But power is the rate at which something does work (see post 16.20). This is another way of saying that it is the rate at which something converts one form of energy to another form of energy (see post 16.21). You might then expect to measure power in joules per second (J/s or J.s-1, see posts 16.12 and 16.13 for more explanation about units). But this unit has a special name – the watt (abbreviated to W). Dungeness B nuclear reactor, in the South of England, is used to produce electricity. Its power is 1.05 GW which means that it converts nuclear energy into 1.05 GJ of electrical energy every second. (To remind yourself about the prefixes G and k, see post 16.12).
Power is sometimes expressed as horsepower. Originally this was the rate at which a horse could work. It is now defined to be 0.746 kW. The power of a car’s engine (before energy is dissipated by the gearbox and drive chain) is usually called its brake horsepower (abbreviated to bhp). A Volkswagen Golf engine has a brake horsepower of about 100 (the exact value depends on the model) which is a power of over 70 kW. So, it’s about 100 times more powerful than a horse.
It’s easy to work out your power. Measure the time, t, it takes you to run up a lot of steps, in seconds. Now measure your mass, m, (in kilogrammes) and the total vertical height, h, of all the steps (in metres). In the time t, you increased your potential energy from zero to mgh (see post 16.21). So your average power, during this time period, was mgh/t (measured in watts).