Energy vs Power: What’s the difference?

Here at Voltio we often hear people use the words “energy” and “power” interchangeably (we’re guilty of it too, the Power Purchase Agreement (PPA) we sell to customers is actually an agreement for energy, not power). This isn’t a problem, we’re happy to provide these people remarkable solar solutions – but technically speaking these two words represent two different things.

What is Energy?

Energy measures the capacity to do work or the total quantity of work completed. In other words, energy is how much fuel is contained within something or the amount of fuel used by something.

For example, one Tim-Tam contains 500 kilojoules (kJ) of energy. If you eat the biscuit you transfer the energy into your body allowing you to do work. If you burn the biscuit it will release the energy as heat. In both instances, the energy is converted from potential energy to kinetic energy.

When we talk about how much energy a photovoltaic (PV) solar system can create (or more accurately, convert) we say the system “can generate X-number of kilowatt-hours (kWh)” over a given period of time. For example, a large family home might have enough energy demand to warrant a solar PV system that can produce 30 kWh per day. This means the system has the capacity to create up to 30 kWh worth of energy each day for the household to consume.

What is Power?

Power is a measure of the rate of producing or consuming energy. If energy is measured in kilowatt-hours (kWh) then the rate at which energy is consumed or created is measured in kilowatts (kW). Similarly, if you go for a run after eating that Tim-Tam, you might burn that 500kJ at a rate of 10kJ per minute.

If you remember one thing:

The analogy isn’t exact, but the relationship between energy and power is a bit like the relationship between distance and speed.

Energy, either generation or usage, is similar to distance in that it is a fixed measure from one point to another (like the capacity to do work). Power is like speed because it’s a measure of the instantaneous rate at which energy is being created or used.