How this instrument works
Amp-hours (Ah) measure electric charge capacity: how much current a battery can deliver for how long. That number alone is incomplete, because the same charge delivered at a higher voltage carries more energy — a watt is defined as one volt times one amp, so multiplying a battery's amp-hour rating by its voltage converts charge capacity into watt-hours (Wh), the actual unit of stored energy.
This distinction matters because two batteries can share the same Ah rating and store very different amounts of energy. A 10 Ah battery at 3.7 V (a typical lithium-ion cell voltage) holds about 37 Wh, while a 10 Ah battery at 48 V (a typical e-bike pack voltage) holds about 480 Wh — over ten times more energy for the identical amp-hour number. Watt-hours is the apples-to-apples unit that removes voltage from the comparison, which is why power banks, batteries and shipping regulations increasingly label capacity in Wh rather than Ah alone.
The figure this calculator returns is the nameplate, or nominal, energy capacity, assuming the battery discharges fully and ideally. Real usable energy typically runs somewhat lower, because of internal resistance, voltage sag under load, and manufacturer-recommended depth-of-discharge limits — treat the Wh result as a ceiling to plan against, not a guaranteed delivered figure.
- Enter Capacity (Ah) — the amp-hour rating printed on the battery label or listed in its datasheet.
- Enter Voltage (V) — the battery's nominal voltage.
- Read Energy capacity (Wh) — the total stored energy, directly comparable against any other battery regardless of its voltage.
- Feed this Wh figure into this site's battery life calculator alongside a device's power draw to estimate runtime.
Worked example — a 10 Ah, 12 V battery
Enter 10 into Capacity (Ah) and 12 into Voltage (V) — a common small sealed lead-acid battery size. Energy capacity reads 120.00 Wh: 10 Ah × 12 V = 120 Wh, meaning the battery can theoretically deliver 120 watts for one hour, 12 watts for ten hours, or any other combination of watts and hours that multiplies to 120 watt-hours.
Questions
Why isn't amp-hours alone enough to compare two batteries?
Because amp-hours ignore voltage, and voltage is exactly what determines how much energy each amp-hour actually carries. A 100 Ah battery at 3.7 V stores 370 Wh, while a 50 Ah battery at 24 V stores 1,200 Wh — over three times more energy despite a smaller amp-hour rating — so comparing raw Ah numbers across batteries of different voltage is misleading without also multiplying through by voltage.
How do I find my battery's voltage and Ah rating?
Both are usually printed directly on the battery label or listed in its datasheet — common nominal voltages include 3.6-3.7 V for a single lithium-ion cell, 12 V for lead-acid and many power tool packs, and 24 V or 48 V for larger e-bike or solar battery banks. If you only see a Wh figure already listed, that's the number this calculator produces, so you can skip straight to it.
Is the watt-hour figure the same as the energy a battery actually delivers?
It's the nameplate figure, assuming ideal, complete discharge. Real-world usable energy is typically somewhat lower, because of internal resistance, voltage sag as the battery empties, and manufacturer-recommended depth-of-discharge limits meant to protect battery life. Treat the Wh result as an upper bound when planning runtime, not a guarantee of delivered energy.
Why do power banks and battery packs increasingly list Wh instead of just mAh?
Because mAh alone doesn't reveal total energy — 1,000 mAh at 3.7 V is very different from 1,000 mAh at 5 V once you account for the actual voltage doing the work. Listing Wh gives a single number describing true energy content regardless of internal cell voltage, which is why it's the figure worth comparing across different power banks, and why many shipping and airline rules reference Wh specifically.
Does this formula work for any battery chemistry?
Yes — Wh = Ah × V is a plain unit conversion that holds for any battery chemistry, because it follows directly from the definition of a watt as one volt times one amp. Use the pack's actual nominal voltage, whether it's a 3.7 V lithium-ion cell, a 12 V lead-acid battery, or a 48 V e-bike pack, and the calculation is identical in every case.