Which expression correctly represents energy in joules when power is in watts and time is in seconds?

• A. E = Time / Power
• B. E = Power x Time
• C. E = Power / Time
• D. E = Power x Energy

Answer: (B)

Power tells us how much energy is used each second, so energy is found by multiplying power by the time the device runs. Since a watt is a joule per second, the energy in joules equals power in watts times time in seconds: E = P × t. For example, at 60 watts for 5 seconds, energy is 60 × 5 = 300 joules. The other forms don’t match how energy, power, and time relate: dividing time by power would give a unit that isn’t joules, dividing power by time isn’t energy, and multiplying power by energy would mix units in a way that doesn’t represent energy.