# Energy Efficiency

> How much of the energy we put in actually does the job we want? Calculating and improving efficiency.

From HSC Physics on WhizzNotes, free study notes for NSW students: https://whizznotes.com/physics/energy-efficiency/. Updated 2 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The light bulb swap
Lucas's family got a shock when the electricity bill arrived. His dad walked around the house counting the old-style light bulbs. There were 24 of them.

Lucas held his hand near one. It was hot. "Dad, these are more like heaters than lights."

He was right. They switched every bulb to LEDs. The rooms were just as bright, the bulbs stayed cool, and the next bill was noticeably smaller. The house needed the same amount of light. The new bulbs simply wasted less energy getting it.

**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:

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

### Worked example: The old bulb and the LED
An old filament bulb takes in 100 J of electrical energy and gives out 10 J of light.

$$\text{efficiency} = \frac{10}{100} \times 100\% = 10\%$$

An LED takes in 100 J and gives out 40 J of light.

$$\text{efficiency} = \frac{40}{100} \times 100\% = 40\%$$

The LED is four times as efficient.

### Key idea: No device is 100% efficient
Some energy is always lost, usually as heat from friction or electrical resistance. An efficiency above 100% would mean creating energy, which breaks the law of conservation of energy. If you ever calculate more than 100%, check your working.

## Typical efficiencies

| Device | Approximate efficiency |
|---|---|
| Filament light bulb | 5 to 10% |
| Petrol car engine | 25 to 30% |
| Solar panel | 15 to 22% |
| Coal power station | 35 to 40% |
| LED light | 30 to 50% |
| Electric motor | 80 to 95% |
| Large transformer | about 99% |

## 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).

### Watch out: Efficiency is not power
A 2000 W heater is more **powerful** than a 1000 W heater: it uses energy faster. That says nothing about how **efficient** it is. Power is about speed. Efficiency is about the fraction that is useful.

## 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.

**Mind map**

```text
Efficiency
  Energy out
    Useful: what we want
    Wasted: usually heat
  Formula
    useful out ÷ total in
    × 100%
    Always under 100%
  Improving
    Lubricate
    Streamline
    Insulate
    Better devices
  Environment
    Less fuel burned
    Renewable sources
    Lower bills
```

### Year 11: Efficiency of machines and motors
For a motor lifting a load: useful output is the gain in GPE, input is electrical energy, $E = Pt$.

**Example.** A 500 W motor lifts a 40 kg crate 6 m in 8 s ($g = 9.8$).

- Useful output: $mgh = 40 \times 9.8 \times 6 = 2352$ J
- Input: $Pt = 500 \times 8 = 4000$ J
- Efficiency: $\dfrac{2352}{4000} \times 100\% \approx 59\%$

When devices are linked in a chain, multiply efficiencies: a 40% efficient power station feeding a 90% efficient motor gives an overall efficiency of $0.40 \times 0.90 = 36\%$.

### Year 12: Why heat engines can never be perfect
Any engine that turns heat into work must dump some heat into a cooler place. The best possible efficiency (the Carnot limit) depends only on the hot and cold temperatures in kelvin:

$$\eta_{\max} = 1 - \frac{T_{\text{cold}}}{T_{\text{hot}}}$$

For a power station with steam at 800 K and a river at 300 K, $\eta_{\max} = 1 - \tfrac{300}{800} = 62.5\%$. Real stations reach about 40%. This is a law of nature (the second law of thermodynamics), not just poor engineering.

## Quick summary

- Devices give out useful energy and wasted energy (mostly heat).
- Energy in = useful out + wasted.
- Efficiency $= \dfrac{\text{useful out}}{\text{total in}} \times 100\%$.
- 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

Try these questions from the chapter quiz.

1. A kettle takes in 2000 J and transfers 1800 J to the water. Its efficiency is:
   - A. 80%
   - B. 90%
   - C. 111%
   - D. 10%
   - Answer: B. 90%. 1800 ÷ 2000 × 100% = 90%.

2. In most devices, wasted energy is mainly:
   - A. Light
   - B. Chemical energy
   - C. Heat
   - D. Kinetic energy
   - Answer: C. Heat. Friction and electrical resistance turn energy into heat.

3. A motor is 75% efficient. If it takes in 400 J, how much useful energy does it give out?
   - A. 75 J
   - B. 533 J
   - C. 300 J
   - D. 100 J
   - Answer: C. 300 J. 0.75 × 400 = 300 J.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/physics/energy-efficiency/#quiz