---
title: "Electrical Power and the Cost of Running Appliances"
description: "Power in a circuit is potential difference times current, and also current squared times resistance. Energy transferred is power times time, which is what the kilowatt-hour on an electricity bill coun"
canonical: https://lightmysky.com/learn/science/electrical-power-and-the-cost-of-running-appliances-mt_099G0drIbk
source: https://lightmysky.com/learn/science/electrical-power-and-the-cost-of-running-appliances-mt_099G0drIbk.md
retrieved: 2026-09-12
---

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# Electrical Power and the Cost of Running Appliances

Power in a circuit is potential difference times current, and also current squared times resistance. Energy transferred is power times time, which is what the kilowatt-hour on an electricity bill counts.

Subject: Science · Area: Energy · Ages 14 to 15
Page: https://lightmysky.com/learn/science/electrical-power-and-the-cost-of-running-appliances-mt_099G0drIbk

## Ready when they can

- Calculates the power of an appliance from its voltage and current, or from its current and resistance
- Works out energy transferred in joules and in kilowatt-hours over a stated time
- Costs a week of using an appliance given a price per kilowatt-hour

## Lesson: Watts, units, and the bill

Power is how fast energy moves, measured in watts. In a circuit it equals voltage times current, and for a resistor that also gives current squared times resistance. A 230 V kettle pulling 10 A runs at 2300 W, all ending up heating the water.

Energy is power stretched over time. Multiply watts by seconds to get joules. Bills use a bigger unit, the kilowatt hour: 1000 W running for one hour, which is 3.6 million joules.

**Example.** A 2 kW heater running 3 hours a day uses 6 units a day. Over a week that is 42 units. Multiply 42 by the price per unit on the bill to cost the week.

**Tip.** Read watts off the label, then divide by 1000 to get kilowatts before multiplying by hours. Forgetting that step makes the answer 1000 times too big. High power devices need thick wiring, because big currents heat thin wires.

**Recap.** Volts times amps gives watts, watts times hours gives units, and units times price gives the bill.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Resistance in Series and Parallel](https://lightmysky.com/learn/science/resistance-in-series-and-parallel-mt_B29m3h5B75)
- [Power: watts and energy per second](https://lightmysky.com/learn/science/power-watts-and-energy-per-second-mt_ckpA3oZQ44)

## Opens up

- [Electromotive Force, Internal Resistance and the Potential Divider](https://lightmysky.com/learn/science/electromotive-force-internal-resistance-and-the-potential-divider-mt_g3zYJ9qC53)
- [Mains Electricity: a.c., Earthing and Fuses](https://lightmysky.com/learn/science/mains-electricity-a-c-earthing-and-fuses-mt_jKTotBL78P)