Output Power - Different Units of Measure

Motors as per ISO/IEC has now output in HP as well as in KW (in fact we use a lot of them where the HP is not mentioned)

The power in HP for motor is purely due to the historical reasons I will say - people were more comfortable saying My car has ... BHPs, etc, ever heard that My car engine is ... KW ?

The electrical equipments - you mean generators are also so. All these are used for converting electrical energy into useful work and vice versa.

In case of transformers, the case is a bit different, it converts a voltage and current into another V & I. It is not bothered about the power factor and the coils are designed to the current it has to carry and insulators to the voltage.

Here based on the Pf the current may be more (since it is demanded by load and not created by the transformer) so transformers are rated for its load - ie VI (the unit is VA ofr KVA)

Motor and generators are rated for their load (or input) - ie W,KW,MW etc

kashif_sabeeh,

It has to do with history and changes in the basis for measurements. Earlier methods were centered around human experience or size; later methods were made independent of the humans or their locations of observation. This allowed for a standard of measurement to be applied the same everywhere.

Consider that an early definition of a "foot" (measurement of length) was the average of the lengths of the right feet of the first twelve men to leave church on that particular Sunday. Consider the "cubit", the length of your forearm, the "mile" as 1000 (note the Latin "mille" for a thousand) paces of a centurion. The French introduced the Metric system, basing these various things on more universal standards, so the meter was calculated as one millionth of the diameter of the earth at the equator.

When mechanical power was being introduced to replace animal power, the first logical way to measure it was in terms of the number of horses this mechanical power source was equal to. Hence, "horsepower" or hp. Most people knew from experience or ready observation what a horse could do. Therefore, rating steam engines (and later gasoline or electric motors) in terms of horsepower was a reasonable step.

Since this was a measurement of energy, a derived measurement of energy in Metric system units was also able to be applied to the same equipment. The derived Metric unit was Watt, and the comparison between the two was standardized at 745.7 Watts = 1 hp. (Note the historical honoring of Isaac Watts who perfected the steam engine, by giving his name to a unit of energy.)

In the international system, 1 watt is the energy produced by a current of 1 ampere flowing through a wire with a resistance of 1 Ohm. Thus, 1W = 1VA. Applying the prefixes for size, kilo (k for one thousand), mega (m for one million), giga (g for one thousand million) gives units like you mentioned kW, mW, gW, etc.

In alternating current circuits, the current and voltage are usually not peaking at the same instant (they are out of phase with each other, with voltage usually leading). The useful power is still the Watt, but the mathematical conversion is Power = (volts)*(amps)*(cos-theta), where theta is the phase angle between the peaks of voltage and current. Since the full current flow through a transformer is seen as heat energy, output limits for the transformer are therefore expressed as VA, kVA, mVA, etc. instead of the somewhat smaller amounts of useful power W, kW, mW, etc.

I strongly suspect that similar historical backgrounds exist for other units of measurement that had local beginnings in various parts of the world.

--JMM