Watt
James Watt sold steam engines to mine and mill owners. He had to tell buyers how many horses an engine would replace. Watt watched a horse walk in a circle turning a mill drive and worked out its work per minute. It came to 33 000 foot-pounds. He called this rate a horsepower.
A horsepower is not a force but a power: how much work is done per unit of time. A century later the unit of power was named after Watt himself. One watt is one joule per second. Watt's horsepower is almost 746 watts, and a person at rest uses about a hundred.
| Notation | W |
| Definition | joule per second · J/s · kg·m²·s⁻³ · volt times ampere |
| Reference values | 1 hp — 745.699 87 W · 1 PS — 735.498 75 W · 1 kW for an hour — 3.6 MJ |
| Introduced | 1882, British Association · in the SI since 1960 |
| Standards | ISO 80000-4 · IEC 80000-6 · SI Brochure 9th ed. |
| Typical values | 100 W — a person at rest · 746 W — one horsepower · 1 GW — a nuclear power unit |
01 · Definition
A joule per second
Power P shows how fast work is done or energy is transferred. One watt is one joule per second. The same work can be done by a weak motor in an hour or by a strong one in a minute. The work is the same, the power is different.
In electricity, power is voltage times current. A 230 V lamp drawing 0.26 A takes 60 W. Hence the familiar kilowatt-hour: an appliance of one kilowatt running for an hour uses 3.6 MJ of energy.
Where the watts of an incandescent lamp go
Four lamps give the same light, about 800 lm. The bar height is the power the lamp draws from the mains. The yellow part is what goes into visible light, the red part is heat. The slider sets how many hours a day the lamp is on. Below is the yearly consumption.
The watt is the rate at which energy is used, the kilowatt-hour is the energy itself. It is like speed and distance travelled. A 2 kW kettle uses 0.1 kWh in three minutes. A 9 W LED lamp takes eleven hours to use the same 0.1 kWh.
Who lifts the bucket faster: a human, a horse or Watt's engine
Two identical shafts, 50 m deep. Each has a bucket with the same load, a headframe with a sheave on top and an engine with a drum beside it. Choose an engine for each shaft. The work of lifting is the same; the time depends only on the power. Time on the scene is sped up so that the fast hoist finishes in six seconds. The clocks above the shafts show the real lifting time. On the right are two wattmeters with a logarithmic scale.
Choose the engines and watch who lifts the bucket first.
02 · Conversion
| Unit | Value | Where it is met |
|---|---|---|
| {n} | {v} | {note} |
There are several horsepowers: mechanical 745.7 W, metric 735.5 W, electrical 746 W.
A 12 000 BTU/h air conditioner removes about 3.5 kW of heat but draws a third of that from the mains.
The boiler horsepower is 9.8 kW, thirteen times the ordinary one.
03 · Orders of magnitude
04 · Measuring instruments
05 · Writing rules
06 · Neighbouring units
07 · Historical section
There are several horsepowers. The mechanical one, after Watt, is 33 000 foot-pounds per minute, or 745.699 87 W. The metric one is 75 kilogram-force metres per second, that is 735.498 75 W. The electrical one is exactly 746 W. The boiler horsepower described a steam boiler and is about 9.8 kW.
Watt set the rate with a margin: a horse at a mill worked more slowly, but the buyer of an engine must not lose out. In EU documents engine power is written in kilowatts, and horsepower is allowed only in brackets.
Catalogue of quantities
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