Watt-Hours Calculator — Energy Usage & Cost
Quick Answer
A watt-hour (Wh) is the energy used by a 1-watt device running for 1 hour; 1,000 Wh equals 1 kilowatt-hour (kWh). A 100-watt bulb used 5 hours a day consumes 500 Wh (0.5 kWh) daily, or 15 kWh monthly, per the US Energy Information Administration's standard residential methodology.
Also searched as: watts to watt-hours, wh to kwh, electricity cost calculator, power consumption calculator
Wh per day
—
kWh per month
—
Monthly cost
—
Annual cost
—
Annual CO2 (kg)
—
How Watt-Hours Work
A watt-hour is a unit of energy equal to one watt of power flowing continuously for one hour. Because electricity is bought and sold by energy, not by power, every residential and commercial meter records cumulative watt-hours (or, more practically, kilowatt-hours), and your monthly bill is simply the difference between two meter readings multiplied by the tariff. The US Energy Information Administration reports that the average American household used 10,791 kWh in 2022, or about 29.6 kWh per day — see the EIA's electricity consumption FAQ for the full breakdown.
To compute your own watt-hours, multiply a device's wattage (printed on its label or nameplate) by the hours per day you use it. Scale up to monthly and yearly figures by multiplying by days per month and by 12. Dividing by 1,000 converts watt-hours to kilowatt-hours, the unit your utility bills in. Finally, multiply kWh by your per-kWh tariff (check a recent bill) to get cost, and by a grid emissions factor (about 0.4 kg CO2 per kWh in the US) to estimate your carbon footprint. See also our electricity bill calculator and Ohm's law calculator.
The Watt-Hours Formula
The core formula is: Wh = W × h, where W is the device's power rating in watts and h is the number of hours it operates. To get kilowatt-hours, divide by 1,000: kWh = W × h ÷ 1,000. Monthly cost is cost = kWh × days × rate, and annual CO2 is CO2 = annual kWh × 0.4 kg. For example, a 100 W television run 5 hours per day consumes 100 × 5 = 500 Wh = 0.5 kWh daily. Over 30 days that is 15 kWh, and at the 2024 US average of $0.1629 per kWh (EIA), about $2.44 per month or $29.30 per year. Annual CO2 is 180 kWh × 0.4 = 72 kg of CO2 equivalent.
Key Terms You Should Know
Watt (W): SI unit of power, equal to 1 joule per second. Watt-hour (Wh): SI-derived unit of energy equal to 3,600 joules. Kilowatt-hour (kWh): 1,000 watt-hours, the standard billing unit. Nameplate rating: the maximum power a device draws, printed on its label. Duty cycle: the fraction of time a device actively draws its rated power (refrigerators cycle on and off, so their average draw is lower than nameplate). Grid emissions factor: the kilograms of CO2 emitted per kWh of electricity generated, which varies by country and year.
Household Appliance Wattage — Reference Data
The table below shows typical wattages and monthly kWh for common household devices, based on figures from the US Department of Energy and EIA. Wattages vary by brand and model; the table is intended as a rough starting point. Notice how refrigerators, despite their low nameplate wattage, use a lot of electricity because they run 24/7.
| Appliance | Wattage | Typical use | Monthly kWh |
|---|---|---|---|
| LED bulb | 10 W | 5 h/day | 1.5 |
| Incandescent bulb | 60 W | 5 h/day | 9 |
| Laptop | 50 W | 8 h/day | 12 |
| Desktop + monitor | 200 W | 8 h/day | 48 |
| LED TV (55") | 100 W | 5 h/day | 15 |
| Refrigerator | 150 W | 24 h/day (40% duty) | 43 |
| Clothes dryer | 3,000 W | 1 h/day | 90 |
| Central AC | 3,500 W | 6 h/day | 630 |
Practical Examples
Example 1 — LED vs incandescent. Replacing a 60 W incandescent bulb with a 10 W LED, used 5 h/day for a year, cuts energy use from 109.5 kWh to 18.25 kWh — saving about $14.85/year at US rates. Example 2 — Space heater. A 1,500 W space heater running 8 h/day in winter (say 90 days) uses 1,500 × 8 × 90 ÷ 1,000 = 1,080 kWh, costing $176 at $0.1629/kWh. That is why utilities warn against using portable electric heat as a primary source. Example 3 — Phone charger. A 5 W phone charger left plugged in 24/7 but only actively charging 2 hours a day consumes roughly 12 kWh per year, or about $2 — small, but a common myth-busting figure cited by the UK DESNZ.
Tips and Best Practices
Check the nameplate, not the outlet type. Devices do not use the full wattage shown on their label unless they run at maximum — average draw is usually lower. Use a plug-in meter for accuracy. A $20 Kill-A-Watt or similar energy monitor measures real watt-hours over time, giving much better numbers than estimates. Look for Energy Star ratings. Certified appliances use 10–50% less electricity than minimum-standard models, per the EPA's Energy Star program. Attack your biggest loads first. Heating, cooling, and water heating account for about 51% of a typical US home's electricity use according to the EIA. Shift use off-peak. If you are on a time-of-use tariff, running dishwashers and laundry overnight can cut your cost by 30% or more.
Frequently Asked Questions
What is a watt-hour?
A watt-hour (Wh) is a unit of energy equal to one watt of power maintained for one hour. It describes the total amount of electrical energy used, not the instantaneous rate of use. One thousand watt-hours equals one kilowatt-hour (kWh), the unit most utilities use for billing. A 60-watt bulb running for one hour consumes 60 Wh, or 0.06 kWh, according to the US Energy Information Administration.
How do you convert watts to watt-hours?
Multiply watts by hours of operation: watt-hours = watts × hours. A 1,500-watt space heater running for 4 hours consumes 1,500 × 4 = 6,000 Wh, which equals 6 kWh. To convert watt-hours to kilowatt-hours, divide by 1,000. This is the same conversion used by every North American and European electricity meter, as specified in the ANSI C12 standards published by NEMA.
How much electricity does my device use per month?
Monthly kWh = wattage × hours per day × days per month ÷ 1,000. For a 100-watt device used 8 hours a day for 30 days, monthly use is 100 × 8 × 30 ÷ 1,000 = 24 kWh. At the US average residential rate of 16.29 cents per kWh (EIA, 2024), that costs about $3.91 per month. This calculator automates the math and adds CO2 output for the environmental footprint.
What is the average electricity rate?
The US average residential electricity rate was 16.29 cents per kWh in 2024, according to the Energy Information Administration. The UK average was about 24 pence per kWh under the April 2024 Ofgem price cap. Rates vary widely by state and supplier: Hawaii averaged 41 cents per kWh and Louisiana about 12 cents. Enter your actual tariff for an accurate cost estimate.
How much CO2 does a kilowatt-hour produce?
The US average grid emissions factor is about 0.4 kg of CO2 equivalent per kilowatt-hour, per the EPA's eGRID 2022 data. That figure has been falling as coal is replaced by natural gas and renewables. By comparison, the UK average is about 0.21 kg CO2 per kWh (DESNZ 2024 figures), and fully nuclear or renewable grids emit less than 0.05 kg CO2 per kWh. This calculator uses the 0.4 US figure by default but you can think of it as a typical value.
What is the difference between watts and watt-hours?
A watt is a unit of power, i.e., the instantaneous rate of energy use. A watt-hour is a unit of energy, the total amount of energy used over time. Think of it like driving: watts are like mph (rate) and watt-hours are like miles (total distance). A 1,000-watt microwave drawing power for 6 minutes uses 1,000 × 0.1 = 100 Wh of energy. Electricity bills charge for watt-hours, not watts.