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Electric power
Beginner
Rate of energy use.
P = VI = I²R = V²/RP = VI = I²R = V²/R. Energy used E = Pt. A 2 kW heater on 230 V draws 8.7 A.
A 60 W bulb at 120 V has R = 240 Ω.
Use the form matching the known quantities.
In more detail
Electric power is the rate at which electrical energy is converted, P = VI = I²R = V²/R. It is measured in watts.
Real-world uses
- Appliance labels show power to help estimate running costs.
- Power lines are designed to limit losses using high voltages.
- Electricians check that a socket can supply the power of a kettle, using P equals V times I.
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Frequently asked questions
Which formula should I use?
Use P = VI generally, and P = I²R or V²/R when only two quantities are known.
Why is high voltage used for transmission?
It reduces the current for the same power, cutting losses in cables.
Which power formula should I use?
Use P = VI when you know both values, P = I²R when you know the current and resistance, and P = V²/R when you know the voltage and resistance.
Related topics in Current Electricity & Circuits
Resistors in series · Resistors in parallel · Colour code of resistors · Heating effect of current · Electrical energy and cost · EMF and internal resistance