Watts and kWh: power is not energy
The watt (W) measures power: how much energy an appliance draws at any given moment. The kilowatt-hour (kWh) measures energy: what was used over time, and it is what your utility bills you for. One kWh is what a 1,000 W (1 kW) appliance uses when it runs for one hour.
A car is a good comparison: power is like speed, energy is like distance traveled. A high-power appliance you use for a few minutes can use less than a small one left on all day.
The math: kWh = kW × hours
Convert watts to kilowatts by dividing by 1,000, then multiply by the hours of use. A 100 W (0.1 kW) appliance left on for 10 hours uses 0.1 × 10 = 1 kWh. A 500 W appliance for 4 hours uses 0.5 × 4 = 2 kWh. If you only use it some days, multiply by the number of days as well.
For the cost, multiply the kWh by the price per kWh on your bill. The numbers in this article use an assumed price of $0.20 per kWh, only so there is something to calculate with: yours may be much higher or lower, so always replace it with the rate on your own bill.
Where to find an appliance's wattage
Most appliances have a label or nameplate (on the back, the bottom or near the cord), and the manual repeats it. Look for a number followed by «W». If it only lists voltage and current, for example 120 V and 8 A, multiply them: 120 × 8 = 960 W. This is a fair approximation for resistive appliances (heaters, kettles, irons); for motors and electronics the real draw can be somewhat lower.
Two cautions. The wattage on the label is usually the maximum, not what the appliance draws all the time. And the typical values you see in lists, including the one in the calculator, are rough: the model, its age and how you use it change the result.
Duty cycle and standby power
Refrigerators and air conditioners do not run at full power 24 hours a day: the compressor turns on, cools and turns off. The share of time they actually draw power is the duty cycle. The calculator calls it «Percentage of time it draws power» and suggests about 40% for a refrigerator. That is a reference value: in summer, with the door opened often or with a heavily used unit, it goes up.
Standby power is what devices use when they look off but are still plugged in: TVs, game consoles, cable boxes, plugged-in chargers. Each one draws little, but it runs 24 hours a day. A 5 W device on standby all day adds up to 0.12 kWh per day, about 44 kWh per year.
Worked examples with an assumed price
Every case uses $0.20 per kWh (an example price). The month is calculated as 52 weeks divided by 12, and the year as 52 weeks, just like the calculator does.
| Appliance and use | kWh per month | Cost per month | kWh per year | Cost per year |
|---|---|---|---|---|
| 9 W LED bulb, 5 h per day | 1.365 | $0.27 | 16.38 | $3.28 |
| 60 W incandescent bulb, 5 h per day | 9.1 | $1.82 | 109.2 | $21.84 |
| 1,200 W microwave, 20 min per day | 12.13 | $2.43 | 145.6 | $29.12 |
| 200 W computer, 8 h per day, 5 days per week | 34.67 | $6.93 | 416 | $83.20 |
| 200 W (nameplate) refrigerator, 24 h, draws power 40% of the time | 58.24 | $11.65 | 698.88 | $139.78 |
| 1,500 W air conditioner, 8 h per day, draws power 60% of the time | 218.4 | $43.68 | 2,620.8 | $524.16 |
| 5 W device on standby, 24 h per day | 3.64 | $0.73 | 43.68 | $8.74 |
To see how one row works: the computer uses 0.2 kW × 8 h = 1.6 kWh per day of use; over 5 days that is 8 kWh per week, and 8 × 52 = 416 kWh per year. At $0.20, that is $83.20 per year. Notice that the air conditioner, even running only 60% of the time, outweighs all the rest together.
When it pays to replace an appliance with a more efficient one
In the calculator, the «Compare with a more efficient appliance» section asks for the new appliance's wattage and, optionally, its price. It returns the savings per year and how many months it takes to pay for itself. Two cases with made-up prices for the example:
Bulb: swapping the 60 W incandescent for the 9 W LED, 5 hours a day, saves 109.2 − 16.38 = 92.82 kWh per year, or $18.56 at the example price. If the LED bulb cost $3, it would pay for itself in 3 ÷ (18.56 ÷ 12) ≈ 1.9 months.
Refrigerator: if an old 200 W unit and a new 100 W unit both run 40% of the time, 24 hours a day, the savings are 698.88 − 349.44 = 349.44 kWh per year, or $69.89. If the new one cost $600, it would pay for itself in about 103 months, almost 8.6 years. There the decision depends on whether the old one is already failing, on service life and other reasons, not just on the bill.
The rule of thumb: the more hours a day an appliance runs, the faster the swap pays off. A bulb that is rarely on takes a long time to repay the investment. If the energy label of the new appliance lists kWh per year, compare that figure with your estimate for the old one.
How to use the electricity usage calculator
- Open the electricity usage calculator and, for a starting point, pick an appliance under «Example appliance»: it loads typical power, hours and duty cycle.
- Replace the values with those of your appliance: power in watts (you can change the unit), hours of use per day, days per week and quantity.
- If the appliance does not draw power all the time, adjust «Percentage of time it draws power».
- Enter the «Price per kWh» from your bill to also see the cost per hour, day, week, month and year.
- If you are thinking of replacing it, open «Compare with a more efficient appliance» and fill in the new one's power and price.
It assumes no price: if you leave the kWh price empty, you only see the energy. The calculations run in your browser and what you type is not sent anywhere.
Your bill: what is and is not in the calculation
The calculator multiplies kWh by a price. A real bill adds more: fixed charges, taxes and, in many places, tiered rates where the kWh costs more beyond a certain amount of use. If your bill has tiers, try the price of the tier you are in, since that is what you would pay for each extra kWh.
A useful check: add up your monthly estimates for the big appliances and compare with the kWh for the period on your bill. If it falls far short, some appliance uses more than you thought, or the billing period has more days. Do not expect it to match to the cent.
Modest habits that help
- Replace first what stays on many hours: the living room or kitchen lights, before a closet bulb.
- Use power strips with a switch for the TV, console and cable box, and turn them off at bedtime.
- Clean the air conditioner filters and avoid extreme temperature settings: every degree of difference makes the compressor work more.
- Check that the refrigerator door seals well, let food cool before putting it in, and keep the unit away from the wall and from heat sources.
- Run the washing machine with a full load instead of several half loads.
- Take advantage of natural ventilation and fans, which use far less than an air conditioner.
Frequently asked questions
How much power does a 1,000 W appliance use?
If it runs for one hour at that power, it uses 1 kWh. Used 3 hours a day, that is 3 kWh per day. The cost comes from multiplying by the price per kWh on your bill.
How do I convert amps and volts to watts?
Multiply them: watts = volts × amps. For example, 120 V and 8 A is 960 W. It is an approximation; for appliances with a motor or electronics the real draw can be lower.
Why doesn't my refrigerator use what its label says for 24 hours?
Because the compressor turns on and off to hold the temperature. Use the percentage of time it draws power (the calculator suggests about 40%) to get closer to reality.
Do appliances switched off with the button still use power?
Those left on standby or with a pilot light draw a few watts, while ones that are unplugged or turned off with a physical switch draw none. Each is small, but they run all day.
Will my bill match the calculator?
Not exactly. The calculator only multiplies energy by the price you enter; your bill adds fixed charges, taxes and tiered rates. It is for comparing and estimating, not for replacing the bill.