Battery Capacity Calculator
Calculate the battery capacity (Ah) needed to run a load for a target amount of time.
Calculate
Enter your values below.
Result
Required capacity
0
Formula used
Capacity (Ah) = Runtime (hours) × Load current (A)
Example: 5-hour runtime at 4 A load → 20 Ah battery needed.
How to use this calculator
- Enter Desired runtime (hours), Load current (A).
- Press Calculate to see your result instantly.
FAQ
Frequently asked questions
What is Ah (amp-hour)?
A unit of battery capacity — a 20 Ah battery can theoretically supply 20 amps for 1 hour, or 4 amps for 5 hours.
Does this account for battery efficiency losses?
No, it's a theoretical calculation — real-world usable capacity is often somewhat lower, especially at high discharge rates.
Should I buy a bigger battery than calculated?
Many people add a safety margin (e.g. 20-30%) to account for battery aging and efficiency losses.
How is this different from Battery Runtime Calculator?
This solves for capacity given a target runtime; the Runtime Calculator solves for runtime given a known capacity.