What it does
Power tells you how fast work is done. Two people can climb the same stairs and do the same work, but the one who gets there sooner has more power. This page races two runners and then turns the idea into the units on a real electricity bill. The rate per kWh is a made-up example, so check your own bill for the real one.
It is a teaching model with round numbers. For work done by a changing force, see variable force work, and for how current heats wires and why fuses blow, see fuse heating.
How to use it
- In the Stair race, set mass, height and the two times.
- Read the work and power under the picture.
- Switch to Electricity bill, set hours a day for each appliance and the number of days.
- Answer the quiz.
What each setting does
- Mass and height: together with g = 9.8 m/s² they set the work, W = mgh.
- Runner times: power P = W ÷ t, so a shorter time gives a larger power.
- Hours a day and days: energy in kWh = power in kW × hours.
- Example rate: the made-up price per kWh used for the bill.
Worked example
A 60 kg student climbs 3 m in 6 s. Work = 60 × 9.8 × 3 = 1,764 J. Power = 1,764 ÷ 6 = 294 W. A 1000 W iron used for 1 hour uses 1 kW × 1 h = 1 kWh, which is one “unit” on the bill.
FAQs
Is a kWh a unit of power? No. It is a unit of energy: power (kW) multiplied by time (hours).
How many joules in 1 kWh? 3,600,000 J.
Why does the same work need different power? Because power depends on how quickly it is done.
Is the bill real? No. The rate is only an example, and real bills may add fixed charges and taxes.
Where the numbers come from
- P = W ÷ t and 1 kWh = 3.6 million J are standard definitions. The rate per kWh and appliance hours are made-up examples. Drawings are fibub's own.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-10.