Battery Capacity Calculator
Estimate battery-bank capacity from load, runtime, voltage, depth of discharge and system efficiency
Required battery capacity
How we calculated it
About the Battery Capacity Calculator
Battery capacity sizing works backward from the energy a load must receive and the amount of time it must run. Voltage, load power, system efficiency and usable depth of discharge all affect the amp-hour or watt-hour capacity required.
A calculated nominal capacity is a starting point, not a guarantee. Battery chemistry, temperature, aging, discharge rate and manufacturer limits can reduce the energy actually available.
How to Use the Battery Capacity Calculator
- Enter the average load power or current.
- Enter required runtime.
- Enter system voltage when sizing in amp-hours.
- Apply realistic inverter efficiency and usable depth-of-discharge limits, then calculate required nominal capacity and add design margin.
How the Calculation Works
Required delivered energy = load watts × runtime hours. Required nominal watt-hours = delivered energy ÷ (efficiency × usable fraction).
If capacity is expressed in amp-hours, Ah = required nominal Wh ÷ battery voltage. When current is known directly for a DC load, a simplified estimate is Ah = current × hours adjusted for usable capacity and losses.
Worked example
Battery Capacity Calculator Worked Example
500 W load for 4 hours on 24 V, 90% efficient, 80% usable
Delivered energy = 500×4 = 2,000 Wh. Nominal energy = 2,000÷(0.9×0.8) ≈ 2,777.8 Wh. At 24 V, required capacity ≈ 2,777.8÷24 = 115.7 Ah before additional aging margin.
Result guide
Understanding Your Battery Capacity Calculator Result
The result is the minimum nominal capacity under the entered assumptions. Designing with no reserve can lead to inadequate runtime as batteries age or conditions worsen.
Before you rely on the answer
Assumptions and Limitations
- Average load and runtime are representative.
- Voltage remains suitable for the connected system.
- Efficiency and depth-of-discharge assumptions match the actual chemistry and equipment.
Practical tip
Using This Result in Practice
For critical backup systems, add aging and temperature margin and verify the result against manufacturer discharge curves. Lead-acid capacity in particular can fall at high discharge rates.
Related calculations
Useful Calculations to Continue With
If battery capacity is already known and you want the expected runtime instead, use the Battery Runtime Calculator.
If the battery will supply AC appliances, verify the inverter’s continuous and surge size separately with the Inverter Size Calculator.
Common questions
Battery Capacity Calculator FAQ
How do I convert watt-hours to amp-hours?
Divide watt-hours by battery voltage, provided the voltage basis is appropriate for the battery system.
Why include depth of discharge?
Many batteries should not use 100% of their nominal stored energy during normal operation.
Should I size exactly to the calculated Ah?
Usually not for important systems. Additional margin helps account for aging, temperature and real load variation.