Welcome to the island. Its people need electricity for lights, fridges, phones, pumps and heaters. Before you can supply it, you have to count it, and counting needs two ideas that people often mix up.

• Energy is the amount of work that can be done. Its SI unit is the joule (J).
• Power is how fast energy is used or delivered. Its unit is the watt (W), and 1 W = 1 J per second.

The link is P = E / t, or E = P × t. Energy is like the water in a tank. Power is like the speed the water runs out of the tap.