The use of electric energy has a price that must be paid by the consumer. The more electricity consumed, the greater the price paid. The electricity consumption of electric devices is calculated with the following formula:

Consumption (energy consumed)= Power * time.

For example, the consumption of a 100 W light bulb which is turned on for 10 hours is:

100 watts = 0.1kW (the number of watts is equal to the number of joules consumed per second)
Consumption during 10 hours = 0.1kw *10h = 1 kWh.

The consumption is measured in kW·h (read kilowatt hour, and not kilowatt times hour)
Remember not to write kW / h !

The cost of a kWh, in Spain in 2007, was around 0.1 euros.
Therefore, if you wished to have a 100 W light bulb on for 10 hours per day for a whole month, then the consumption would be

Consumption = 0.1 kW· 10 h. 30 = 30 kWh and
the price would be = 30 kwh· 0.1 € /kWh = 3 €.

The greenhouse effect is a serious global problem which is a consequence of the combustion of fossil fuels (coal and petrol). If you squander energy, you are contributing to the greenhouse effect. Turn the lights out! Unplug devices that you are not using! Save water by controlling the time you spend in the shower! The planet and humanity will thank you.
The following visual teaches you how to use a spreadsheet to find out the consumption of any electrical device.