What is one unit of electricity?

One unit is 1 kWh, the energy a 1,000 W appliance uses in one hour.

Why convert units first?

Power in kW instead of W changes the answer by a factor of 1,000.

S3 Physics: Household Electricity, Power and Cost (10 Basic Items)

2026-11-08

Tutor Prep Guide · S3 Physics

Ten basic items on power, time and energy, kWh to joule conversion, electricity cost and fuse power limits.

Focus　Write down the power and the time first, multiply for the energy, and convert units before calculating.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	30
Difficulty mix	Foundation 10
Chapter	Chapter: S3 Physics: Household Electricity, Power and Cost
Level	S3

02　Questions (10 items)

FoundationA 100 W lamp is left on for 5 hours. Find the energy used in Wh and in kWh.
FoundationA 1,500 W kettle runs for 0.4 hour. Find the energy used in kWh.
FoundationA 2 kW air conditioner runs for 3 hours. Find the energy used in kWh.
FoundationEach unit of electricity costs $0.9 and 5 kWh is used. Find the cost.
FoundationA 60 W fan runs for 10 hours at $1.5 per unit. Find the energy used and the cost.
FoundationHow many joules are there in 1 kWh?
FoundationHow long must a 500 W appliance run to use 1 kWh?
FoundationAn appliance of 1,100 W is connected to 220 V. Find the current.
FoundationA 13 A fuse is used on a 220 V circuit. What is the maximum power it allows?
FoundationWrite 1,000 W and 0.75 kW in the same unit and compare them.

03　Answers and one-line notes

#	Answer	One-line note
1	Energy = 100 W × 5 h = 500 Wh = 0.5 kWh.	Energy = power × time; 1,000 Wh equals 1 kWh, one unit of electricity.
2	1,500 W × 0.4 h = 600 Wh = 0.6 kWh.	Multiply to get Wh, then divide by 1,000 to get kWh.
3	2 kW × 3 h = 6 kWh.	With power in kW, multiplying by hours gives kWh directly.
4	5 kWh × $0.9 = $4.5.	Cost = number of units × price per unit.
5	60 W × 10 h = 600 Wh = 0.6 kWh; 0.6 × $1.5 = $0.9.	A low-power appliance used for many hours can still use little energy.
6	1 kWh = 1,000 W × 3,600 s = 3,600,000 J = 3.6 × 10⁶ J.	One hour has 3,600 seconds and one watt is one joule per second.
7	1,000 Wh ÷ 500 W = 2 hours.	Time = energy ÷ power, with matching units (Wh with W).
8	I = P ÷ V = 1,100 ÷ 220 = 5 A.	Power = voltage × current, so current = power ÷ voltage.
9	P = V × I = 220 × 13 = 2,860 W.	A fuse limits current; convert to power by multiplying by the voltage.
10	1,000 W = 1 kW and 0.75 kW = 750 W, so 1 kW is larger.	1 kW = 1,000 W; convert first, then compare.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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