Calculator
Inverter and battery size calculator
Tick what you want running during a cut and how long it has to last. You get the inverter VA, the battery Ah and the honest backup time — with the arithmetic shown, so you can check it in the shop.
Power Watch · Updated 20 September 2026
What do you need to run?
Counts start at an ordinary two-bedroom flat. Change them and every number below moves.
Your load
213 W
Inverter
600 VA
Battery
100 Ah
Real backup
3.4 hr
Working: 213 W ÷ 0.8 × 1.2 = 320 VA · 213 W × 3 hr ÷ (12 × 0.6) = 89 Ah
What the two numbers mean
VA is how much you can switch on at the same moment; cross it and the inverter beeps, then trips. Ah is the tank — it decides how long, and nothing else. The market sells a generous inverter with the cheapest battery that fits the budget, because the brand name is on the inverter. Where cuts are long, that is backwards: the battery is the purchase.
The backup time above is deliberately lower than the one on the box. A 150 Ah battery holds about 1,800 watt-hours on paper, but the inverter loses some in conversion and draining a lead-acid battery flat shortens its life sharply, so roughly 60% of the nominal figure is what you can actually plan around. Treat the result as year one, in a ventilated room — a three-year-old battery in a hot cupboard gives less.
How many hours should you pick?
Not the worst evening you remember, and not what your neighbour bought. In your area, how long is the power actually out and at what time of day? Live reports for more than a hundred and thirty cities carry the duration people recorded and the hour they filed it, which is the number to put into the box above.
Three things to get right in the shop
- Pure sine wave, not square. Fans hum on a square wave, motors run hotter, and some televisions and chargers refuse it outright. The price gap used to justify the compromise and mostly no longer does.
- Tubular for long cuts. Flat-plate is cheapest and dies fastest under deep discharge. Tubular is built to be drained deeply and often, and lasts several years if you top up the water every couple of months. Lithium (LiFePO4) costs two to three times more, takes charge much faster and needs a matching charging profile — worth it mainly where cuts are frequent and the recharge window is short.
- Read the watts, not the model name. VA figures on the carton are optimistic. The number that matters is the continuous output in watts on the specification sheet, which is usually well below the VA the model is named after.
One thing that breaks the whole calculation
An inverter is also a charger, and it only charges from mains inside its input voltage window. On a line that sags to 170 or 180 volts through the evening, it keeps rejecting the supply and running off the battery while the household believes it is charging. The battery is emptying, and the blackout arrives anyway. If your lane dims rather than dies, that is a supply fault with a compensation figure attached to it in your state's rules: low voltage is a fault, not weak power.
Where to look
Category listings, not named models — prices move and manufacturers revise a model without changing its name, so a guide that names one product ages into being wrong. Match what you find against the numbers above.
- Pure sine wave home inverters
Check the continuous watt rating, not the VA in the name
- Tubular inverter batteries
Match the Ah the calculator gave you, or go one size up
- Mini-UPS for a Wi-Fi router
If your cuts are short, this is the cheapest useful thing here
- Voltage stabilisers
Protects a fridge on a sagging line — treats the symptom, not the fault
Questions people ask
Can I run an air conditioner on an inverter?
Not on a normal home system, which is why it is not in the calculator. A 1.5-ton unit draws around 1,200 to 1,500 watts running and much more at startup — three to four times what a typical home inverter is sized for, and it would flatten a 150 Ah battery in under an hour. The same applies to geysers, water pumps, induction hobs and microwaves. Running those through a cut means a high-capacity hybrid inverter and a battery bank, usually with solar behind it.
Is a bigger battery always better?
No. Capacity you never discharge is money spent and unused, and a large battery habitually left half-charged — because the supply is never on long enough to fill it — degrades faster than a smaller one completing its cycles. Size it to the cuts you actually get, then check your daily hours of supply can refill it.
How long should an inverter battery last?
A tubular battery, watered every couple of months and kept somewhere ventilated, usually gives three to five years; flat-plate gives less. Heat, repeated deep discharges and low electrolyte are what shorten it.
Why is the backup time lower than the shop's figure?
Because shop figures usually multiply amp-hours by 12 volts and stop there. That ignores conversion losses and assumes you will drain the battery flat, which is exactly what kills it. The 0.6 factor used above is what is left after both, and it is roughly what an owner sees in year one.
Check your own area before you buy
The hours you picked above decide most of this purchase. If your area looks quiet on the map, reporting the next cut yourself is the fastest way to start the record.
If what you actually want to know is why the cuts happen at all — why the next street keeps its lights while yours goes dark at the same hour — the answer is in how feeders are switched: why your neighbour has power and you don't.
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How this is calculated
Appliance watts are typical nameplate ratings for equipment sold in India; your own rating plate is the authority for your own house. Inverter VA is your total watts divided by a 0.8 power factor with 20% headroom for motor startup, rounded up to a size actually sold. Battery Ah is watts × hours ÷ (12 × 0.6), where 0.6 covers inverter efficiency and the half of a lead-acid battery it is safe to use.
Prices and models change; the arithmetic does not. If something here no longer matches what is on the shelf, tell us.