The energy transferred equations

There are two energy transferred equations in GCSE Physics. E = Pt: energy transferred (J) = power (W) × time (s). E = QV: energy transferred (J) = charge flow (C) × potential difference (V). Use E = Pt when you know how powerful something is and how long it runs, and E = QV when you know how much charge flows through a potential difference. A 2000 W heater switched on for 60 s transfers 2000 × 60 = 120 000 J.

Energy transferred = power × time

Power is the rate of energy transfer: 1 watt is 1 joule per second. So a 60 W lamp transfers 60 J every second it is on, and in 10 s it transfers 600 J. AQA's GCSE specification puts it simply: the energy an appliance transfers depends on how long it is switched on and on its power.

Rearranging E = Pt

The graph shows the energy transferred by a 2000 W appliance. Here x is the time in seconds and y is the energy transferred in kJ.

The line is straight because the energy goes up by the same amount every second. Its gradient, energy ÷ time, is the power. A more powerful appliance gives a steeper line.

Energy transferred = charge × potential difference

Potential difference is the energy transferred per unit charge: 1 volt is 1 joule per coulomb. So every coulomb that passes through a 230 V appliance transfers 230 J. AQA describes this as work being done when charge flows in a circuit.

Linking E = QV to E = Pt

Charge flow is current × time, Q = It. Substituting into E = QV gives E = ItV = (VI)t. Since power P = VI, this is just E = Pt. The two equations are the same physics seen two ways. Our guides to charge and current and potential difference cover those quantities in more detail.

Try a question on charge and energy, then press Mark to check it in the course.

Energy transferred worked examples

Example 1: a kettle

A 2.2 kW kettle is switched on for 3 minutes. How much energy does it transfer?

Convert first: 2.2 kW = 2200 W and 3 minutes = 180 s.

That is 396 kJ.

Example 2: finding the time

A phone charger has a power of 18 W. How long does it take to transfer 97 200 J?

5400 s is 90 minutes.

Example 3: using E = QV

A 4.5 V battery pushes 60 C of charge round a circuit. How much energy does it transfer?

Example 4: from current and time

A 6.0 V supply drives a current of 0.25 A through a motor for 2 minutes. How much energy is transferred?

First the charge: Q = It = 0.25 × 120 = 30 C. Then E = QV = 30 × 6.0 = 180 J. Check with power: P = VI = 6.0 × 0.25 = 1.5 W, and E = Pt = 1.5 × 120 = 180 J. Both routes agree.

Example 5: kilowatt-hours

Electricity bills measure energy in kilowatt-hours, kWh: the energy a 1 kW appliance transfers in 1 hour. It is the same equation, E = Pt, with power in kW and time in hours. A 3 kW heater used for 2.5 hours transfers:

One kilowatt-hour is 1000 W × 3600 s = 3 600 000 J, so 7.5 kWh is 2.7 × 10⁷ J. If electricity costs 25p per kWh, this costs 7.5 × 25 = 187.5p, about £1.88.

Example 6: energy transferred by a force

When a force moves something, the energy transferred is the work done, W = Fs. Pushing a trolley with a force of 40 N for 12 m transfers 40 × 12 = 480 J. Work done and energy transferred are measured in the same unit, the joule, because they are the same thing.

Example 7: efficiency

A motor is supplied with 1200 J every second and transfers 900 J of it usefully to its kinetic store. Find its efficiency.

The efficiency is 75%. The other 300 J each second ends up in thermal stores, warming the motor and the air around it.

The Work, Energy and Power Calculator works out energy, power, time, work done and efficiency, and shows the steps.

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Work, Energy and Power Calculator

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Which energy transferred equation should I use?

Every one of them gives an energy in joules. For heating, our guide to specific heat capacity covers ΔE = mcΔθ.

Common mistakes

• Time in minutes or hours. In E = Pt with P in watts, t must be in seconds: 3 minutes is 180 s.
• Power in kW. 2.2 kW is 2200 W. Only use kW with hours, to get kWh.
• Charge in mC. 500 mC is 0.5 C.
• Mixing up Q and q in other topics. In electricity, Q is charge in coulombs; in some thermal physics books, Q means heat energy.
• Using E = QV with current. Q is charge, not current. If you are given a current, find Q = It first.

Are these equations on the equation sheet?

Yes. AQA's GCSE Physics equations sheet for June 2026, checked in November 2026, gives both: energy transferred = power × time, E = Pt, and energy transferred = charge flow × potential difference, E = QV. It also gives power = energy transferred ÷ time and power = potential difference × current. AQA's specification asks students to recall and apply both equations, so learn them anyway. Other boards publish their own sheets, so check yours.

Practise physics in a short timed paper

GCSE Physics Higher and GCSE Physics Foundation each give you a short timed paper every day, with 8 questions worth 25 to 35 marks across the course, so energy and electricity questions turn up regularly but not in every paper. The Electricity course covers charge, current and potential difference step by step.

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Physics · Electricity and magnetismWhat is potential difference? Definition, formula, examplesPotential difference is the energy transferred per unit charge between two points. The GCSE and A-level definitions, E = QV, pd vs emf, with examples.