Practice problems

Electrical power practice problems

Power is how fast a circuit uses energy, and it tells you whether a resistor will overheat, how long a battery lasts and what something costs to run. These ten problems use all three power formulas, then move on to energy in watt-hours, battery life and a year of standby.

The method

P = V × I

P = I² × R when you know the current and resistance
P = V² over R when you know the voltage and resistance
E = P × t for energy: watts × hours gives watt-hours (Wh), and 1 kWh = 1000 Wh
Battery life ≈ capacity in mAh over current in mA hours

Keep the units in step: volts × amps gives watts, and volts × milliamps gives milliwatts.

The problems

Type your answer and tap Check. Every one has the working, step by step.

Problem 1 of 10
A 12 V fan draws 0.5 A. How much power does it use?
Show the working
1. You know the voltage and current, so use P = V × I.
2. P = 12 V × 0.5 A = 6 W
Answer: 6 W
Problem 2 of 10
A 3.3 V sensor draws 15 mA. How much power does it use, in milliwatts?
Show the working
1. Volts times milliamps gives milliwatts, so there's no need to convert.
2. P = 3.3 V × 15 mA = 49.5 mW
Answer: 49.5 mW
Problem 3 of 10
30 mA flows through a 1 kΩ resistor. How much power does it give off as heat?
Show the working
1. You know the current and resistance, so use P = I² × R, with 30 mA = 0.03 A and 1 kΩ = 1000 Ω.
2. P = 0.03² × 1000 = 0.0009 × 1000 = 0.9 W. That's over three times a 0.25 W resistor's rating.
Answer: 0.9 W
Problem 4 of 10
A 47 Ω resistor sits across 5 V. How much power does it give off?
Show the working
1. You know the voltage and resistance, so use P = V² over R.
2. P = 5² over 47 = 25 over 47 ≈ 0.532 W
Answer: 0.532 W
Problem 5 of 10
A 2 kW kettle runs on 230 V mains. What current does it draw?
Show the working
1. Rearrange P = V × I to get I = P over V, with 2 kW = 2000 W.
2. I = 2000 W over 230 V ≈ 8.70 A
Answer: 8.7 A
Problem 6 of 10
A 10 W LED bulb is left on for 6 hours. How much energy does it use, in watt-hours?
Show the working
1. Energy is power × time. Watts times hours gives watt-hours (Wh).
2. E = 10 W × 6 h = 60 Wh
Answer: 60 Wh
Problem 7 of 10
A 2 kW heater runs for 3 hours. Electricity costs 25 pence (or cents) per kWh. What does it cost to run, in pence?
Show the working
1. Energy in kilowatt-hours: E = 2 kW × 3 h = 6 kWh.
2. Cost = 6 kWh × 25 = 150 pence.
Answer: 150
Problem 8 of 10
A battery is rated 2000 mAh. A circuit draws a steady 250 mA from it. Roughly how many hours will it last?
Show the working
1. mAh means milliamps × hours, so divide the capacity by the current.
2. t = 2000 mAh over 250 mA = 8 h. Real batteries give a little less, especially at high currents.
Answer: 8 h
Problem 9 of 10
A 3.7 V, 3000 mAh lithium cell powers a 1.5 W light. Roughly how many hours will it last?
Show the working
1. Turn the capacity into energy: 3.7 V × 3 Ah = 11.1 Wh.
2. Divide by the power: t = 11.1 Wh over 1.5 W = 7.4 h
Answer: 7.4 h
Problem 10 of 10
A TV on standby draws 5 W, all day, every day, for a year. How much energy is that, in kilowatt-hours?
Show the working
1. A year is 365 × 24 = 8760 hours.
2. E = 5 W × 8760 h = 43,800 Wh
3. 43,800 Wh ÷ 1000 = 43.8 kWh
Answer: 43.8 kWh

Build it in a free lesson

“Power and heat” is lesson 6 of 6 in Copper Flats, Electronics World 1. Free with an account. Lessons unlock in order, so you’ll do the 5 before it first.

Free lesson

Now build it yourself

Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.

Try it yourself: free lesson

“Power and heat” is lesson 6 of 6 in Copper Flats, Electronics World 1. Free with an account. Lessons unlock in order, so you’ll do the 5 before it first.

Or start from the very first lesson, no account needed

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The ideas behind it

Questions people ask

Which power formula should I use?

Use the two values you know. Voltage and current: P = V × I. Current and resistance: P = I² × R. Voltage and resistance: P = V² ÷ R. All three give the same answer, because each comes from P = V × I and Ohm's law.

What is the difference between watts and watt-hours?

Watts measure power: how fast energy is used. Watt-hours measure energy: power multiplied by time. A 10 W bulb uses 10 Wh in one hour and 60 Wh in six. Electricity bills charge for energy, in kilowatt-hours (kWh).

How do I work out battery life from mAh?

Divide the capacity in mAh by the current in mA to get hours: 2000 mAh at 250 mA lasts about 8 h. If you know the power instead, multiply the voltage by the capacity in Ah to get Wh, then divide by the watts.