Power is how fast energy is used, and energy is how much is used in total. To compare appliances, multiply power by time for each one and compare the energy.
This lesson closes the four lessons of electrical reasoning from diagrams and measurements. For the formulas themselves, see calculating electrical energy and power.
Worked example: a fictional appliance table
This is an original table with invented appliances. Energy in kWh = power in kW × time in hours.
| Appliance | Power | Time used | Energy (kWh) |
|---|---|---|---|
| A: fan | 60 W = 0.06 kW | 5 h | 0.06 × 5 = 0.30 |
| B: rice cooker | 600 W = 0.6 kW | 0.5 h | 0.6 × 0.5 = 0.30 |
| C: LED lamp | 10 W = 0.01 kW | 8 h | 0.01 × 8 = 0.08 |
The rice cooker has ten times the power of the fan, yet both use 0.30 kWh. The lamp has the lowest power and also uses the least energy, 0.08 kWh, even though it runs the longest.
How do I write the comparison?
Write the quantity that answers the question. If the question asks which uses more energy, rank by kWh. If it asks which is the most powerful, rank by watts.
A good sentence reads: “The rice cooker has the highest power, but the fan runs ten times as long, so both use 0.30 kWh.” It names both quantities and keeps them apart.
The mistake that costs marks
The common slip is to rank by power. A student says the rice cooker uses the most energy because 600 W is the largest number.
| Claim | Why it fails |
|---|---|
| “600 W is largest, so the cooker uses most energy” | Ignores time |
| “The lamp is on longest, so it uses most energy” | Ignores its very low power |
| Correct: “Compare E = Pt for each appliance” | Uses both quantities |
The error is easy to make because the table shows power but not energy. Add an energy column before you decide.
Check yourself
Appliance D is a 1 500 W heater used for 20 minutes. Appliance E is a 100 W lamp used for 6 hours. Which uses more energy, and by how much?
Answer
D: 1.5 kW × (20 ÷ 60) h = 1.5 × 0.333 = 0.50 kWh.
E: 0.1 kW × 6 h = 0.60 kWh.
Appliance E uses more, by 0.60 − 0.50 = 0.10 kWh, even though its power is 15 times smaller.
What to study next
Test all four lessons in the section practice set. If unit conversion is where slips happen, the units and significant figure checker checks the conversions on your working.
For a teacher to go through energy and power questions with you, see online one-to-one Physics tuition or try the one-hour trial class (from RM50).