Power Converter

Power is a rate, energy is a quantity

A watt is one joule per second. That distinction governs everything here: a 2 kW heater does not consume 2 kW, it consumes at 2 kW, and running it for an hour uses 2 kWh of energy. Electricity bills charge for energy, while appliance ratings state power. Confusing the two is the reason people are surprised by what a high-wattage appliance actually costs.

There are several horsepowers

Mechanical horsepower is 745.7 watts. Metric horsepower, written PS or CV and used across most of Europe, is 735.5 watts, about 1.4 percent less. Electrical horsepower is defined as exactly 746 watts, and boiler horsepower is an entirely different unit at 9,810 watts. A car quoted at 200 hp in one market and 200 PS in another is not the same car, and specification sheets routinely fail to say which.

Air conditioning uses two odd units

Cooling capacity is rated in BTU per hour, almost always shortened to BTU in marketing, which conflates a rate with a quantity. A ton of refrigeration is 12,000 BTU/h, derived from the rate of cooling produced by a ton of ice melting over 24 hours. Both are rates despite neither sounding like one, and 1 kW is about 3,412 BTU/h.

Input power is not output power

An appliance's rated power is usually what it draws, not what it delivers. A motor rated at 1 kW input at 85 percent efficiency produces about 850 W of mechanical work; the rest becomes heat. Amplifier and generator ratings are notorious for quoting whichever figure looks better, so check whether a number is input, continuous output or peak.

Volts times amps is not always watts

For direct current it is. For alternating current, apparent power in volt-amperes exceeds real power in watts by the power factor, typically 0.8 to 0.95 for motors and older equipment. Uninterruptible power supplies are rated in VA for this reason, and sizing one by treating VA as watts leaves it undersized.

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