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Battery Capacity Calculator: How Many Ah Do I Need?

Enter the load, the backup hours you want and the system voltage. You get the Ah required and a suggested set of 12 V batteries.

Battery capacity needed

184Ah

Energy needed
1,500 Wh
Suggested batteries
1 × 200 Ah 12 V
Layout (series × parallel)
1S × 1P

Estimate only. Real results depend on your equipment, its age and conditions.

S = batteries in series per string, P = strings in parallel. Allow extra for battery ageing.

Show working
  1. 500 W × 3 h = 1,500 Wh
  2. 1,500 ÷ (12 × 0.85 × 0.8) = 183.8 Ah

How to use

  1. Enter the total load in watts.
  2. Enter how many hours of backup you want.
  3. Choose 12, 24 or 48 V system voltage to see Ah and battery set.

Good to know

Ah needed = load (W) × hours ÷ (system volts × inverter efficiency × usable depth). For 500 W for 3 hours at 12 V with 85% efficiency and 80% usable depth: 1,500 ÷ 8.16 = 184 Ah.

The suggestion uses common Indian inverter battery sizes (100–220 Ah, 12 V). A 24 V system uses 2 batteries in series, 48 V uses 4; if one string is not enough, strings are added in parallel.

Add margin for battery ageing (capacity drops over the years) and for heavy loads. Do not mix old and new or different-Ah batteries in one bank. Match the battery to your inverter’s system voltage and charger.

Questions

What battery do I need for 500 W for 3 hours?

About 184 Ah at 12 V (with 85% inverter efficiency and 80% usable depth), so a 200 Ah battery.

Is a higher Ah battery always better?

More Ah gives more backup but needs a charger strong enough to refill it; check your inverter’s charging current.

12 V or 24 V system?

Small home inverters (up to about 1,100 VA) are usually 12 V; 1,500–2,500 VA often 24 V; larger systems 48 V.

Can I mix a new battery with an old one?

Not advisable. The weaker battery drags the bank down and both wear out sooner.

Why is usable depth only 80%?

Lead-acid batteries last longer if not fully drained. Lithium allows deeper use; change it under More options.

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