What this calculator does
A battery amp hour calculator answers the sizing question that comes before buying a battery: given a device or load that draws a certain current, and how many hours it needs to run for, how many amp-hours (Ah) of capacity does the battery need. Amp-hours are simply current multiplied by time, so a 5 amp load running for 4 hours needs 20 Ah of capacity to run flat out.
Sizing a battery on the bare number leaves no room for error, since batteries lose some usable capacity over their life, and rarely deliver 100% of their rated capacity in practice. This calculator adds an adjustable safety margin on top of the bare figure, so the result is a more realistic capacity to shop for rather than the absolute theoretical minimum.
The formula
Multiply the current draw in amps by the runtime in hours to get the bare amp-hour requirement, then increase that figure by the safety margin percentage to allow for battery losses and ageing.
| Term | Meaning |
|---|---|
| Amp-hour (Ah) | A unit of electric charge equal to one amp flowing for one hour, the standard way battery capacity is rated. |
| Current draw | The steady current, in amps, that the device or load pulls while running. |
| Safety margin | An extra percentage added on top of the bare calculation to account for battery ageing, temperature effects and not fully discharging the battery. |
The inputs explained
| Field | What to enter |
|---|---|
| Current draw (A) | The steady current draw of the device or load, in amps. |
| Hours of runtime needed (h) | How many hours you need the load to run for on a single charge. |
| Safety margin (%) | Extra capacity to add on top of the bare figure, as a percentage. |
When to use it
Sizing a battery for camping or off-grid gear
Adding up the current draw of lights, a fridge or a pump, then multiplying by the hours needed between charges, gives the minimum battery capacity to carry or install.
Choosing a backup or UPS battery
Knowing the load a backup battery needs to support, and for how long, translates directly into the Ah rating to look for when comparing battery options.
Checking whether an existing battery is big enough
Comparing a battery's rated Ah capacity against the bare requirement plus a safety margin shows whether it has enough headroom for the intended use, or whether it is being pushed close to its limit.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much capacity is needed at different current draws?
A fixed 4 hour runtime requirement, at a range of current draws.
| Current draw | Battery capacity needed | Bare capacity (no margin) |
|---|---|---|
| 1 A | 4.80 Ah | 4.00 Ah |
| 2 A | 9.60 Ah | 8.00 Ah |
| 5 A | 24.00 Ah | 20.00 Ah |
| 10 A | 48.00 Ah | 40.00 Ah |
| 15 A | 72.00 Ah | 60.00 Ah |
| 20 A | 96.00 Ah | 80.00 Ah |
Questions
Why add a safety margin at all?
A battery rarely delivers its full rated capacity in real use. Depth-of-discharge limits, capacity loss as the battery ages, and reduced performance in cold weather all eat into the usable capacity, so sizing to the bare theoretical minimum risks running out early.
How is this different from the battery runtime calculator?
The battery runtime calculator works forwards, from a known battery capacity to how many hours it will last. This calculator works backwards, from a required runtime to the capacity you need to buy or install.
What margin should I use?
There is no universal figure. A common starting point is 20 to 30% for general use, with a larger margin for critical backup applications or batteries that will be used in cold conditions.
Does voltage matter for amp-hours?
Amp-hours describe charge, not energy, so the calculation here does not need a voltage. If you want the equivalent energy figure in watt-hours, or want to convert between mAh, Ah and Wh at a given voltage, use the battery capacity unit converter instead.
To convert the resulting Ah figure into mAh or Wh at a given voltage, see the battery capacity unit converter. To work the other way, from a known battery capacity to expected runtime, see the battery runtime calculator.