The four units
| Unit | Measures | The everyday analogy | On your bill? |
|---|---|---|---|
| Volt (V) | Electrical pressure | Water pressure in the pipe | No |
| Amp (A) | Rate of flow | Litres per second through the pipe | No |
| Watt (W) | Rate of energy use — volts × amps | How fast the tap is running | No |
| Kilowatt-hour (kWh) | A quantity of energy — 1,000 watts for one hour | How much water ended up in the bucket | Yes. This is the only one you pay for. |
The whole confusion lives in that last column. A label tells you watts, because that is a property of the appliance. A bill charges kilowatt-hours, because that is what you consumed. Getting from one to the other requires knowing how long it ran — and for anything with a thermostat, how much of that time it was actually drawing.
The conversions worth memorising
The last row is the most useful line on this site. There are 8,760 hours in a year, so one watt drawn continuously is 8.76 kWh — $1.61 at the US average rate.
| To get | Do this | Example |
|---|---|---|
| Watts, from a label with amps | amps × volts | 12 A on a 120 V circuit = 1,440 W |
| Kilowatt-hours | watts × hours ÷ 1,000 | 1,440 W for 2 hours = 2.88 kWh |
| Cost | kWh × your rate | 2.88 kWh × 18.34¢ = $0.53 |
| Annual cost of anything always on | watts × $1.61 | A 45 W pump = $72 a year |
Where labels mislead
- Microwave “wattage” is output, not input. A 1,000 W microwave delivers 1,000 W of cooking power and draws around 1,500 W from the wall. Sizing from the advertised number understates it by half.
- A power supply’s rating is its maximum, not its draw. An 850 W PSU in a computer using 400 W draws 400 W plus conversion losses. Estimating from the PSU label roughly doubles the answer.
- Air conditioner BTU is cooling output. Divide BTU by the CEER rating to get watts: 10,000 BTU at CEER 12 is about 833 W.
- “Equivalent” wattage on lighting and grow lights is marketing. Buy on lumens for light and on actual rated power for cost — a “1,000 W equivalent” LED fixture may draw 200 W.
- Nameplate watts is what it draws while drawing. For anything thermostatic, that is a small fraction of the time. This is the big one.
Worked examples
Four calculations that come up constantly, done end to end.
At the US average of 18.34¢/kWh. Substitute your own rate — it varies by more than four times across the country.
| Question | Working | Answer |
|---|---|---|
| A 1,500 W heater for 6 hours | 1,500 × 6 ÷ 1,000 = 9 kWh; 9 × 18.34¢ | $1.65 |
| Boiling 1 litre of water | Raising 1 kg by 80°C needs 0.093 kWh; 0.093 × 18.34¢ | $0.02 |
| A 12 W router, always on | 12 × 8,760 ÷ 1,000 = 105 kWh a year | $19 |
| Driving 100 miles at 3.5 mi/kWh | 100 ÷ 3.5 = 28.6 kWh, plus ~10% charging loss | $5.76 |
What amps tell you that watts do not
Amps matter for safety rather than cost. A circuit breaker trips on current, not on energy, which is why the two units are used for different questions. A 15 A circuit at 120 V can carry 1,800 W — and by convention a continuous load should not exceed 80% of that, or 1,440 W. This is precisely why two space heaters on one circuit trip the breaker: 3,000 W is 25 A on a 15 A circuit.
Nothing here is a substitute for an electrician, and none of it appears on your bill. It is the arithmetic that explains why some appliances need their own circuit.
| Circuit | Maximum | Continuous limit (80%) | What that is |
|---|---|---|---|
| 15 A at 120 V | 1,800 W | 1,440 W | One 1,500 W heater is already over the continuous limit |
| 20 A at 120 V | 2,400 W | 1,920 W | A kitchen small-appliance circuit |
| 30 A at 240 V | 7,200 W | 5,760 W | A clothes dryer |
| 50 A at 240 V | 12,000 W | 9,600 W | An electric range, or a Level 2 EV charger |
Reading your own bill
- Find the kilowatt-hours. They are usually near the meter reading, and they are the actual quantity you consumed.
- Divide the total amount due by those kilowatt-hours. That is your true effective rate, including the fixed monthly charge and every rider.
- Use that figure, not a state average, in any calculation you care about. It is usually a little higher than the published average — and the less electricity you use, the more the fixed charge weighs on it.
The published state averages on this site’s rate pages are average revenue per kilowatt-hour across every residential customer in the state. They are the right number for comparing states and the wrong number for your own arithmetic, if you have a bill to hand.
Reading the meter itself
A meter counts kilowatt-hours and nothing else. Whether it is a spinning disc or a digital display, the number it shows is cumulative — it never resets — so any measurement you take is a difference between two readings.
That makes one experiment available to anyone, with no equipment at all. Read the meter last thing at night, read it again first thing in the morning, and divide the difference by the hours between. The result is your average draw in kilowatts while the house was asleep, and multiplying it by 8,760 gives the annual consumption of everything that never switches off. It is the single most informative measurement in domestic energy and it costs nothing.
Why none of this is on the appliance label
A manufacturer knows the rating and cannot know the hours, so the label states the only figure it can. That is a reasonable division of labour, and it becomes a problem only when someone treats the half they were given as the whole answer — which is exactly what nameplate-times-hours arithmetic does, and exactly what this site exists to correct.
Questions
How do I convert watts to kilowatt-hours?
How much does 1 kWh cost?
How many watts is an amp?
What is the difference between kW and kWh?
All guides · How the figures are computed · Break down your own bill