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A Exhaust Fan draws about 40W, so a 150Ah 12V tubular battery runs one of them for roughly 15 hours 45 minutes on its own. That comes from 630Wh usable energy divided by 40W. Kitchen and bathroom exhaust fans, usually run in short bursts.
Kitchen and bathroom exhaust fans, usually run in short bursts. The 40W figure used across this page is a typical mid-range Indian retail unit — check the nameplate on yours and put the real number into the calculator, because a 10W difference on an appliance you run for six hours is 60Wh out of a budget of only a few hundred.
Category: Cooling. People search for this as kitchen exhaust fan, ventilation fan — all the same question, and all answered by the same arithmetic.
Usable energy is Ah x 12V x 0.7 efficiency x 0.5 depth of discharge. Divide that by the appliance draw and you have the runtime with nothing else connected.
| Battery | Usable energy | Runtime with one unit | Runtime with two |
|---|---|---|---|
| 100Ah | 420Wh | 10 hours 30 minutes | 5 hours 15 minutes |
| 120Ah | 504Wh | 12 hours 36 minutes | 6 hours 18 minutes |
| 135Ah | 567Wh | 14 hours 11 minutes | 7 hours 5 minutes |
| 150Ah | 630Wh | 15 hours 45 minutes | 7 hours 53 minutes |
| 165Ah | 693Wh | 17 hours 19 minutes | 8 hours 40 minutes |
| 180Ah | 756Wh | 18 hours 54 minutes | 9 hours 27 minutes |
| 200Ah | 840Wh | 21 hours | 10 hours 30 minutes |
| 220Ah | 924Wh | 23 hours 6 minutes | 11 hours 33 minutes |
| 250Ah | 1050Wh | 26 hours 15 minutes | 13 hours 8 minutes |
Nothing stops you connecting more units — the limit is runtime, not the inverter, until you hit the surge ceiling below.
| Quantity | Continuous draw | Runtime | Band |
|---|---|---|---|
| 1 | 40W | 15 hours 45 minutes | Low load |
| 2 | 80W | 7 hours 53 minutes | Low load |
| 3 | 120W | 5 hours 15 minutes | Low load |
| 4 | 160W | 3 hours 56 minutes | Moderate |
| 5 | 200W | 3 hours 9 minutes | Moderate |
| 6 | 240W | 2 hours 38 minutes | Moderate |
Very few appliances run flat out for a full hour. A fridge compressor cycles, a mixer runs in bursts, a pump fills a tank and stops. Battery Buddy asks for minutes run per hour and multiplies the wattage by that fraction — which is why its answers are usually longer, and more honest, than a calculator that assumes continuous draw.
| Minutes run per hour | Average hourly draw | Runtime on a 150Ah battery |
|---|---|---|
| 15 min | 10W | 63 hours |
| 30 min | 20W | 31 hours 30 minutes |
| 40 min | 26.7W | 23 hours 38 minutes |
| 45 min | 30W | 21 hours |
| 60 min | 40W | 15 hours 45 minutes |
This is a surge-sensitive appliance. Battery Buddy applies a 1.5x start-up multiplier to a Exhaust Fan, so one unit spikes to about 60W for a second or two at switch-on. Your inverter has to survive everything else already running plus that spike — not just the average load.
Peak demand = total continuous load + the single largest start-up surge. The sizing advisor runs that check automatically for whatever mix you enter.
About 40W while running. Kitchen and bathroom exhaust fans, usually run in short bursts. Always confirm against the nameplate rating on your own unit before sizing a battery around it.
Roughly 15 hours 45 minutes with nothing else connected: 630Wh usable divided by 40W. Add anything else to the inverter and that time drops proportionally.
Battery Buddy applies a 1.5x multiplier, so a single unit peaks near 60W at switch-on before settling to 40W. That spike, not the running load, is what trips undersized inverters.
Yes — one unit runs about 10 hours 30 minutes on a 100Ah 12V battery at 70% efficiency. Whether that is enough depends on how long your outages actually last and what else shares the inverter.
The figures assume 70% system efficiency. An older battery, low electrolyte, corroded terminals or thin cabling all reduce the effective factor. Measure yours with the efficiency estimator and re-run the numbers — it takes one discharge test.
Battery Buddy is free, needs no sign-up, sells no batteries and shows every formula it uses. It is built by Flocci Technologies, an India-first AI-native software studio behind 30+ apps that share one Flocci identity.