Power Converter

About power units

Power is the rate at which energy is transferred or work is done — energy divided by time. Its SI unit is the watt (W), equal to exactly one joule per second. A 60 W light bulb consumes 60 joules of energy every second; a 100 kW electric motor delivers 100,000 joules per second to its load. Horsepower is the traditional unit in automotive engineering, but it hides an important ambiguity: the metric horsepower (PS, from German Pferdestärke) equals 735.5 W, while the mechanical (imperial) horsepower equals 745.7 W — a difference of about 1.4% that matters when comparing engine specs across regions.

History & origin

The watt was named in 1882 in honour of James Watt, the Scottish engineer who dramatically improved the steam engine in the 1760s–1780s. Watt himself coined the term 'horsepower' around 1783 as a marketing unit to help customers compare his engines with the draft horses they replaced, estimating one horse could lift 33,000 foot-pounds of load per minute. The kilowatt became the standard billing unit for electricity once alternating-current systems became widespread in the 1890s.

Tips

The two definitions of horsepower catch out anyone comparing car specs across countries. European manufacturers and DIN ratings traditionally use metric horsepower (1 PS ≈ 735.5 W); American manufacturers use mechanical horsepower (1 hp ≈ 745.7 W). A car advertised at 200 PS in Germany and 200 hp in the US are not the same machine — the US figure is slightly more powerful. Always check which horsepower a spec sheet is using before making a direct comparison.

Common questions

What is the difference between metric horsepower and mechanical horsepower?

Metric horsepower (PS or CV) is defined as the power needed to lift 75 kg at 1 m/s, equal to 75 × 9.80665 W ≈ 735.5 W. Mechanical (imperial) horsepower is defined as 550 foot-pounds per second ≈ 745.7 W. The two differ by about 1.4%, meaning a car rated 300 PS is approximately 296 mechanical hp — close but not identical.

How many watts are in a kilowatt, and how does that relate to electricity bills?

1 kilowatt = 1,000 watts. A 1 kW device running for 1 hour uses 1 kilowatt-hour (kWh), which is the unit on your electricity bill. A 2,000 W (2 kW) electric kettle running for 3 minutes uses 2 × (3/60) = 0.1 kWh of electricity.