Quick answer
A watt (W) is the unit of power: the rate at which electrical energy is used or produced at any moment, equal to one volt times one amp.
One kilowatt (kW) is 1,000 watts. Wattage tells you how hard a device works right now. Multiply it by time and you get energy, the kilowatt-hours on your bill. The unit is named after James Watt.
Quick facts
The key facts about Watt, with sources:
Key takeaways
- Watts measure power: how fast a device uses energy at this moment.
- Watts = volts × amps. On a 120 V outlet, every 120 watts is one amp.
- Watts are not what you pay for. Your bill counts kilowatt-hours: watts over time.
- Heat-making appliances are the big wattage users; a heater can draw 1,500 W, a lamp 10 W.
- Wattage decides which circuit an appliance needs, and whether a circuit will overload.
A watt in everyday terms
A watt measures how fast energy flows. The EIA explains that electricity is measured in units of power called watts, named for James Watt, the inventor of the steam engine 1. One watt is a small amount. A phone charger uses a few watts; a hair dryer uses well over a thousand.
Because a watt is so small, bigger amounts use kilowatts. The EIA notes that one kilowatt equals 1,000 watts 1. Power plants and large loads use megawatts, a million watts.
Think of a car. Watts are your speed at a given moment. Kilowatt-hours are the miles you drove. A fast car parked in the garage uses nothing.
How watts relate to volts and amps
NIST defines the volt as one watt per ampere 2. That gives the rule used on every label: watts equal volts times amps. Virginia Tech Extension uses the same rule to work out an appliance’s watts from its amps and voltage 3.
So a 120-volt appliance drawing 10 amps uses about 1,200 watts. A 240-volt appliance drawing 10 amps uses about 2,400 watts. Same current, double the power, because the push is twice as strong.
For AC motors and electronics, the real watts can be a bit lower than volts times amps. That gap is called power factor. For heaters and old-style bulbs, volts times amps is close enough.
Watts = Volts × Amps → kWh per day = Watts × Hours used ÷ 1,000
Watts- Power at a moment
Hours used- How long it runs each day
÷ 1,000- Turns watt-hours into kilowatt-hours
Kinds of watt ratings on labels
| Rating | Meaning | Example |
|---|---|---|
| Rated (nameplate) watts | The most the device draws in normal use; Virginia Tech calls it the maximum 3 | Microwave label: 1,100 W |
| Running watts | What it draws once running | A fridge compressor while it cycles |
| Starting (surge) watts | A short burst when a motor starts | Used for generator sizing |
| Standby watts | What it draws while “off” but plugged in | Desktop asleep: 30 W or less 3 |
| Kilowatts (kW) | 1,000 watts; used for big loads and chargers 1 | Level 2 EV charger: 2.9 to 19.2 kW 5 |
Example: what a space heater costs to run
A 1,500 W heater is the top of the 750 to 1,500 W range Virginia Tech lists for portable heaters 3. The price is the EIA US average residential rate of 18.31 cents per kWh for July 2026 4.
| Step | Math | Result |
|---|---|---|
| Daily energy, 5 hours | 1,500 W × 5 h ÷ 1,000 | 7.5 kWh |
| Daily cost | 7.5 kWh × $0.1831 | About $1.37 |
| 30 days | 7.5 kWh × 30 | 225 kWh, about $41.20 |
| Current drawn at 120 V | 1,500 ÷ 120 | 12.5 A |
This is example arithmetic with a national average rate. Use your own rate from your bill, or the outage cost tool. The 12.5 A row shows why a heater and a hair dryer on one circuit trip the breaker.
Where you see watts
On appliance nameplates, usually on the back or inside the door. On bulb boxes, next to lumens, which measure light. On generator spec sheets, as running and starting watts. On EV chargers, often in kilowatts.
On your bill you will not see watts. You will see kilowatt-hours, which the EIA says is how utilities measure electricity 1. Some bills also show a demand charge in kilowatts, the highest power you drew in a short window.
What wattage tells you, and what it hides
Useful for
- Checking whether two appliances can share a circuit.
- Sizing a generator or battery backup.
- Comparing bulbs or heaters on energy use per hour.
- Estimating running cost when you know the hours.
Not useful for
- Telling you light output: use lumens for bulbs.
- Telling you cost on its own: you need hours and a rate.
- Showing motor start-up surges unless the label lists them.
- Showing real draw for cycling loads such as fridges 3.
Limits of a nameplate wattage
Virginia Tech Extension notes that nameplate wattage is the maximum, and refrigerators cycle on and off. It suggests dividing plugged-in hours by three for a fridge to estimate time at full wattage 3. Many appliances use less than their label most of the time.
A plug-in watt meter gives a better reading of real use. It is safe for any 120-volt plug-in appliance. Hard-wired loads need an electrician to measure.
How wattage drives cost
We do not quote equipment or labor prices. Wattage affects cost in these ways. Our project cost tool explains the drivers.
- Running cost. Watts × hours × your rate. High-watt, long-hour loads cost the most.
- Circuit needs. High-watt appliances may need a dedicated circuit, sometimes at 240 V.
- Service capacity. Many large new loads can push a home past its service rating.
- Backup sizing. Generators and batteries are sized by the watts you want to keep on.
- Lifespan. Wattage does not set lifespan by itself. Heat from running near a limit can shorten it.
How to total the watts on a circuit
Checking an appliance label for watts is a reading exercise, not wiring work.
- Find which outlets and lights a breaker controls, using the panel directory.
- List the nameplate watts of everything on those outlets.
- Add the watts for things that may run at once.
- Divide by 120 (or 240 for 240 V circuits) to get amps.
- Compare with the breaker rating. If you are close, move a load or ask an electrician about a new circuit.
Wattage habits that keep a home safe
- Use bulbs at or below the max wattage printed on each light fixture.
- Plug heaters and kettles straight into the wall.
- Unplug high-watt devices when not in use.
- Check the wattage before buying a new appliance for an older home.
Risks of too many watts
Too many watts on one circuit means too many amps, and that means heat. The CPSC warns against plugging several high-current products, such as heating appliances, into a low-amp circuit 6. Signs include warm plugs, dimming lights and tripping breakers.
A bulb above a fixture’s wattage limit can overheat the fixture and its wiring. Check the label inside the shade or socket.
Heaters are the usual culprit
A 1,500 W heater uses most of a 15 A circuit on its own. Do not run it from an extension cord or power strip. If plugs or outlets feel hot, unplug the heater and have an electrician check the circuit.
Rules that use watts
Code load rules often use volt-amperes, which is close to watts for most loads. The 2023 NEC 210.23 says that in no case may the load exceed the branch-circuit ampere rating 7. Electricians turn appliance watts into amps to meet that rule. The 2026 NEC is free to read online 8.
Federal efficiency rules also use watts, such as the limits on standby power for some products. Your state may adopt an energy code with lighting watt limits for new homes.
Watt compared with related units
Myths about watts
- Myth More watts means a brighter bulb.
- Reality Lumens measure brightness. An LED gives the same light as an old bulb at far fewer watts.
- Myth A 1,000 W device uses 1,000 kWh.
- Reality It uses 1 kWh per hour of running. Watts are a rate.
- Myth Off means zero watts.
- Reality Many devices draw standby power. Virginia Tech lists computers asleep at up to 30 W 3.
What changes by state
A watt is the same everywhere, but what a watt-hour costs is not. In July 2026 the EIA reported average home prices from 12.72 cents per kWh in Louisiana to 48.00 cents in Hawaii 4. Running the same 1,500 W heater costs almost four times as much in Hawaii.
When wattage matters to you
Check watts before you buy a heater, a window AC, a kitchen appliance or a generator. Check them when a breaker trips. Total them before you add an EV charger, using the home load calculator.
Rule of thumb for 120 V
Every 120 watts is about one amp on a standard outlet. A 15 A circuit tops out near 1,800 W; a 20 A circuit near 2,400 W.
Questions about Watt
What is a watt?
A watt is the unit of power. It measures how fast energy is used or made at a given moment. The EIA notes electricity is measured in watts, named after James Watt. One watt equals one volt times one amp, so a 120-volt device drawing one amp uses 120 watts.
What is the difference between watts and kilowatt-hours?
Watts measure power, the rate of use. Kilowatt-hours measure energy, the total used over time. A 1,000-watt heater running for one hour uses one kilowatt-hour. Your bill charges for kilowatt-hours, not watts.
How do I convert watts to amps?
Divide watts by volts. On a 120-volt outlet, 1,200 watts is 10 amps. On a 240-volt circuit, the same 1,200 watts is 5 amps. This is how electricians check whether appliances fit on one circuit.
How many watts can a 15 amp circuit handle?
A 15-amp, 120-volt circuit carries up to 1,800 watts before its breaker should trip. For loads that run for long periods, plan to stay well below that. A 20-amp circuit carries up to 2,400 watts.
How many watts does a house use?
Usage changes all day, so it is easier to talk about energy. The EIA reports an average US home bought about 10,791 kWh in 2022. That works out to an average of roughly 1,230 watts across every hour of the year, with big peaks when heating, cooling or cooking.
How much does it cost to run 1,000 watts for an hour?
It costs whatever one kilowatt-hour costs on your rate. At the EIA US average of 18.31 cents per kWh for July 2026, that is about 18 cents. In Hawaii, at 48 cents, it is about 48 cents.
What is a kilowatt?
A kilowatt is 1,000 watts. The EIA uses it for larger loads. A Level 2 EV charger delivers 2.9 to 19.2 kilowatts, according to the DOE. A kilowatt is power; a kilowatt-hour is energy.
Do watts or amps matter more for safety?
Amps decide whether a wire overheats, but watts are easier to read off labels. Convert watts to amps at your circuit voltage, then compare with the breaker rating. Staying within the rating keeps wires within safe heat limits.
Which home appliances use the most watts?
Heat-making appliances use the most. Virginia Tech Extension lists 40-gallon water heaters at 4,500 to 5,500 watts and clothes dryers at 1,800 to 5,000 watts. Dishwashers reach 2,400 watts and hair dryers 1,875 watts. Electronics are small by comparison, with a laptop near 50 watts.
How many watts does a refrigerator use?
Virginia Tech Extension lists a 16 cubic foot frost-free fridge at about 725 watts. A fridge cycles on and off, so it does not draw that all day. The extension suggests dividing the hours it is plugged in by three to estimate time at full wattage.
Sources
- U.S. Energy Information Administration, Electricity explained: Measuring electricity, retrieved .
- NIST, SI Units: Electric Current (volt definition), retrieved .
- Virginia Cooperative Extension, How to Estimate Appliance Energy Use (pub. 2901-9014), retrieved .
- U.S. Energy Information Administration, Electric Power Monthly, Table 5.6.A (July 2026 preliminary), retrieved .
- U.S. DOE Alternative Fuels Data Center, Developing Infrastructure to Charge Electric Vehicles, retrieved .
- U.S. CPSC, safety information on circuit breakers and fuses (1983, rev. 2011), retrieved .
- NEC 2023 210.23 Permissible Loads, Multiple-Outlet Branch Circuits, ElectricalLicenseRenewal.com, retrieved .
- NFPA 70, National Electrical Code, 2026 edition (free access), retrieved .
Expert review
Written by the HyreElectrical Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Definitions from EIA and NIST; appliance wattages from Virginia Cooperative Extension; price from EIA Table 5.6.A; heater cost is labelled example arithmetic.
Suggest a correction. We fix errors and say what changed.