Understanding power
Energy, per second.
The watt for almost everything; horsepower for habit.
The watt and its multiples.
A watt is one joule per second. The metric prefixes apply cleanly: kilowatts for appliances, megawatts for power stations, gigawatts for grids. Every step is a thousand.
Horsepower.
James Watt invented "horsepower" in the 1780s as a marketing tool: how many of the brewery's draft horses one of his steam engines could replace. Two definitions survive — the mechanical horsepower (about 745.7 W) used in the United States, and the metric horsepower or PS (about 735.5 W) used in continental Europe. They differ by less than two percent.
1 hp (mech) ≈ 745.6999 W · 1 PS (metric) ≈ 735.49875 W
Three worked conversions.
200 hp to kilowatts
1 hp ≈ 745.7 W
Multiply by the factor and convert to kW (÷ 1000).
200 × 745.7 ÷ 1000 ≈ 149.14
= 149.14 kW
1500 W to horsepower
1 hp ≈ 745.7 W
A watt is smaller than a horsepower, so divide.
1500 ÷ 745.7 ≈ 2.012
= 2.012 hp
100 kW to BTU/h
1 W ≈ 3.412 BTU/h
Multiply (kW first to W) by the factor.
100 × 1000 × 3.412 ≈ 341 200
= ≈ 341 200 BTU/h
Power vs energy.
Power is the rate at which energy moves, not energy itself. A 1500 W heater running for an hour uses 1.5 kWh of energy; the same heater running for two hours uses 3 kWh. The wattage stays the same; the energy on the bill doesn't.
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