Watts are a rate, kilowatt-hours are an amount
This is the single most important idea in the whole subject, and the source of most confusion. A watt measures how fast energy is used; a kilowatt-hour measures how much was used.
Energy (kWh) = Power (kW) × Time (h)
A 1,500 W kettle used for 2 minutes: 1.5 kW × (2/60) h = 0.05 kWh. Run continuously it would use 36 kWh a day, but nobody does that. The kettle is high-power and short-duration; an LED lamp is low-power and long-duration. What matters is the product.
Converting between power units
- 1 kW = 1,000 W. The prefix is decimal and unambiguous.
- 1 hp (mechanical) = 745.7 W. Defined as 550 ft·lbf/s, so it is a real constant, not an estimate. An engine rated at 100 hp is 74.57 kW.
- 1 hp (metric, PS) = 735.5 W. The difference is 1.4%, which is enough to matter in specifications but not in an estimate.
- 1 hp (boiler) = 9,000 W thermal. A third definition again, used in heating, and about 12x a mechanical horsepower. Comparing it to a motor rating is a common and expensive mistake.
Note also that the three horsepower figures are all output ratings. A 100 hp motor typically draws 75-85 kW of electrical input, because motors are 85-95% efficient. The nameplate is output, the bill is input.
A worked running-cost example
A 1,500 W desktop computer runs 8 hours a day at $0.28 per kWh.
- Daily energy = 1.5 kW × 8 h = 12 kWh
- Daily cost = 12 × 0.28 = $3.36
- Monthly = 12 × 30.4 = 365 kWh → $102.19
- Yearly = 12 × 365 = 4,380 kWh → $1,226.40
Two things are worth noticing. A machine that draws 1,500 W is not unusual for a desktop, and it is always on: the same computer left on 24 hours a day costs $102.19 × 3 = about $307 a month, and standby power is the part that quietly dominates. Second, the 125% figure in the circuit note is about power draw, not energy — but the two are linked through time.
Tariffs, and why the daily number is not the whole story
Residential tariffs are often time-of-use: a cheap overnight or off-peak rate, and an expensive peak rate. Under such a tariff the same 12 kWh can cost three or four times as much depending on when it is used. Three practical consequences:
- Running appliances at night is often near-free. Charging a device overnight on an off-peak rate at 8p versus a peak rate of 28p is a fourfold saving on the same energy.
- A home battery or a controllable EV charger pays for itself exactly by shifting load into cheap hours, and this is the single largest available saving on most UK and European tariffs.
- Your bill is per kWh, so removing load is worth the full rate. Replacing a 60 W incandescent with a 6 W LED saves 0.054 kW × hours, and at 8 hours a day for 300 days that is 130 kWh a year — about 4.3% of the 3,000 kWh a typical home uses.
And the standing charge matters. A typical residential bill includes a fixed daily charge regardless of consumption, so the marginal cost of the next kWh is the tariff rate but the average cost is higher. Cutting consumption from 3,000 to 2,000 kWh a year does not cut the bill by a third.
Frequently asked questions
How do I convert watts to kilowatts?
Divide by 1,000. 1,500 W is 1.5 kW, 90 W is 0.09 kW. The prefix is decimal, so there is no ambiguity — unlike the hour, where metric and international horsepower differ by 1.4%.
How much does 1000 W cost to run?
1 kW running for 1 hour uses 1 kWh. Multiply by your tariff: at $0.28 per kWh that is $0.28. Running for 8 hours a day costs about $68 a month.
Is a 100 hp motor the same as 75 kW?
Not quite. 100 mechanical hp is 74.57 kW of mechanical output, but a motor is 85-95% efficient, so it draws 78-88 kW of electricity. The nameplate is output; the meter measures input.
How do I calculate the running cost of an appliance?
Take its power in watts, divide by 1000 for kW, multiply by hours of use to get kWh, then multiply by the price per kWh. For an accurate annual figure use 365 days, and remember that time-of-use tariffs make the price depend on when you run it.