# Energy and power consumption.

In science, the words work and energy are used interchangeably. Both are measured in Joules (J) named after the nineteenth-century British scientist James Joule.

It would take approximately 10j of work to lift a 1kg object through a metre. The lift is achieved by applying force to an object, causing it to move. Work is done when a forces point of application moves. At the end of the lift, the object would be said to have gained 10 J of energy which it could realise as work by descending through a metre and driving a mechanism. In this example, it doesn’t matter how fast the object rises or falls, in each case the work or energy would be the same.

There is clearly a difference between raising the 1kg quickly and slowly. What is different is the power required for the lift. A quick lift requires more power than a slow lift. Power is the rate at which the work is done, and the unit of power is the watt (W). 1 W is the power that is required to do 1 J of work in 1 second.

Power = Work / Time

Doing 1 J of work in one-tenth of a second requires a power of 10w, doing the same work over 10 seconds needs 0.1 W.

The power of devices we use every day varies enormously. The electrical power of an LED bulb is between 5-15W, a filament bulb would have used 8 times that, most of which would have radiated as heat, rather than light. A kettle uses around 2 kW, which is really high, but typically used only briefly – the energy consumed from the kettle would be comparable with running a  15 W lightbulb for a few hours. A washing machine might use 2 kW heating water, 250 W during the wash and 500 W to spin.

Continuous power consumption of 1 W by a mains powered device would not make much difference to your bill, but a typical battery-powered 1 W device would exhaust its battery in a few hours, to a few days. The max power consumption of a smartphone is about 3 W but would only operate that power intermittently.

Solar panels on roofs generally can generate about 200W depending on the strength of sunlight, and a typical house installation has several panels.

Technology Device Energy use per hour Scientific Expression Comparison
Wi-Fi Laptop 2 kJ 2 x 10³ 100/100
ZigBee Hub 20 mJ 20 x 10⁻³ 1/100000
Green Power Light switch 200 µJ 20 x 10⁻⁶ 1/10000000
Other Unit Joules (J)
1 kJ (kilojoule) = 1000 joule (J)
1 mJ (millijoule) = 0.001 joule (J)
1 microjoule (µJ) 1.0 E⁻⁶ joule (J)

Useful energy unit comparison at unitconverters.net

If the Laptop uses 2kJ in one hour, then we can work out the watts used. First we need to convert the units to joules (J) and the time to seconds.

2kJ = 2 x 1000 joules = 2000 joules.
1Hr = 60 x 50 = 3600 seconds

If power / time = energy then; 2000 J / 3600 s = 0.56 w (to 2 significant figures)