Watt Converter and Power Calculator

What a watt measures: a rate, not a total

A watt is a rate of energy flow, not a quantity of energy. One watt is one joule every second, written 1 W = 1 J/s, and the SI definition fixes it in base units as one kilogram times one square metre per second cubed. The BIPM SI Brochure lists the watt among the derived units that carry a name, so a power figure can be traced back to mass, length and time rather than to a comparison with a spinning shaft.

That distinction is where most confusion starts. A 100 W lamp delivers 100 joules each second, and 3,600 joules each hour. The lamp does not burn up 100 watts; it draws 100 watts at that moment, and the bill is settled on the accumulated energy, which is watt-hours. NIST SP 811 prints the joule and the watt among its conversion factors, so the chain from joules to watts to kilowatt-hours is exact rather than a rule of thumb.

For electrical power the same watt falls out of volts and amps: P = V x I, so 120 V x 10 A = 1,200 W. That relation is exact for a resistive load on DC, and it holds on AC when current and voltage stay in phase. A motor or a ballast can draw the same volts and amps and still deliver fewer watts, and the shortfall is the power factor, which the watts to amps calculator works through in detail.

The conversion factors: kW, horsepower and BTU per hour

Every watt on this page can be expressed in the other units through the fixed factors below. The watt is the pivot: everything else is a number attached to it.

The horsepower figure comes from James Watt's own definition: 1 hp is 550 foot-pounds of work per second, and 1 foot-pound is 1.3558179483314 joule, so 550 x 1.3558179483314 = 745.699872 W. That is why the number carries so many digits. It is a conversion of two exact definitions, not a measurement.

The metric horsepower, sold in Europe as PS or CV, is a different animal: it is 75 kilograms-force moved one metre per second, which is 735.49875 W. A mechanic's shop in Milan and a workshop in Ohio mean something about 1.4 percent apart when they write horsepower, and mixing the two is a common source of small, stubborn errors.

The BTU/h unit is a heating rate. One British thermal unit is 1,055.056 joule, so a 1 W device moving 1 joule per second moves 3.412141633 BTU per hour. Furnace and air-conditioner ratings in North America use BTU/h, while electricians and appliance labels use watts, and the two meet on the same wall.

Worked conversions from everyday loads

Four examples cover the range of numbers that show up on a bill, a spec sheet or a product listing.

• A 1,500 W space heater: 1,500 / 1,000 = 1.5 kW, then 1,500 x 0.00134102209 = 2.0115 hp, and 1,500 x 3.412141633 = 5,118.21 BTU/h. On a 120 V circuit it draws 1,500 / 120 = 12.5 A.
• A 745.699872 W tool motor: that is exactly 1 hp, and 745.699872 x 3.412141633 = 2,544.43 BTU/h.
• A 2,000 W kettle: 2 kW, 2,000 x 0.00134102209 = 2.682 hp, and 2,000 x 3.412141633 = 6,824.28 BTU/h.
• A 100 W bulb: 0.1 kW, and 100 x 3.412141633 = 341.21 BTU/h. A 60 W incandescent bulb doing the same job as an 8 W LED draws 60 / 8 = 7.5 times the power.

Notice how the horsepower column stays small. A kettle pulls about 2.7 hp of electrical power, which sounds odd next to a car engine but is correct: a car engine turns a shaft slowly and loses a lot of the power it makes, while a kettle turns electricity into heat almost perfectly.

Typical appliance power: a reference table

Nameplate wattage is the number to look for when you are sizing a circuit, a power bank or a generator. The table below lists common household loads with the range most models fall into.

Two numbers on a label deserve care. The first is the maximum draw, which is what a breaker has to tolerate. The second is the average, which is what a bill sees. A refrigerator rated at 400 W might average 100 W because the compressor runs a third of the time, and a dimmable LED rated at 10 W draws about 3 W when turned down.

Sizing a generator or inverter from wattage

Once you know the wattage of each load, the sizing rule is short. Add the running watts of everything that could switch on at the same time, add the largest motor surge, and keep a margin of about 20 percent.

A worked case: kettle 2,000 W plus microwave 1,200 W plus lights 300 W gives 2,000 + 1,200 + 300 = 3,500 W. A portable generator sold as 3,600 W running and 5,000 W surge would cover that, but only if the microwave and kettle never overlap with the washing machine. Most camping and job-site generators are quoted at running and surge separately for exactly this reason.

Inverters follow a stricter pattern. A 1,500 W inverter may be rated for 3,000 W of surge for a few seconds, which covers a compressor starting but not a kettle boiling and a microwave running at the same time. A 1,500 W space heater on a 1,500 W inverter is at the edge of the rating, and a hair dryer on the same inverter is the classic way to trip it.

Small electronics are gentler than they look. A phone charger draws 5 to 20 W, and a laptop charger is usually rated at 30 to 65 W even though the laptop itself takes less. The charger rating is a ceiling, not a promise.

Where the unit came from

The watt is named for James Watt, who in the 1780s was busy improving the steam engine for Boulton and Watt in Birmingham. His company quoted an engine's output in horsepower, where 1 hp was the work of 550 foot-pounds per second, an estimate of what a strong shire horse could manage through a pulley system. Watt also worked out a way to express the rate of a horse's work in a unit that did not depend on the animal, and that quantity is the watt.

The watt did not become an electrical unit until the 1860s, once the volt and the ampere were pinned down. The physical link was settled by James Prescott Joule in the 1840s, whose careful measurement of the mechanical equivalent of heat showed that a steady mechanical output of one joule per second is the same thing as a steady electrical power of one watt.

The modern definition arrived with the SI. The watt is a derived unit, and the 2019 revision of the kilogram changed how the unit is realised without changing the unit itself, because the watt is built from the kilogram, the metre and the second. The BIPM SI Brochure gives the current wording, and the horsepower definition that anchors the 745.699872 factor comes from the same lineage of exact definitions.

Where watt figures go wrong

Five mistakes account for most of the bad watt numbers that circulate.

• Mixing metric and mechanical horsepower. 1 PS is 735.49875 W and 1 hp is 745.699872 W. Using the wrong one on a 200 hp engine gives 147,100 W instead of 149,140 W, a gap of about 2,000 W that nobody notices until a spec sheet disagrees with a measurement.
• Reading car horsepower as electrical watts. A 100 hp engine is rated at the crank and much of it becomes heat, so 100 x 745.699872 = 74,570 W describes the fuel energy, not the electricity a car draws. An electric car charging at 7,200 W is using about 9.7 hp.
• Confusing BTU/h with BTU. The per hour is the whole point. A 10,000 BTU/h furnace moves 10,000 BTU every hour, so it draws 10,000 / 3.412141633 = 2,930.7 W, which lands near a 3 kW heater.
• Treating kW as kWh. A 1.5 kW heater used for 4 hours consumes 6 kWh of energy. The kW number stays on the nameplate, the kWh number goes on the bill.
• Trusting a peak figure. Audio amplifiers are sold in peak watts at 8 ohms, and a 100 W amplifier can deliver far less continuous power into a real speaker. For everything else on this page, treat the nameplate number as the running figure.

A quick sanity check catches most of the others. One watt is one joule per second, one kilowatt is a thousand of them, and one horsepower is 745.699872 of them. If a converted figure cannot be traced back to one of those three, go back to the labels.

Numbers worth memorizing

A short list of anchors carries most of the arithmetic on this page.

Everything else on this page is one of those anchors multiplied or divided. That is the useful habit: convert to watts, do the arithmetic there, then convert back into the unit the question asked for.