The energy a device uses is its power multiplied by the time it runs. With power in watts and time in hours the answer is in watt-hours, and dividing by 1 000 gives kilowatt-hours.
This is the first lesson in the electricity and energy cluster. The next lesson, interpreting an energy-flow diagram, uses the same unit habits on a diagram.
Worked example: a classroom in one day
An invented classroom uses four devices on a school day.
| Device | Power (W) | Use time |
|---|---|---|
| Ceiling fan | 70 | 6 h |
| LED lamp | 12 | 4 h |
| Projector | 250 | 90 min |
| Laptop charger | 45 | 2 h |
Step 1: make the time units match. The projector runs for 90 minutes, which is 90 ÷ 60 = 1.5 h.
Step 2: calculate each energy in Wh.
- Fan: 70 × 6 = 420 Wh
- Lamp: 12 × 4 = 48 Wh
- Projector: 250 × 1.5 = 375 Wh
- Charger: 45 × 2 = 90 Wh
Step 3: add them. 420 + 48 + 375 + 90 = 933 Wh.
Step 4: convert to kWh. 933 ÷ 1 000 = 0.933 kWh.
Step 5: find a cost under an assumption. Assume the school pays RM0.45 per kWh. Cost = 0.933 × 0.45 = RM0.42 to the nearest sen.
The assumption about the rate is written as part of the answer. Without it the cost has no stated basis.
The mistake that costs marks
The common slip is to use the projector’s 90 minutes as if it were 90 hours, or to write 250 × 90 = 22 500 Wh. That is a factor of 60 too large.
The fix is the conversion line before any multiplication. Writing “90 min = 1.5 h” takes five seconds and removes the error. Also check that the answer is sensible: 22 500 Wh for one projector is far more than the 933 Wh the whole classroom uses.
Check yourself
An invented workshop runs a 40 W fan for 8 hours, a 15 W lamp for 5 hours and a 150 W monitor for 30 minutes. Find the total energy in kWh.
Answer
Fan: 40 × 8 = 320 Wh. Lamp: 15 × 5 = 75 Wh. Monitor: 30 min = 0.5 h, so 150 × 0.5 = 75 Wh.
Total = 320 + 75 + 75 = 470 Wh = 0.47 kWh.
What to study next
Read interpreting an energy-flow diagram with consistent units. Check your unit handling with the units and significant-figure checker.
If you would like a teacher to go through your tables with you, see online one-to-one Science tuition. Record any slips in the mistake log and paper-error review.