Amp Hour Run Time Calculator and Battery Formula

An amp-hour run time calculator estimates how long a battery can power a device from battery capacity, load current, and usable capacity. Enter those values to get an estimated run time in hours and minutes; the result is a planning estimate, not a guarantee of actual battery performance.

Formula

Run time (hours) = battery capacity (Ah) × usable-capacity fraction ÷ load current (A). With usable capacity entered as a percentage, the calculator uses:

t = C × (U ÷ 100) ÷ I

• t is estimated run time in hours.
• C is the battery’s rated capacity in amp-hours (Ah).
• U is the portion of capacity you intend to use, as a percentage.
• I is the device’s average current draw in amperes (A).

The second output converts the same estimate to minutes. Amp-hours and amperes are compatible units: 1 Ah represents 1 amp supplied for 1 hour under the conditions used to rate the battery. In metric terms, 1 Ah equals 1,000 mAh, and 1 A equals 1,000 mA. Convert units before calculating: for example, 5,000 mAh is 5 Ah, not 5,000 Ah.

Worked Example

Using the calculator’s default values—100 Ah capacity, a 5 A load, and 80% usable capacity:

1. Convert the usable percentage to a fraction: 80 ÷ 100 = 0.8.
2. Find the usable capacity: 100 Ah × 0.8 = 80 Ah.
3. Divide by current: 80 Ah ÷ 5 A = 16 hours.
4. Convert to minutes: 16 × 60 = 960 minutes.

The calculator displays an estimated run time of 16.00 hours, or 960 minutes. The minute output is a conversion of the estimate, not a more precise prediction.

How to Choose the Inputs

Use the battery’s capacity rating and the load’s actual average current draw. Check the battery label or product documentation for capacity, and the device documentation or a suitable meter for current. If several devices share a battery, add their currents when they operate at the same time. If they run in cycles, estimate the average draw over a representative period rather than using only the peak current.

• Capacity: Enter amp-hours (Ah). If the rating is in milliamp-hours (mAh), divide by 1,000.
• Load current: Enter amperes (A). Convert milliamps to amps by dividing by 1,000.
• Usable capacity: Enter the fraction of the rated capacity you expect to use. Use less than 100% when you need a reserve or the battery should not be fully discharged.

There is no universal usable-capacity percentage. It depends on battery chemistry, manufacturer guidance, temperature, age, operating conditions, and how much reserve you want. Follow the battery maker’s instructions rather than treating the example’s 80% as a rule. If you enter 100%, the calculation assumes the entire rated capacity is available to the load.

Understanding What Amp-Hours Mean

Amp-hours measure electric charge capacity, not energy by themselves. Battery voltage matters when comparing energy needs: watt-hours (Wh) are approximately volts (V) × amp-hours (Ah) for a battery with a stated nominal voltage. A 12 V, 100 Ah battery is nominally 1,200 Wh, but that does not mean every device will receive all 1,200 Wh. Conversion losses, voltage limits, battery conditions, and the device’s power demand affect what is usable.

This distinction matters when a load is specified in watts rather than amps. For a reasonably stable voltage, estimate current as I = P ÷ V, where power P is in watts and voltage V is in volts. For example, a 60 W load on a nominal 12 V supply would draw about 5 A in an ideal calculation. Real systems may draw more because of conversion losses or varying voltage. Use the equipment’s actual input specifications where possible, and do not assume a device’s output wattage equals its battery-side input.

Limits and Common Sources of Error

The formula assumes a steady load and that the stated capacity is available at the current draw and conditions in use. Actual battery capacity can differ from its label rating depending on discharge rate, temperature, age, condition, and battery chemistry. In some batteries, pulling current faster reduces the capacity available compared with a slow discharge. Inverters and other converters also consume energy, so a load powered through one may run for less time than a direct-current calculation suggests.

• Mixing Ah and mAh: Convert to matching units first; 2,000 mAh is 2 Ah.
• Using rated capacity as guaranteed usable capacity: Reserve and discharge limits can reduce the portion available.
• Using peak current for a variable load: Peak draw may describe startup rather than normal operation; average draw is more useful for a basic runtime estimate.
• Ignoring voltage and conversion losses: A watt-rated device may need a current estimate based on the battery-side voltage and system efficiency.
• Reading decimals as certainty: A result such as 16.00 hours does not account for changing conditions or measurement uncertainty.

Amp-hour arithmetic is useful for rough planning, but it cannot establish a safe operating limit or guarantee that a battery can supply a particular current. Check the battery and device specifications, and use appropriate protections and qualified guidance for system design.

Frequently Asked Questions

How do I calculate battery run time from amp-hours?
Divide usable battery capacity in Ah by the load’s average current in A. If you are not using all rated capacity, multiply the rating by the usable fraction first.

How long will a 100 Ah battery last at 5 amps?
At a steady 5 A draw, the ideal estimate is 20 hours if all 100 Ah is usable. If only 80% is used, the estimate is 16 hours. Real run time can be shorter.

Does a higher voltage make amp-hours last longer?
Not by itself. Amp-hours describe charge, while watt-hours include voltage. Compare energy demand and available energy in compatible units, and account for conversion losses and operating limits.

Can I calculate run time from watts instead of amps?
Estimate current by dividing watts by volts, then use the amp-hour formula. This is approximate when voltage changes or a converter has losses; use actual input specifications when available.

Why does my battery run out sooner than the calculator predicts?
The load may draw more than expected, usable capacity may be below the rating, or temperature, age, discharge rate, and conversion losses may reduce available energy. The calculation assumes a steady load and idealized capacity.

Related

• Australia Post Calculator: Sending Costs & Times
• Australia Post Quote Calculator: How to Use It
• Pallet Configuration Calculator: How to Plan Loads

in cap | Battery capacity | Ah | 100 | 0 | 100000
in load | Load current | A | 5 | 0 | 100000
in use | Usable capacity | % | 80 | 0 | 100
out hrs | Estimated run time | h | cap * use / 100 / load | 2
out mins | Estimated run time | min | cap * use / 100 / load * 60 | 0