Softonic

Amp-Hour Calculator

Step 1 / 3 Runtime
Next

How long will a battery run a given load, and how much energy does it actually store? Enter the battery capacity in amp-hours (Ah), its nominal voltage in volts (V) and the current your device draws in amps (A). The calculator instantly returns the estimated runtime as hours and minutes, together with the stored energy in watt-hours (Wh) and kilowatt-hours (kWh). It is ideal for sizing off-grid solar packs, RV and marine house batteries, UPS units, drone packs and any 12 V or 48 V build where you need an honest figure before you buy.

How to use the calculator

  1. 1

    Enter capacity and voltage

    Type the battery capacity in amp-hours (Ah) and its nominal voltage in volts (V).

  2. 2

    Add the load current

    Enter the average current your device draws in amps (A). Use the steady draw, not the brief peak.

  3. 3

    Read runtime and energy

    See the runtime in hours and minutes, plus the stored energy in watt-hours and kilowatt-hours, updating as you type.

The formulas

Two simple relationships drive this tool. Runtime is capacity divided by load current:

runtime (hours) = capacity (Ah) ÷ load current (A)

Stored energy is capacity multiplied by voltage:

energy (Wh) = capacity (Ah) × voltage (V)

and kilowatt-hours are just kWh = Wh ÷ 1000. Because Ah ÷ A cancels the amp unit, the runtime comes out directly in hours; the decimal part is converted to minutes (× 60).

Worked example

A 100 Ah, 12 V battery powers a fridge that draws 5 A:

  • runtime = 100 ÷ 5 = 20 hours = 20h 0m
  • energy = 100 × 12 = 1200 Wh
  • kWh = 1200 ÷ 1000 = 1.2 kWh

Halve the load to 2.5 A and the runtime doubles to 40 hours, while the stored energy stays at 1.2 kWh because energy depends only on capacity and voltage.

Capacity vs energy at a glance

Capacity (Ah) Voltage (V) Energy (Wh) Energy (kWh)
50 12 600 0.60
100 12 1200 1.20
100 24 2400 2.40
200 48 9600 9.60

Pitfalls

  • Usable capacity is lower. Lead-acid cells should only be drained to ~50%; lithium (LiFePO₄) to ~80–90%. Multiply the rated Ah by that depth-of-discharge before trusting the runtime.
  • Peukert’s effect. High discharge rates reduce effective capacity, especially on lead-acid — a 100 Ah cell at a heavy draw may behave like 70 Ah.
  • Voltage is nominal. A “12 V” pack swings from ~14.4 V charged to ~10.5 V flat, so Wh is an approximation around the nominal value.
  • Average vs peak current. Use the steady running current; sizing from a momentary surge badly underestimates runtime.

Frequently Asked Questions

An amp-hour is the charge delivered by one amp of current flowing for one hour. A 100 Ah battery can, in theory, supply 1 A for 100 hours, 5 A for 20 hours, or 10 A for 10 hours. It measures charge capacity, not energy, so you also need the voltage to know the watt-hours.

Multiply amp-hours by the nominal voltage: Wh = Ah × V. A 100 Ah battery at 12 V stores about 1200 Wh, or 1.2 kWh. Watt-hours let you compare packs that run at different voltages on equal footing.

The tool gives the ideal figure from rated capacity. In practice you lose runtime to depth-of-discharge limits, Peukert losses at high currents, temperature, ageing and inverter inefficiency. Treat the result as an upper bound and derate it for your battery chemistry and load.

No. The calculation runs entirely in your browser session and nothing you type is uploaded, saved or shared. The numbers exist only for the current page and disappear when you leave.

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