Power Calculator

One watt, many unit names

A watt is one joule per second, which makes it a rate rather than an amount. Every unit on this page measures that same rate in a different trade's language: horsepower for engines and motors, BTU per hour for furnaces and air conditioners, tons of refrigeration for cooling equipment, and kilowatts for the electric meter. Convert the input to watts, then convert watts to the unit you want, and the arithmetic never changes shape.

The watt is a derived SI unit, defined as kg m² s^-3 from the base units of mass, length and time. Nothing about that definition matters at appliance level. What matters is that 1 W is exactly 1 J/s, so a 1,500 W heater releases 1,500 joules of heat every second. NIST lists the SI base and derived units for the formal version.

Most confusion comes from units that look similar and are not. A kilowatt is 1,000 watts. A kilowatt hour is not a power at all but an amount of energy, equal to one kilowatt running for one hour. A BTU per hour is a rate, while a BTU is an amount. The calculator only converts rates, so the energy units stay out of the dropdown on purpose.

Conversion factors, to enough digits to matter

These are the numbers the calculator uses. Each one converts the named unit to watts, and dividing goes the other way.

The small differences in the table are large in practice. Mechanical and metric horsepower sit 1.4 percent apart, which is 10.2 kW on a 1,000 hp engine, and that gap is bigger than the output of a small car. The electric horsepower of exactly 746 W is close to the mechanical figure and is a separate definition, not a rounding of it.

Rounding early is the other quiet error. The mechanical horsepower is 745.6999 W, and using 0.746 kW instead costs 1.5 kW on a 5,000 hp marine engine: 5,000 x 0.746 = 3,730 kW against 3,728.5 kW from the exact factor.

Horsepower comes in three sizes

Mechanical horsepower is 550 foot-pounds per second, or 745.6999 W. It is the number behind US engine ratings and the one most people mean by hp. Metric horsepower, written PS, is 75 kilogram-force meters per second, or 735.49875 W, and it is what a German or Japanese spec sheet usually quotes.

A car review that says 300 hp means 223.71 kW. A European sheet that says 300 PS means 220.65 kW. The two figures differ by 3 kW, which is enough to change a tax bracket in some countries and enough to look like a typo in a comparison table.

Electric horsepower, 746 W, appears on motor nameplates sold in the US and is close enough to mechanical horsepower that the two are often used interchangeably in conversation. In a spec sheet they are not interchangeable, and a 1 percent difference on a 500 hp motor is 3.7 kW of shaft power.

BTU per hour, tons, and cooling capacity

A BTU is the heat needed to raise one pound of water by one degree Fahrenheit, about 1,055 joules. BTU per hour turns that amount into a rate, and one BTU per hour is 0.2930711 W. Air conditioning adds a second unit: one ton of refrigeration is 12,000 BTU/h, a figure that comes from the heat absorbed by melting a ton of ice in a day, and that works out to 3,516.85 W.

So a 3-ton central air conditioner moves 36,000 BTU/h, which is 10,551 W, or 10.55 kW. A 10,000 BTU/h window unit is 2,931 W and 0.83 tons. An 80,000 BTU/h furnace is 23,446 W of input, and its output is lower because furnaces are rated with an efficiency percentage, usually 80 to 96 percent.

Cooling capacity in BTU/h is not the same as electrical draw. A 3-ton system moving 36,000 BTU/h of heat draws roughly 3,500 W of electricity, because a heat pump moves heat rather than making it. The ratio of those two numbers is the efficiency rating on the label, and it is why the cooling figure is several times the electrical figure.

Kilowatts, kilowatt hours, and the bill

Power and energy are different quantities, and the bill charges for energy. A kilowatt hour is one kilowatt running for one hour, so energy in kWh is power in kW times hours. A 1,500 W space heater running 8 hours uses 1.5 x 8 = 12 kWh. At 16 cents per kWh that is $1.92.

The 16 cents is an example rate, not a national average. The EIA energy conversion pages and its monthly state averages are the place to check a current figure, since residential rates range from about 11 cents to over 40 cents per kWh depending on the state. The arithmetic transfers unchanged: watts divided by 1,000 gives kW, and kW times hours gives kWh.

Standby draw adds up in a way that a single reading hides. Ten devices drawing 3 W each, all day, use 0.72 kWh in a day and about 22 kWh in a month, which is more than a month of LED lighting. The Department of Energy guide to appliance use covers meters and nameplate reading in more detail.

Nameplate math: amps and volts into watts

US appliances list watts, or amps and volts, and multiplying gives watts for direct current and for resistive loads: 120 V x 12.5 A = 1,500 W. A 240 V dryer drawing 24 A is 5,760 W. Those two numbers are the ones to look for on the label before trusting any estimate.

Motors are the exception, in two ways. The nameplate horsepower is shaft output, and the electrical input is larger by the efficiency: a 1 hp motor at 85 percent efficiency draws about 877 W. And alternating current carries a power factor, so the current a clamp meter reads is higher than watts divided by volts. That 877 W motor on a 120 V circuit at a power factor of 0.8 draws roughly 9.1 A, not the 7.3 A that watts alone would suggest.

Starting current is the third wrinkle. A compressor or a motor can pull several times its running current for a fraction of a second, which is why a 1,500 W unit on a 15 A circuit can trip a breaker at startup even though 1,500 divided by 120 is 12.5 A.

Mistakes that survive the calculator

Unit mix-ups are the usual failure, and the numbers still look plausible. A 2.5-ton air conditioner and a 2.5 kW heater are not comparable: one is 8,792 W of cooling capacity and the other is 2,500 W of heat. Reading the unit before reading the number prevents most of these.

Volt-amperes are not watts. A 1,000 VA uninterruptible power supply at a power factor of 0.9 delivers 900 W, and a motor load at 0.8 delivers 800 W. Sizing a generator or a UPS from watts alone understates the current it must supply.

Kilowatts and kilowatt hours get swapped in conversation constantly. A 2 kW heater run for 3 hours uses 6 kWh, and the 2 kW rating never changes. The same distinction explains why a 100 W bulb left on all year uses 876 kWh, which is more energy than a 1,500 W heater used an hour a day for the same year.

Last, the boiler horsepower on old steam equipment is 9,810.55 W, not 745.7 W. It is a thermal rating for steam output, and applying the mechanical factor to it understates the unit by a factor of about 13.