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Understand power and energy. Multiply watts by seconds to get joules.
Watts are a measure of power, or how fast energy is used (energy over time). Joules is a measure of energy. In order to convert from watts to joules, you need to specify a length of time. The longer a current flows, the more energy it uses. A 1 Watt device consumes 1 Joule of energy every 1 second. If you multiply the number of watts by the number of seconds, you'll end up with joules. To find out how much energy a 60W light bulb consumes in 120 seconds, simply multiply (60 watts) x (120 seconds) = 7200 Joules. This formula works for any form of power measured in watts, but electricity is the most common application.