Required Battery Capacity Calculator

About this calculator

Battery shopping starts with a load and an hour count, not with a capacity. Work out what you need first and the Ah figure to shop for follows; start from a battery somebody recommended and you find out whether it was enough during the next power cut.

The result is rounded up to a size you can actually buy, with the exact figure alongside so the rounding is visible.

How to use this calculator

1. Add up everything you want backed up. Fans, lights, a television and a router come to 200–400 W in most Indian households.
2. Enter the backup time you want.
3. Choose the battery voltage — 12 V for a single battery, 24 V for two in series.
4. Set the efficiency. For a lead-acid inverter battery, 40–45% gives the honest answer.

The formula

Energy needed   = Load x Backup time
Battery energy  = Energy needed / Efficiency
Capacity needed = Battery energy / Voltage
Buy at least    = that, rounded up to the next 10 Ah

The rounding is deliberate and upward. Batteries come in stocked sizes, and a requirement of 167 Ah is met by a 180 Ah battery rather than a 160 Ah one.

Worked example

A 300 W household load for 3 hours at 12 V, with lead-acid allowed for at 45%:

Energy needed   = 300 x 3      = 900 Wh
Battery energy  = 900 / 0.45   = 2,000 Wh
Capacity needed = 2000 / 12    = 166.7 Ah  -> buy 170 Ah or more

Which is why 150 Ah and 180 Ah are the sizes Indian inverter batteries actually come in.

Notes and limits

• Round up, and then allow a margin for age. A battery at the end of its life holds noticeably less than a new one.
• Two smaller batteries can be wired in series for a higher voltage or in parallel for more amp-hours. They should be matched in type, capacity and age, since the weakest limits the string.
• Lithium tolerates deeper discharge, so a lithium bank of this nominal capacity gives more real backup than a lead-acid one — which is part of why it costs more per Ah and less per usable Ah.
• Check the inverter can supply the load as well. Sizing the battery is only half the job; the inverter has its own wattage rating and its own surge limit.
• To verify, put the answer into the inverter backup calculator and see that the hours come back.

What this calculation assumes

• The load is steady and everything to be backed up is included.
• The efficiency you choose combines inverter loss with usable depth of discharge.
• The rounded figure is a minimum; battery age and cold both argue for more.