METRONMETHOD. REASON. LEARN.
Electricity · 12 min

Electrical power and energy

Energy bills charge mainly for energy used over time, while appliance labels often state power. Distinguishing the two makes bills easier to interpret.

01Why this matters

Energy bills charge mainly for energy used over time, while appliance labels often state power. Distinguishing the two makes bills easier to interpret.

02See it

power 1.5 kWtime 2 hE = P × tenergy 3 kWh
Read left to right: power 1.5 kW → time 2 h → E = P × t → energy 3 kWh. Each panel connects one part of the example.

03Understand it

Power is the rate of energy transfer. Electrical power in a steady direct-current circuit is voltage multiplied by current.

Use P = VI for a steady DC load. With constant power, E = Pt. Watts multiplied by seconds give joules. Kilowatts multiplied by hours give kilowatt-hours. A kilowatt-hour is an energy unit, not a power unit. Actual appliance consumption can vary as controls cycle on and off.

04Guided example

A 1.5 kW heater runs at constant power for 2 h. Find energy use.

  1. Power is already in kilowatts and time is in hours.
  2. Multiply: E = 1.5 × 2 = 3 kWh.
  3. Convert if needed: 3 × 3.6 million = 10.8 million J.

05Try it

DC voltage 12 V, current 2 A. Power?
A little hint

Multiply V by I.

06Practise it

Power 2 kW for 3 h. Energy?
A little hint

Multiply power by time.

Energy 4 kWh at $0.25/kWh. Usage cost?
A little hint

Multiply energy by price per unit.

07Use it in the real world

At an illustrative energy rate of $0.30 per kWh, 3 kWh costs $0.90 for usage alone. Supply charges and other tariff components are additional.

Go a little deeper

For AC circuits, average real power can depend on power factor; multiplying RMS voltage by RMS current alone gives apparent power. Household cost estimates here start from stated real power or measured energy.

08Quick check

Power 10 W for 6 s. Energy?
A little hint

Watts times seconds gives joules.

09Summary

Power is the rate of energy transfer. Electrical power in a steady direct-current circuit is voltage multiplied by current. Convert if needed: 3 × 3.6 million = 10.8 million J.

Before moving on, explain the worked example in your own words. Use the hint on any question you want to revisit.

10Key idea

P = VI; E = Pt

P: W; V: V; I: A; t: s; E: J. Alternatively use kW and h to get kWh.

t = E/P; I = P/V for non-zero divisors

Find this in the formula library →