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Electricity is billed by the kilowatt-hour (kWh) — one "unit" is 1000 watts running for one hour. Multiply an appliance's power by how long it runs to get the energy, then multiply by your tariff to get the cost. The tariff symbol here is ₹, but the maths works for any currency.
| Quantity | Formula |
|---|---|
| Energy per day | kWh = P(W) × hours × quantity / 1000 |
| Daily cost | kWh/day × tariff |
| Monthly cost | daily cost × 30 |
| Yearly cost | daily cost × 365 |
A handy rule: a 1000 W (1 kW) device running for 1 hour uses exactly 1 kWh — one unit of electricity.
Running cost is energy multiplied by tariff, and energy is power multiplied by time. The part people underestimate is the time: a small load left on permanently accumulates more than a large load used briefly.
A device drawing a steady 5 W consumes 5 × 8,760 = 43,800 Wh, or 43.8 kWh, over a year. A 2 kW heater used for half an hour a day consumes 2 × 0.5 × 365 = 365 kWh. The heater is 400 times the power but only about eight times the annual energy — and a household with a dozen 5 W standby loads is burning over 500 kWh a year on equipment nobody is using.
A watt is a rate; a watt-hour is a quantity. A 100 W lamp does not consume "100 W per hour" — it consumes 100 watt-hours in an hour. The distinction matters whenever a bill or a specification has to be interpreted, because tariffs are charged per kilowatt-hour of energy, not per kilowatt of power.
Some commercial tariffs do also charge for peak demand in kilowatts, billed on the highest power drawn in any interval during the period. For those customers, reducing simultaneous load can cut the bill even when total energy is unchanged — which is the entire basis of load-shifting and peak-shaving schemes.
The savings from replacing an inefficient device scale with how long it runs, so the economics follow usage rather than rating. Replacing a continuously running pump or fan motor with a high-efficiency unit repays the difference quickly; replacing something used a few minutes a week rarely does, whatever its nameplate efficiency.
The same logic applies to power supplies. A 90%-efficient adapter wastes 10% of whatever it draws, but if it is left plugged in with no load, what matters is its no-load consumption — which is why modern standards regulate standby draw separately from loaded efficiency.
A kilowatt-hour is the energy of 1000 watts running for one hour. It is the "unit" your electricity meter counts and your bill charges for.
Multiply its power in kilowatts by the hours used to get kWh, then multiply by your tariff. For example 1.5 kW × 2 h × ₹8 = ₹24.
Divide watts by 1000 to get kilowatts, then multiply by hours of use. So 100 W for 8 h is 0.1 kW × 8 = 0.8 kWh.
Many do, but motors, fridges, and air-conditioners cycle on and off, so their average power is lower than the nameplate. Use the average running power for a realistic estimate.
Use the per-unit (per-kWh) rate from your electricity bill. Some tariffs are tiered or time-of-use, in which case use the rate for the relevant slab or time.
Bills often add fixed charges, taxes, and tiered rates on top of the energy cost. This calculator gives the pure energy cost of the appliances you enter.
Small but constant — a device drawing 5 W on standby all year uses about 44 kWh. Multiply by your tariff to see it adds up across many gadgets.
Use efficient appliances (LED lighting, inverter ACs), run high-power devices less, switch off standby loads, and shift usage to cheaper time-of-use periods where available.
Check the label or nameplate (often in watts, or volts × amps). A plug-in energy meter gives the most accurate real-world figure including cycling.
Yes — on an electricity bill "1 unit" means 1 kWh.
Multiply the kWh added to the battery by your tariff. Charging losses add roughly 10–15%, so add a little for a real-world figure.
Only if it runs as long. Cost depends on power × time, so a high-wattage device used briefly can cost less than a low-wattage one left on all day.
Multiply power in kilowatts by hours of use to get kilowatt-hours, then multiply by the tariff per kWh. A 2 kW heater for half an hour uses 1 kWh.
A steady 5 W draw uses about 43.8 kWh a year. Several such devices in one home easily exceed 500 kWh annually, spent on equipment that is never actively used.
A watt is a rate of energy use; a watt-hour is an amount of energy. A 100 W lamp uses 100 watt-hours per hour. Electricity is billed by energy, so the kilowatt-hour is the unit on the meter.
A component of some commercial tariffs billed on the highest power drawn during any interval in the period, rather than on total energy. Reducing simultaneous load lowers it even if total consumption is unchanged.
When it runs for long enough for the savings to accumulate. Continuously running pumps, fans and lighting repay the investment quickly; rarely used appliances seldom do regardless of their efficiency rating.
Yes, though modern designs reduce it considerably. No-load consumption is regulated separately from loaded efficiency precisely because adapters spend most of their lives idle but connected.
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