Energy Saving: Become a Smart Electricity User!
Extension topic – this whole chapter is Extension content in the 2025 junior secondary science curriculum, linked to the "Energy" topic in junior secondary Geography.
Charging your phone, running the air conditioner, boiling the kettle – all use electrical energy, and someone pays for it! In this chapter you will learn to work out how much energy an appliance uses, how efficient it is and how much it costs to run, how to read energy labels, and how to use electricity in a low-carbon way. These skills save your family money and help protect our planet.
1. Power: How Fast Energy Is Used
What is power?
Power is the amount of energy an appliance uses (or transfers) every second. The unit of power is the watt (W).
\(1\ \text{W} = 1\ \text{J/s}\) – one watt means one joule (J) of energy every second.
Think of a water tap: a fully open tap (high power) fills a bucket quickly; a dripping tap (low power) fills it slowly. The water in the bucket is like the total energy used.
Watts and kilowatts
For large powers we use the kilowatt (kW):
\(1\ \text{kW} = 1000\ \text{W}\)
To change W into kW, divide by 1000: a 2000 W kettle is 2 kW, and a 60 W fan is 0.06 kW.
Typical power of home appliances
The power rating is printed on the appliance's label or in its manual. Typical values (real models vary):
LED bulb – about 5 W to 10 W
Electric fan – about 40 W to 60 W
Air conditioner – about 1 kW to 2 kW
Kettle or hair dryer – about 1 kW to 2 kW
Key takeaway: Power tells us how fast an appliance uses energy, in W or kW.
2. How Much Energy? Power and Time Together
The energy an appliance uses depends on its power and how long it is on.
\(\text{energy} = \text{power} \times \text{time}\), or in symbols \(E = P \times t\)
So a high-power appliance used briefly can use less energy than a low-power one left on for hours!
Method A: energy in joules (J)
If power is in watts (W) and time is in seconds (s), the energy is in joules (J):
\(\text{energy (J)} = \text{power (W)} \times \text{time (s)}\)
Worked example 1: a hair dryer
A 1500 W hair dryer is used for 5 minutes. How much energy does it use, in joules?
Step 1: Time in seconds: \(5 \times 60 = 300\ \text{s}\)
Step 2: \(E = P \times t = 1500\ \text{W} \times 300\ \text{s} = 450\,000\ \text{J}\)
Method B: energy in kilowatt-hours (kWh)
A joule is tiny, so power companies use a bigger unit, the kilowatt-hour (kWh). If power is in kW and time is in hours (h), the energy is in kWh:
\(\text{energy (kWh)} = \text{power (kW)} \times \text{time (h)}\)
1 kWh is the energy used by a 1 kW appliance in 1 hour. The kWh is a unit of energy, not power.
Worked example 2: the same hair dryer in kWh
Step 1: Power in kW: \(1500 \div 1000 = 1.5\ \text{kW}\)
Step 2: Time in hours: \(5 \div 60 = \dfrac{1}{12}\ \text{h}\)
Step 3: \(E = 1.5\ \text{kW} \times \dfrac{1}{12}\ \text{h} = 0.125\ \text{kWh}\)
Changing kWh into joules
\(1\ \text{kWh} = 1000\ \text{W} \times 3600\ \text{s} = 3\,600\,000\ \text{J}\)
Check: \(0.125 \times 3\,600\,000 = 450\,000\ \text{J}\) – the same as Worked example 1!
Memory aid: "W with s gives J" and "kW with h gives kWh". Never mix the pairs!
Common mistake: Putting watts and minutes straight into the kWh formula. 1500 W is 1.5 kW, and 30 minutes is 0.5 h. Convert before you calculate.
Did you know? Your home's electricity meter counts kWh, and people often call 1 kWh "one unit" of electricity. 1 kWh can light a 10 W LED bulb for 100 hours!
Key takeaway: \(E = P \times t\). Use W and s to get J, or kW and h to get kWh. \(1\ \text{kWh} = 3\,600\,000\ \text{J}\).
3. Efficiency: How Much Energy Is Useful?
No appliance turns all its input energy into the form we want. Some is always wasted, usually as heat.
Useful and wasted energy
A bulb changes electrical energy into light (useful) and heat (wasted). A fan gives kinetic energy (useful) plus some heat and sound (wasted). (See the chapter "Household electricity" for energy changes.)
Energy cannot be created or destroyed, so:
\(\text{energy input} = \text{useful energy output} + \text{wasted energy}\)
The efficiency formula
Efficiency is the percentage of the input that becomes useful output:
\(\text{efficiency} = \dfrac{\text{useful power output}}{\text{power input}} \times 100\%\)
Using energy instead of power gives the same answer. Since useful output can never exceed input, real efficiency is always less than 100%. Higher efficiency means less waste.
Worked example 3: an electric kettle
A 2000 W kettle gives 1800 W of useful heating power to the water. Find its efficiency.
\(\text{efficiency} = \dfrac{1800\ \text{W}}{2000\ \text{W}} \times 100\% = 90\%\)
The other 10% (200 W) is wasted heating the kettle body and the air.
Comparing LED, CFL and halogen bulbs
In the lab, compare bulbs of about the same brightness: measure power input with a power meter, and light output with a light sensor at the same distance from each bulb (a fair test). Bulb boxes often give light output in lumens (lm).
Example figures for bulbs each giving about 2 W of useful light (real bulbs vary):
Halogen bulb – power input 40 W, efficiency \(\dfrac{2}{40} \times 100\% = 5\%\)
CFL (compact fluorescent lamp) – power input 10 W, efficiency \(\dfrac{2}{10} \times 100\% = 20\%\)
LED bulb – power input 5 W, efficiency \(\dfrac{2}{5} \times 100\% = 40\%\)
The halogen bulb wastes most energy as heat – that is why it gets so hot. The LED gives the same light with one-eighth of the power, so it saves the most energy. CFLs contain a little mercury, so recycle used ones properly.
Common mistake: Dividing input by output and getting over 100%. Put the useful output on top.
Key takeaway: Efficiency compares useful output with input. For the same brightness, LED bulbs are the most efficient.
4. Paying the Bill: The Cost of Electricity
Hong Kong's power companies, CLP Power and HK Electric, charge households by the kWh used.
The cost formula
\(\text{cost} = \text{energy used (kWh)} \times \text{price per kWh}\)
Here we use an example price of \(\text{HK}\$1.5\) per kWh – a made-up figure for practice. Real bills are more complicated, but the idea is the same.
Worked example 4: an air conditioner
A 1.2 kW air conditioner runs 8 hours a night. Find the cost for 1 and 30 nights.
Step 1 – Energy for one night: \(E = 1.2\ \text{kW} \times 8\ \text{h} = 9.6\ \text{kWh}\)
Step 2 – Cost for one night: \(9.6 \times 1.5 = 14.4\), so the cost is \(\text{HK}\$14.40\)
Step 3 – Cost for 30 nights: \(14.4 \times 30 = 432\), so the cost is \(\text{HK}\$432\)
Worked example 5: one year of lighting
A bulb is on 4 hours a day for 365 days: \(4 \times 365 = 1460\) hours. Using the bulbs from Section 3:
Halogen (40 W = 0.04 kW): \(0.04 \times 1460 = 58.4\ \text{kWh}\); cost \(58.4 \times 1.5 = 87.6\), so \(\text{HK}\$87.60\)
CFL (10 W = 0.01 kW): \(0.01 \times 1460 = 14.6\ \text{kWh}\); cost \(14.6 \times 1.5 = 21.9\), so \(\text{HK}\$21.90\)
LED (5 W = 0.005 kW): \(0.005 \times 1460 = 7.3\ \text{kWh}\); cost \(7.3 \times 1.5 = 10.95\), so \(\text{HK}\$10.95\)
Swapping one halogen bulb for an LED saves \(\text{HK}\$76.65\) a year here. Imagine the savings for a whole school!
Key takeaway: Find the kWh first, then multiply by the price. Every kWh saved is money saved.
5. Reading Energy Labels
In Hong Kong, many common appliances, such as air conditioners and refrigerators, must carry an energy label under the Mandatory Energy Efficiency Labelling Scheme.
What the label tells you
1. Energy efficiency grade – Grade 1 (most efficient) to Grade 5 (least efficient).
2. Energy consumption – the expected kWh used, often per year.
3. Other details – brand, model, size or capacity.
Using the label to compare models
Two refrigerators of the same type and size (example figures):
Model A (Grade 1): 300 kWh per year, yearly cost \(300 \times 1.5 = 450\), so \(\text{HK}\$450\)
Model B (Grade 4): 450 kWh per year, yearly cost \(450 \times 1.5 = 675\), so \(\text{HK}\$675\)
Model A saves 150 kWh and \(\text{HK}\$225\) a year. Even if it costs more in the shop, it can save money over its life – and causes less carbon emission.
Key takeaway: Grade 1 is the most efficient. Compare kWh figures of similar models.
6. Low-Carbon Living: Shrinking Your Carbon Footprint
Why does saving electricity help the planet?
Much of Hong Kong's electricity comes from power stations burning fossil fuels such as natural gas and coal. This releases carbon dioxide (CO2), a greenhouse gas that adds to global warming.
Your carbon footprint is the total greenhouse gas released because of your activities, measured in kg of carbon dioxide equivalent (kg CO2e). Less electricity means a smaller footprint.
Worked example 6: a family's yearly carbon emission
A family uses 300 kWh a month. Use an example emission factor of 0.5 kg CO2e per kWh (made up; real values depend on the power company and year).
Step 1 – Yearly energy: \(300 \times 12 = 3600\ \text{kWh}\)
Step 2 – Yearly emission: \(3600 \times 0.5 = 1800\), so about 1800 kg CO2e a year
If the family cuts its electricity use by 10%, it avoids 180 kg CO2e every year.
Ways to use electricity the low-carbon way
1. Switch off at the wall – appliances on standby still use a little power.
2. Choose efficient appliances – pick Grade 1 labels and LED bulbs.
3. Use air conditioners wisely – the Hong Kong government has long suggested setting them to about 25.5 °C. Close windows, clean filters, and use a fan on milder days.
4. Use natural light – open the curtains in the daytime.
5. Use the refrigerator smartly – close the door quickly and cool hot food first.
6. Wash and dry smartly – wash full loads and hang clothes to dry.
7. Support clean energy – solar and wind power produce much less CO2.
Key takeaway: Less electricity used means less CO2 released. Small habits add up!
Quick Review
1. Power is the energy used per second; \(1\ \text{kW} = 1000\ \text{W}\).
2. \(E = P \times t\): W and s give J; kW and h give kWh; \(1\ \text{kWh} = 3\,600\,000\ \text{J}\).
3. \(\text{efficiency} = \dfrac{\text{useful output}}{\text{input}} \times 100\%\), always below 100%. LED beats CFL, which beats halogen.
4. \(\text{cost} = \text{kWh} \times \text{price per kWh}\).
5. Energy label: Grade 1 is the most efficient.
6. Switch off, choose efficient appliances and set the air conditioner sensibly to shrink your carbon footprint.
Great work! Try finding the power of three appliances at home and working out their monthly cost – you might be surprised!