Electrical energy
When a current flows through a component, electrical energy is converted into other forms such as heat and light. The electrical energy, E, transferred is given by E = V I t, measured in joules (J). Using V = IR, this can also be written as E = I² R t or E = V² t / R.
Electrical power
Electrical power, P, is the rate of transfer of electrical energy — the energy converted per second. It is measured in watts (W), where 1 W = 1 J s⁻¹.
Key formula
P = V I = I² R = V² / R
and E = P t = V I t. P in watts, E in joules, t in seconds.
The kilowatt-hour
Electricity companies charge for energy in kilowatt-hours (kWh), a larger unit. One kilowatt-hour is the energy used by a 1 kW appliance in 1 hour. Energy (kWh) = power (kW) × time (h). The cost is then energy in kWh multiplied by the tariff per kWh.
Worked example
An electric heater is rated 240 V, 2000 W. Find the current it draws and the energy used in 3 hours in kWh.
Current: I = P / V = 2000 / 240 = 8.33 A.
Energy: power = 2000 W = 2 kW, so E = 2 × 3 = 6 kWh.
If electricity costs RM0.40 per kWh, cost = 6 × 0.40 = RM2.40.
Remember
- Power rating '240 V, 2000 W' means 2000 W at 240 V.
- 1 kWh = 3.6 × 10⁶ J.
- Choose P = I²R when I is known, P = V²/R when V is known.