Power Converter

The single most common confusion in this area is between power and energy. Power is how fast energy is being converted or transferred; energy is the total converted over some span of time. A 2,000 W kettle draws energy at a fixed rate whether you run it for one second or ten minutes — what changes is the energy consumed, not the power. The relationship is simply energy = power × time, which is why the electricity meter measures kilowatt-hours (a power multiplied by a duration) rather than kilowatts.

Horsepower is unusual in having two widely used definitions that differ by enough to matter commercially. James Watt coined the original unit in the 18th century to market steam engines against the draught horses they replaced, settling on 550 foot-pounds per second — what is now called mechanical or imperial horsepower, about 745.7 watts. Continental Europe later defined its own metric horsepower as 75 kilogram-force metres per second, which works out to exactly 735.49875 watts.

The result is that the same engine can be legitimately advertised as 150 hp or 152 PS depending on which side of the definition the brochure was written on. Neither figure is wrong, but comparing a PS figure against an hp figure without converting overstates the second engine by about 1.4%.

Heating and air-conditioning equipment is usually rated in BTU per hour rather than watts, and air conditioners are often described in "tons" — a unit with nothing to do with the appliance's weight. One ton of refrigeration is the rate of cooling that would freeze one short ton of water in 24 hours, standardised as exactly 12,000 BTU per hour, or about 3.52 kW. The unit survives from the era when cooling was actually delivered by melting ice, and stuck because the industry's equipment sizing, ducting standards and rules of thumb were all built around it.