Battery Capacity Calculator

Battery capacity calculator for runtime (Ah and Wh)

This Battery Capacity Calculator estimates the battery size you need to power a device for a specific number of hours. The dominant use case is planning a battery setup for a known electrical load (usually expressed in watts), at a known battery or system voltage (such as 12V, 24V, or 48V), where you want a quick, defensible answer in amp-hours (Ah) and watt-hours (Wh). This is the decision most people are trying to make: “How big a battery do I need to run this thing for that long?”

You enter three basics: device load in watts, the battery or system voltage, and your desired runtime in hours. The calculator then converts that into energy required (watt-hours), and finally into battery capacity (amp-hours). If you open the Advanced section, you can include efficiency losses (common when using an inverter or DC to DC conversion) and the usable depth of discharge (how much of the battery you actually plan to use). Those two optional settings are the difference between an optimistic paper result and a practical setup that actually delivers the runtime you expect.

The results are shown as a minimum required battery capacity and a practical recommendation. The minimum tells you the theoretical amount of capacity needed under the assumptions you selected. The recommendation is a simple “round up” guidance because batteries are sold in standard sizes and because real usage is rarely perfectly steady. If you provide a typical single-battery size in the Advanced section (for example a 12V 100Ah battery), the calculator also estimates how many batteries of that type you would need to meet the recommended capacity. This is meant for quick planning, not for detailed electrical design.

Assumptions and how to use this calculator

• The load (watts) is treated as an average steady draw over the full runtime. If your device cycles on and off, use an average wattage.
• If you leave efficiency blank, the calculator assumes 90% efficiency to reflect typical losses in wiring and conversion equipment.
• If you leave depth of discharge blank, the calculator assumes 80% usable capacity. This is a common practical planning value.
• The calculator outputs energy in Wh and capacity in Ah using the relationship Wh = V × Ah. Voltage is assumed constant for planning purposes.
• The optional “number of batteries” estimate assumes identical batteries and is a sizing guide only. Series/parallel wiring, surge currents, temperature, and manufacturer limits are not modeled.

Common questions

What is the difference between Ah and Wh?

Amp-hours (Ah) is capacity at a specific voltage. Watt-hours (Wh) is energy and is easier to compare across voltages. A 12V 100Ah battery is roughly 1,200Wh (12 × 100), but the usable energy depends on how much you are willing to discharge it and on system losses.

Why do efficiency and depth of discharge matter?

Efficiency accounts for losses, especially when converting battery power to another form (like DC to AC via an inverter). Depth of discharge (DoD) reflects that you typically do not use 100% of a battery’s rated capacity in real life. Including both makes the result more realistic and reduces the chance of under-sizing.

My device draw is in amps, not watts. Can I still use this?

This page is locked to the “watts + voltage + hours” sizing decision. If you only have amps, you can convert to watts using watts = volts × amps, then enter the watts here. If you are unsure which voltage applies (device vs inverter vs battery bus), use the system voltage that your battery setup actually runs at.

Why is my result larger than what I expected from the battery label?

Battery labels are usually nominal capacity at ideal conditions. Real-world runtime is reduced by conversion losses, usable discharge limits, and the fact that voltage and available capacity can drop under higher loads. If you want a conservative plan, keep DoD at 80% or lower and do not assume 100% efficiency.

When does this calculator not apply?

This tool is not for precise engineering of battery banks, surge loads, or motor starting currents. It also does not model temperature effects, battery chemistry curves, or manufacturer-specific limits. If you are designing a critical system (medical, safety, or high-current industrial), use detailed manufacturer data and a full electrical design approach.