Energy Efficiency

In short: Energy efficiency is how much of the energy put into a device comes out as useful energy: efficiency = useful energy out ÷ total energy in × 100%. The rest is wasted, mostly as heat, so no real device is 100% efficient.

Useful and wasted energy

Every device takes in energy and gives out energy. Some of the output is what we want. The rest is not.

• Useful energy: the form we want. For a bulb, light. For a fan, kinetic energy.
• Wasted energy: forms we do not want, usually heat and sound. Wasted energy spreads into the surroundings and is very hard to get back.

By conservation of energy:

$$\text{energy in}=\text{useful energy out}+\text{wasted energy}$$

What is efficiency?

Efficiency tells you what fraction of the input energy becomes useful output.

$$\text{efficiency}=\frac{\text{useful energy out}}{\text{total energy in}}\times 100\%$$

It can also be calculated with power:

$$\text{efficiency}=\frac{\text{useful power out}}{\text{total power in}}\times 100\%$$

Typical efficiencies

Making things more efficient

Most energy is wasted as heat. So, to improve efficiency, reduce the unwanted heat:

• Lubricate moving parts (oil, grease) to reduce friction.
• Streamline vehicles to reduce air resistance.
• Use thicker or better conducting wires to reduce heating in cables.
• Insulate homes so heaters waste less heat through walls and roofs.
• Replace old devices with more efficient ones (LEDs, heat pumps, electric motors).

Energy and the environment

Wasting less energy means burning less fuel. That saves money, makes fuel supplies last longer and reduces greenhouse gases like carbon dioxide.

Renewable sources do not run out: solar, wind, hydro, geothermal, tidal. Non-renewable sources will run out: coal, oil, natural gas, uranium.

Quick summary

• Devices give out useful energy and wasted energy (mostly heat).
• Energy in = useful out + wasted.
• Efficiency $=\frac{\text{useful out}}{\text{total in}}\times 100\%$.
• Nothing is 100% efficient. An answer over 100% means a mistake.
• Reduce waste by cutting friction, air resistance and heat loss.
• Efficiency is not the same as power.

Quiz