Coaching Centre
Energy efficiency
Calculating efficiency as a percentage, power in watts, measuring household energy use in kilowatt-hours, comparing the true cost of light bulbs, energy star ratings, and ways to save energy at home.
An LED globe costs more to buy than an old incandescent one, yet over its lifetime it can save you over $100 in electricity. The cheapest thing in the shop is often the most expensive thing to own. Once you can calculate efficiency and running costs, you can see straight through the price tag.
Calculating efficiency
Efficiency tells you what fraction of the energy put into a device comes out in the form you wanted:
efficiency (%) = useful energy out ÷ total energy in × 100
No real device is 100% efficient, because some energy is always transformed into heat (and often sound). The higher the efficiency, the less energy is wasted.
A gymnast lands on a trampoline with 500 J of kinetic energy. The trampoline springs return 400 J to her as she bounces back up. How efficient is the trampoline?
efficiency = 400 ÷ 500 × 100
= 80%. The other 100 J became heat and sound in the springs and mat. To keep bouncing higher, she must add energy with her legs each time.
An electric motor is 75% efficient. It needs to produce 300 J of kinetic energy. How much electrical energy must be supplied?
75% of the input becomes useful output, so 300 J is 75% of the input.
input = 300 ÷ 0.75 = 400 J, with 100 J wasted.
Power and kilowatt-hours
Power is how fast a device transforms energy. It is measured in watts (W): 1 W means 1 joule every second. A 10 W LED uses 10 J each second; a 2000 W (2 kW) kettle uses 2000 J each second.
Power companies charge for energy in kilowatt-hours (kWh). One kilowatt-hour is the energy a 1000 W device uses in one hour, which equals 3.6 million joules.
energy (kWh) = power (kW) × time (hours)
cost = energy (kWh) × price per kWh
A 2.4 kW heater runs for 5 hours a night. Electricity costs 33 c per kWh. What does it cost each night?
2.4 × 5 = 12 kWh
12 × 33 c = 396 c = $3.96 a night, or about $360 over a three-month winter.
The true cost of a light bulb
Different kinds of globes give about the same amount of light while using very different amounts of power, and they last for very different numbers of hours.
| Type | Power for the same light | Typical life | Approximate efficiency |
|---|---|---|---|
| Incandescent | 60 W | 1000 h | About 5% |
| Halogen | 42 W | 2000 h | About 7% |
| Compact fluorescent (CFL) | 14 W | 8000 h | About 20% |
| LED | 9 W | 25 000 h or more | About 40% |
- With the default settings, which globe is cheapest to buy? Which is cheapest after 10 years?
- When does the LED line cross below the incandescent line? That's the point where the LED has paid for itself.
- Change the electricity price. Does a higher price make LEDs more or less worth it?
- Set the hours to 1 a day. Is the LED still the best buy?
Energy star ratings
In Australia, fridges, freezers, washing machines, dishwashers, clothes dryers, TVs, computer monitors and air conditioners must carry an Energy Rating Label. The more stars, the more efficient the appliance compared with others of the same size. The label also shows the expected energy use in kWh per year, which is the most useful number for comparing two models: multiply it by your electricity price to estimate the running cost.
A fridge runs 24 hours a day for 10 to 15 years, so an efficient one can save hundreds of dollars over its life even if it costs more up front.
Efficiency or conservation?
There are two ways to cut energy use, and they're easy to mix up:
- Energy efficiency means using less energy to do the same job: swapping to LEDs, insulating the ceiling, choosing a high-star fridge or a heat-pump hot water system.
- Energy conservation (in everyday language) means choosing to do less of the job: switching off lights in empty rooms, taking shorter showers, drying clothes on a line, setting the air conditioner to 24 °C instead of 20 °C in summer.
Heating and cooling make up about 40% of energy use in a typical Australian home, and hot water another 20% or so, so these are where the biggest savings are. Standby power matters too: devices left on standby can add up to around 5% of a household's electricity bill.
Test yourself
A kettle takes in 2000 J of electrical energy and 1700 J goes into heating the water. Its efficiency is
1700 ÷ 2000 × 100 = 85%.
One watt means
Power is energy per second.
A 1.5 kW air conditioner runs for 4 hours. How much energy does it use?
1.5 × 4 = 6 kWh.
Which is an example of energy efficiency rather than conservation?
The LEDs do the same job (lighting) with less energy.
Why can no device be more than 100% efficient?
This follows from the conservation of energy.
An appliance label says 300 kWh per year. Electricity costs 30 c per kWh. The yearly running cost is about
300 × 0.30 = $90.
Order these globes from least to most efficient.
Is each action energy efficiency or energy conservation?
Efficiency is useful energy out divided by total energy , times 100. Power is measured in . Electricity bills charge for energy in . More on an Energy Rating Label mean a more efficient appliance. The wasted energy is usually .
A family is choosing between two fridges. Fridge A costs $900 and uses 450 kWh per year. Fridge B costs $1200 and uses 250 kWh per year. Electricity costs 30 c per kWh. Which fridge is the better buy over 10 years? Show your working and give one other reason for your choice.
Fridge A: 450 × $0.30 = $135 a year, so $1350 over 10 years. Total = $900 + $1350 = $2250.
Fridge B: 250 × $0.30 = $75 a year, so $750 over 10 years. Total = $1200 + $750 = $1950.
Fridge B is the better buy: it costs $300 less over 10 years even though it is $300 more to buy. It also uses 2000 kWh less electricity over 10 years, so less fuel needs to be burned in power stations, which means fewer greenhouse gas emissions. If electricity prices go up, the saving will be even bigger.