the Battery Capacity Calculator
The Battery Capacity Calculator is a free, browser-based tool for battery capacity, runtime, charging time and series/parallel bank sizing. Estimate required battery capacity in amp-hours for a load and desired runtime. 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.
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:
Ah = W × hours / (V × DoD × efficiency).
Where each input maps to a field in the calculator above:
- Load in W.
- Required Runtime in h.
- Battery Voltage in V.
- Usable DoD in %.
- Efficiency in %.
Change any field above and the result recalculates immediately — nothing is submitted anywhere, so it's safe to experiment freely.
A worked example
For example, start with 500 W for load, 5 h for required runtime, 24 V for battery voltage, 80 % for usable dod and 90 % for efficiency. Press Calculate and the Battery Capacity Calculator returns the result instantly — then change one field at a time to see how sensitive the answer is to that particular input.
When to use the Battery Capacity Calculator
Typical reasons people reach for the Battery Capacity 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: Real-world runtime is usually shorter than a simple capacity ÷ load calculation because of inverter losses, temperature and battery age.
Also worth knowing: 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.
Frequently asked questions
What does the Battery Capacity Calculator calculate?
Estimate required battery capacity in amp-hours for a load and desired runtime.
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 Capacity Calculator runs entirely in your browser and is free to use.
What happens if I get this wrong?
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.
What should I do if the result looks off?
The Battery Capacity 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.