Watts vs. kWh
Last updated 2026-10-21
Watts measure how fast an appliance uses power right now. Kilowatt-hours measure how much energy it used over time, and kWh is what you pay for.
kWh = watts × hours ÷ 1,000
Cost = kWh × price per kWh (U.S. average now 17.99¢)
1. The units
| Unit | Measures | Analogy | Example |
|---|---|---|---|
| Watt (W) | Power: rate of energy use | Speed (mph) | LED bulb 9 W; space heater 1,500 W |
| Kilowatt (kW) | 1,000 watts | Central AC 3 to 4 kW; EV charger 7 to 11 kW | |
| Kilowatt-hour (kWh) | Energy: 1 kW for 1 hour | Distance (miles) | Average home 874 kWh/month |
| Volt (V) | Electrical pressure | Water pressure | Outlets 120 V; dryers and ranges 240 V |
| Amp (A) | Current: flow of electricity | Water flow | Standard circuit 15 or 20 A |
| BTU | Heat energy | 1 kWh = 3,412 BTU | |
| BTU/hr | Heating or cooling rate | 1,500 W heater = 5,118 BTU/hr; 12,000 BTU/hr = 1 "ton" of cooling | |
| VA | Apparent power (volts × amps) | On UPS units and some motors; watts are usually lower |
2. Formulas
| To find | Formula | Example |
|---|---|---|
| Watts | volts × amps | 120 V × 10 A = 1,200 W |
| Amps | watts ÷ volts | 1,500 W ÷ 120 V = 12.5 A |
| kWh | watts × hours ÷ 1,000 | 1,500 W × 4 h ÷ 1,000 = 6 kWh |
| Cost | kWh × ¢/kWh ÷ 100 | 6 kWh × 17.99¢ = $1.08 |
| Monthly cost | watts × hours/day × 30 ÷ 1,000 × price | 100 W × 24 h × 30 = 72 kWh = $12.95 |
| BTU/hr from watts | watts × 3.412 | 1,000 W = 3,412 BTU/hr |
| Watts from cooling BTU (AC) | BTU/hr ÷ EER (or CEER) | 10,000 BTU ÷ 11 EER ≈ 909 W |
| Circuit limit | volts × amps × 0.8 (continuous load) | 120 V × 15 A × 0.8 = 1,440 W |
A 1,500 W space heater on a 15 A circuit is already above the 80% continuous-load guideline, which is why heaters should not share a circuit with other big loads.
3. Reading a nameplate
The label is usually on the back, bottom, inside the door, or on the power adapter.
- Watts listed: use it directly. It is the maximum draw.
- Only volts and amps (for example "120V 60Hz 8A"): multiply, 120 × 8 = 960 W maximum. Real average is usually lower.
- Power adapters: "Input 100-240V 1.5A" is the worst case. "Output 20V 3.25A" = 65 W is closer to actual use.
- AC units: look for BTU/hr and EER/CEER, or "rated input" watts.
- EnergyGuide label (fridges, freezers, water heaters, washers, TVs): annual kWh under test conditions. Divide by 12 for monthly use.
- Heat pumps and central AC: SEER2 and HSPF2 ratings. Higher is more efficient.
Best accuracy: a plug-in power meter (about $15 to $30) shows actual watts and cumulative kWh. For hardwired loads, many smart meters give hourly use on your utility's website.
4. Worked examples
At the U.S. average price of 17.99¢/kWh:
| Appliance | Watts | Hours/day | kWh/day | Per day | Per month |
|---|---|---|---|---|---|
| Space heater | 1,500 | 8.0 | 12.00 | $2.16 | $64.76 |
| Window AC | 900 | 8.0 | 7.20 | $1.30 | $38.86 |
| Refrigerator (runs ~35% of the time) | 150 | 8.4 | 1.26 | $0.23 | $6.80 |
| LED bulb | 9 | 5.0 | 0.04 | $0.01 | $0.24 |
| 65-inch TV | 110 | 5.0 | 0.55 | $0.10 | $2.97 |
| Clothes dryer | 5,000 | 1.0 | 5.00 | $0.90 | $26.98 |
Calculate any appliance with the kWh calculator or the electricity cost calculator, look up hundreds of devices on cost to run, or put two side by side on compare appliances.
5. Common mistakes
- Confusing kW and kWh. A 7 kW EV charger running 3 hours uses 21 kWh.
- Using peak wattage for cycling appliances. A 150 W fridge does not use 150 W × 24 h; it runs a third to half the time.
- Ignoring standby. 10 W of always-on load is 88 kWh a year, about $16. Check yours with the standby power calculator.
- Using the energy price only. Your real price includes delivery and taxes; divide your total bill by kWh. See how to read your bill.
- Thinking a "1500 W" heater on low uses 1500 W. Low is often 750 to 900 W, and the thermostat cycles it.
Frequently asked questions
How many kWh does a 1,000-watt appliance use?
1 kWh per hour of running. At 17.99¢/kWh, that costs about 18¢ an hour.
How do I convert amps to watts?
Watts = amps × volts. A 12-amp vacuum on a 120-volt outlet draws about 1,440 W. For 240-volt appliances (dryers, ranges, water heaters), multiply by 240.
Is the wattage on the label what the appliance actually uses?
The label shows maximum or rated draw. Appliances with thermostats or compressors (fridges, AC, heaters, ovens) cycle on and off, so average use is often 30% to 70% of the rating. For the real number, use a plug-in power meter for a day or check the yellow EnergyGuide label for annual kWh.