Energy
Battery & Inverter Size Calculator
How many batteries a given backup actually needs, and how big an inverter to buy. Depth of discharge, inverter losses and the gap between VA and watts are all in the sum — leave any of them out and the bank runs flat early.
What you are running
The running watts of everything on at once. Fans and lights are easy; a fridge or a pump draws several times its running figure for a moment at start-up, which the inverter has to survive even though it barely touches the battery.
The battery bank
Twelve volts is one battery, twenty-four is two in series, forty-eight is four. The energy stored is the same; a higher voltage carries it at less current, which means thinner cable and smaller losses.
- How much of the battery you are willing to use. Lead-acid taken below half regularly does not last, so half of the rating is what the trade sizes on; lithium is happy at eighty per cent. Sizing on the full rating is the commonest mistake here and it roughly halves the backup you get.
- The rating printed on the battery. Note what it is rated at: inverter batteries in India are usually quoted at C10 — ten hours to empty — while solar and automotive ones are quoted at C20, and the same cell shows a bigger number at C20. Two batteries with the same printed Ah are not necessarily the same battery.
The inverter
- Turning DC into AC costs ten to fifteen per cent on an ordinary unit, and it comes out of the battery. A good sine-wave inverter manages ninety or a little more.
- What turns the VA rating into watts. Indian inverters are almost all sold at 0.8, so a 1500 VA unit delivers 1200 W.
- Spare capacity so the inverter is not run at its limit hour after hour. Twenty per cent is a reasonable habit; more if the load has motors in it.
What to buy
3 × 150 Ah
1 in series × 3 in parallel
- Capacity needed
- 392 Ah
- Capacity installed
- 450 Ah
- Energy the bank holds
- 4706 Wh
- Backup this actually gives
- 4.6 h
- Inverter rating
- 750 VA
- which delivers
- 600 W
VA is not watts
The question most people arrive with. An inverter's rating is apparent power; the real power it delivers is that times its power factor. Buying on the label alone means buying a unit a fifth smaller than the number suggests.
A 1500 VA inverter delivers about 1200 W of real load.
How to use it
- Add up the running watts of everything that must stay on, and decide how long for.
- Choose the bank voltage and the battery type; the depth of discharge moves to that type's usual figure and you can adjust it.
- Enter the amp-hour rating of the battery you are pricing.
- Set the inverter efficiency, power factor and headroom, or leave the defaults.
- Read the number of batteries, the inverter rating in VA, and — importantly — the watts that rating actually delivers.
How it works
The load in watts times the hours gives the energy needed at the socket.
That is divided by the inverter efficiency and by the depth of discharge to get the energy the bank has to hold, which at the bank voltage becomes amp-hours.
Batteries in series reach the bank voltage; strings in parallel reach the capacity, rounded up to whole batteries — so the backup delivered is always at least what was asked for.
The inverter rating is the load divided by the power factor, plus the headroom, and the watts it delivers is that rating times the power factor again.
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Common questions
Why does a 1500 VA inverter only run 1200 W?
Because VA is apparent power and watts are real power, and the ratio between them is the power factor. Indian inverters are almost all rated at 0.8, so multiply the VA figure by 0.8 to get the watts it will actually carry. It is not a trick — the rating is honest — but it is the number people misread most often, and it means an inverter bought to the watt figure is a fifth too small.
Why can I not use the whole battery?
You can, once or twice. A lead-acid battery regularly taken below half its capacity loses life quickly, so the trade sizes on fifty per cent usable and treats the rest as reserve. Lithium iron phosphate is different and will give eighty per cent without complaint, which is most of why a lithium bank of the same usable size is physically smaller.
Does the tool account for start-up surge?
No, and that is deliberate. A fridge or a pump draws several times its running current for a fraction of a second, which the inverter has to survive but which barely registers on the battery. Sizing for it belongs on the inverter's surge rating — a figure the maker publishes separately — not in an energy calculation. The headroom setting is the blunt version of the same idea.
Appliance running cost →What does C10 or C20 mean on the battery?
The discharge rate the capacity was measured at: C10 means it will give its rated amp-hours over ten hours, C20 over twenty. The same cell shows a larger number at C20, so a 150 Ah C20 battery and a 150 Ah C10 battery are not the same battery — the C10 one is better. Inverter batteries in India are usually rated at C10, solar and automotive at C20.
Twelve volts or twenty-four?
The same energy either way, but at half the current on twenty-four volts. Less current means thinner cable, smaller losses and less heat, so higher-voltage banks are usual once the load gets past a kilowatt or so. Below that, twelve volts and one battery keeps it simple.