Watts, horsepower and BTU per hour
Power is the rate of doing work, energy divided by time. The SI unit is the watt, one joule per second. A kilowatt is 1,000 W and a megawatt is a million, the scale utilities use for generating plants. James Watt defined horsepower as 550 foot-pounds per second, which converts to 745.6999 W; that is mechanical or imperial horsepower, the figure on an American engine. Europe and Japan long used metric horsepower, abbreviated PS or CV, defined as 75 kilogram-meters per second, or 735.49875 W. A 300 PS import is therefore 296 hp, a difference big enough to matter in a spec sheet.
Heating and cooling equipment is rated in BTU per hour, where one BTU per hour is 0.29307107 W. Air conditioners are also sold in tons, a holdover from the days of ice: one ton of refrigeration is the rate needed to freeze a short ton of water in 24 hours, standardized as 12,000 BTU/h or 3,516.85 W. Kilocalories per hour show up on European boilers and on exercise machines, at 1.163 W each.
Worked examples
Air conditioning: a 3-ton central system is 36,000 BTU/h, which is 10.55 kW of cooling. It does not draw 10.55 kW from the wall; at a SEER2 rating of 15 it consumes roughly 36,000 / 15 = 2,400 W, about 20 amps on a 120 V equivalent basis or 10 amps at 240 V. A 5,000 BTU/h window unit is 1.47 kW of cooling and pulls around 450 W.
Generators and engines: a 7,500 W portable generator can carry 7.5 kW, which is 10.1 hp of output and 25,600 BTU/h if you were running resistance heat. A 200 hp car engine makes 149 kW at the crank, and a 1,500 W hair dryer is 2.0 hp. Utility scale: a 2 MW wind turbine equals 2,682 hp; a 1,000 MW nuclear reactor is a gigawatt, enough for roughly 750,000 US homes at average demand.
Sizing tips and common mistakes
Do not confuse power with energy. A 1,500 W heater running for three hours uses 4.5 kWh of energy; the watt rating alone says nothing about the bill. Nameplate ratings on motors describe output power, while the electrical input is higher because of efficiency, so a 5 hp motor at 88 percent efficiency draws about 4,237 W. On AC circuits, volt-amps and watts differ whenever the power factor is below one, which is why UPS units are rated in VA.
For quick estimates: 1 hp is about three quarters of a kilowatt, 1 kW is about 3,412 BTU/h, and 12,000 BTU/h is one ton. Generator advice usually adds a starting surge of two to three times the running watts for motor loads such as compressors and well pumps, so size from the surge, not the steady figure.