Why do I need twice the battery capacity?

Because lead acid should not go below fifty percent without shortening its life sharply, and the inverter loses about fifteen percent converting DC to AC. Both derations multiply.

Is lithium worth the cost?

Per usable amp-hour it is much closer than the nameplate price suggests, because it tolerates deep discharge and lasts more cycles.

Construction & HomeenergyIndia only

Inverter and Battery Sizing Calculator

Inverter and battery sizing for a backup load. Nameplate battery capacity is roughly double the energy you need, because a lead acid battery should not be discharged past half and the inverter loses fifteen percent.

Also called: inverter size calculator, battery backup calculator.

Battery capacity needed

392.2

392.2 Ah at 12 V to run 500 W for 4 hours. Nameplate capacity has to be higher than the energy suggests because only 50% is usable and the inverter is 85% efficient. Inverter rating 625 VA. Lead acid and tubular batteries lose life sharply below fifty percent, so half the nameplate is the usable half.

Inverter rating

625

Energy needed

2,000

Usable energy per battery

900

Batteries at 150 Ah

Approximate recharge time

31.4

On depth of discharge

Lead acid and tubular batteries lose life sharply below fifty percent, so half the nameplate is the usable half.

On regulation

This is a sizing estimate. The applicable wiring or building regulation governs and overrides any figure here.

How this is calculated

Method and background

How this is calculated

Energy is load times hours. Two derations follow: the inverter converts DC to AC at eighty-five percent or so, and the battery should not be discharged below its rated depth. Lead acid lasts far longer if kept above fifty percent, while lithium tolerates eighty or ninety, which is most of why lithium costs more per nameplate amp-hour and less per usable one. Inverter VA rating uses the power factor, so a 500 W load needs about 625 VA.

the usable fraction and inverter efficiency both divide, so nameplate capacity is well above the raw energy

DoD: Depth of discharge
eta: Inverter efficiency

Worked examples

Each of these is asserted on every build. If a change to the engine ever moved one of these answers, the build would fail before the page could print it.

500 W for four hours

Load to run: 500 W
Backup needed: 4 hours
Battery voltage: 12 V
Usable depth of discharge: 50%
Inverter efficiency: 85%
Battery type: Tubular

Battery capacity needed392.2

2000 / (12 x 0.85 x 0.5)

Open this example

lithium at 90 percent needs far less nameplate

Load to run: 500 W
Backup needed: 4 hours
Battery voltage: 12 V
Usable depth of discharge: 90%
Inverter efficiency: 85%
Battery type: Lithium

Battery capacity needed217.9

boundary: the depth of discharge difference

Open this example

Method and limits

What it assumes

A resistive load. Motors draw a large starting surge not covered here.

What it deliberately does not model

Battery capacity falls with age, temperature and discharge rate.
Peukert effect means a high discharge rate delivers less than the rated capacity.
Motor starting surges can be several times running current.

Formula version 1.0.0 · definition 1.0.0 · India · Report a problem with this calculator

Frequently asked questions

Why do I need twice the battery capacity?: Because lead acid should not go below fifty percent without shortening its life sharply, and the inverter loses about fifteen percent converting DC to AC. Both derations multiply.
Is lithium worth the cost?: Per usable amp-hour it is much closer than the nameplate price suggests, because it tolerates deep discharge and lasts more cycles.