Power Calculator

GCSE PhysicsAQAEdexcelOCRFoundation & Higher

Find power, energy transferred or time using P = E ÷ t. It works the same for work done (P = W ÷ t). Enter minutes, hours or kilowatts if the question uses them.

Quick answer

Power = energy transferred ÷ time. Use energy in joules and time in seconds to get watts (W). For example, a kettle that transfers 180,000 J in 60 s has a power of 180,000 ÷ 60 = 3000 W (3 kW).

By Mehmood Kakar · Updated

P = E ÷ t
Find:

Use work done (W) here if the question gives work instead of energy.

How to calculate power

Power is how quickly energy is transferred. One watt is one joule every second. Your equation sheet gives two versions of the same idea:

power = energy transferred ÷ time    and    power = work done ÷ time

  • P is power in watts (W)
  • E is energy transferred in joules (J), or W for work done, also in joules
  • t is time in seconds (s)

Use whichever the question gives you; the maths is identical. A kettle that transfers 180,000 J in 60 s has a power of 3000 W, or 3 kW.

Two motors can do the same work, but the more powerful one does it faster. That is the difference between energy and power: energy is how much, power is how fast.

Time causes most lost marks here. Questions often give minutes or hours, and they must become seconds before you calculate. Electrical power has its own equations, P = VI and P = I²R, which use current, potential difference and resistance.

Common mistakes that cost marks

  1. Leaving time in minutes or hours. 5 minutes is 300 s; 2 hours is 7200 s.
  2. Not converting kW to W. 1.2 kW is 1200 W. Multiply by 1000.
  3. Dividing the wrong way round. Power is energy ÷ time. A kettle with a power of 0.0003 W means you divided time by energy.
  4. Mixing up energy and power. Joules measure how much energy; watts measure how fast it is transferred.
  5. Forgetting the unit. Power is in watts (W) and energy in joules (J). Mark schemes often give a mark for the unit.

Practice questions

Try each one on paper first, then reveal the worked answer.

Q1A kettle transfers 180,000 J of energy in 60 s. Calculate its power.[2 marks]
  1. P = E ÷ t
  2. P = 180,000 ÷ 60
  3. P = 3000 W (3 kW)
Q2A 1.2 kW motor runs for 5 minutes. Calculate the energy it transfers.[3 marks]
  1. Convert: P = 1.2 kW × 1000 = 1200 W
  2. Convert: t = 5 min × 60 = 300 s
  3. Rearrange: E = P × t = 1200 × 300
  4. E = 360,000 J (360 kJ)
Q3A 12 W LED lamp transfers 43,200 J of energy. Calculate how long it was switched on, in hours.[3 marks]
  1. Rearrange: t = E ÷ P
  2. t = 43,200 ÷ 12 = 3600 s
  3. 3600 s ÷ 3600 = 1 hour
  4. t = 3600 s = 1.0 hour

Frequently asked questions

What is the equation for power in GCSE Physics?

Power = energy transferred ÷ time (P = E/t), or power = work done ÷ time (P = W/t). Power is in watts (W), energy and work in joules (J) and time in seconds (s).

What is a watt?

A watt is a transfer of one joule of energy every second. A 60 W lamp transfers 60 J of energy each second.

How do I convert kW to W?

Multiply by 1000. For example, 2.5 kW = 2500 W.

What is the difference between energy and power?

Energy is how much is transferred, in joules. Power is how fast it is transferred, in watts. A more powerful device transfers the same energy in less time.