What watts and kilowatt-hours really measure, how to work out an appliance's current and running cost, and how long a battery will last.
These two get mixed up constantly, and the difference explains your electricity bill.
One kilowatt-hour is a 1,000 W load running for one hour. That is the "unit" your meter counts and your bill charges for.
Rearrange P = V × I to get I = P / V. This tells you whether a socket, wire or fuse can cope.
A 1,500 W heater on a 230 V supply:
I = 1500 / 230 ≈ 6.5 A
A 10 W LED bulb on the same supply draws only about 0.043 A.
Add up the currents of everything on one circuit. If the total goes above the wire or breaker rating, the circuit is overloaded. Our Wire Gauge calculator helps check cable size.
Use three steps: convert to kilowatts, multiply by hours, multiply by your tariff. The tariff below is only an example, so use the rate printed on your own bill.
A 1,500 W heater used 3 hours a day for 30 days, at an example rate of ₹8 per unit:
Daily: 1.5 kW × 3 h = 4.5 kWh
Monthly: 4.5 × 30 = 135 kWh
Cost: 135 × 8 = ₹1,080
A 10 W LED bulb used 5 hours a day for 30 days, same rate:
0.01 kW × 5 h × 30 = 1.5 kWh, so about ₹12
| Appliance | Typical power | Daily use | Energy per day |
|---|---|---|---|
| LED bulb | 10 W | 5 h | 0.05 kWh |
| Ceiling fan | 75 W | 10 h | 0.75 kWh |
| Air conditioner (1.5 ton) | 1,500 W | 8 h | 12 kWh |
| Water heater | 2,000 W | 1 h | 2 kWh |
These are typical figures. Check the label on your own appliance, and remember that air conditioners and refrigerators cycle on and off, so real usage is usually lower than the rating times the hours.
Battery energy in watt-hours is voltage times amp-hours. Divide by the load to estimate runtime, then allow for losses.
A 12 V, 100 Ah battery running a 120 W load:
Energy = 12 × 100 = 1,200 Wh
Ideal runtime = 1200 / 120 = 10 h
With roughly 80% usable capacity after losses, plan for about 8 hours. Our Battery Life calculator does this for you.
Combine P = V × I with Ohm's Law and you get P = I² × R and P = V² / R. These tell you how much heat a resistor or wire has to shed, which is why a part with the right resistance can still burn out if its watt rating is too low.
Try each one before you open the answer.
0.06 kW × 4 × 30 = 7.2 kWh, or 7.2 units.
I = 2000 / 230 ≈ 8.7 A.
Energy = 12 × 50 = 600 Wh, so 600 / 60 = 10 h before losses.
A watt measures power, the rate at which a device uses energy. A kilowatt-hour (kWh) measures energy, which is power multiplied by time. A 1,000 W heater running for one hour uses 1 kWh.
On an electricity bill, one unit is one kilowatt-hour.
Divide power by voltage: I = P / V. A 1,500 W heater on 230 V draws about 6.5 A.
The rating is usually the maximum. Many devices only draw full power at peak load, and motors and compressors cycle on and off.
Need quick conversions between kW, kVA and horsepower? Use our power converter.
Open the Power Converter