Power Converter

The watt (W), named after engineer James Watt, is the SI base unit of power — one joule of energy transferred per second. Every unit in this converter is defined as an exact multiple of the watt: a kilowatt is 1,000 W, a megawatt is 1,000,000 W, and so on. Using watts as the common base makes it possible to convert between any two units in a single step: convert the source value to watts, then convert from watts to the target unit. This is exactly how the converter above works internally, and it's why the Reference tab lists every unit's equivalent in watts.

Horsepower dates back to James Watt's marketing of steam engines against horse-drawn mills, and it survives today in three slightly different flavors. Mechanical (imperial) horsepower — the standard in US automotive specs — equals exactly 745.7 W. Metric horsepower (PS, or cheval-vapeur), common in European and Japanese spec sheets, equals about 735.5 W, roughly 1.4% less. Electrical horsepower, used on US motor nameplates, is defined as exactly 746 W. The differences are small, but they matter when comparing a European car's PS rating to a US car's hp rating, or when sizing an electric motor from its nameplate spec.

British Thermal Units per hour (BTU/h) is the unit the HVAC industry uses to rate the cooling or heating capacity of air conditioners, furnaces, and heat pumps — you'll see it on every window unit and central AC spec sheet as a number like 8,000, 12,000, or 36,000 BTU/h. One BTU/h equals about 0.293 W, so a 12,000 BTU/h (a ‘1-ton’ unit) air conditioner corresponds to roughly 3,517 W of cooling capacity. Confusingly, BTU/h describes the rate of heat removal, not the electrical power the unit draws to do it — a well-designed AC removes several times more heat energy per hour than the watts it consumes, which is why efficiency ratings like SEER exist alongside the raw BTU/h number.

Watts and kilowatts dominate electrical and electronic specs — light bulbs, chargers, appliances, and solar panels are all rated in watts because that's what the electric meter measures. Horsepower remains the default for vehicle and engine specs in the US, even though most engineering calculations happen in watts or kilowatts behind the scenes. Megawatts show up at industrial and utility scale: wind turbines, power plants, and grid capacity are always discussed in MW because a wattage figure would be unreadably large. Foot-pounds per second is the least common of the six units here, mostly appearing in older US engineering references and physics textbooks as the ‘native’ imperial power unit before horsepower took over as the practical standard.