Electrical energy

Calculating energy from power and time

If we know the electrical power of a circuit, we can calculate the energy delivered by the circuit over a certain amount of time.

We use this equation:

E = Pt

• E is the energy delivered (in joules, J)
• P is the power of the electrical circuit (in watts, W)
• t is the time for which the circuit is delivering energy (in seconds, s)

We can rearrange this equation, so there are 3 main forms you’ll use it in:

• E=Pt
• P=\frac{E}{t}
• t=\frac{E}{P}

Calculating energy from potential difference, current and time

We know that P=VI, so we can substitute this into the equation for energy to get:

E = VIt

• E is the energy delivered (in joules, J)
• V is the voltage (more specifically, potential difference) across the circuit (in volts, V)
• I is the electrical current flowing through the circuit (in amperes, A)
• t is the time for which the circuit is delivering energy (in seconds, s)

Again, we can rearrange this into 4 different forms:

• E=VIt
• V=\frac{E}{It}
• I=\frac{E}{Vt}
• t=\frac{E}{VI}

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