Electrical power and energy · Grade 9
Electrical power is how fast energy is transferred, measured in watts (1 W = 1 J/s). Appliances that heat things – kettles, heaters, ovens – have the highest power ratings. Power companies charge for energy in kilowatt-hours, so knowing P = IV and E = Pt lets you work out the cost of running anything at home.
Key facts
- P = I × V, P = I²R, P = V² ÷ R.
- E = P × t (J = W × s).
- 1 kWh = the energy a 1 kW appliance uses in 1 hour = 3,600,000 J.
- Cost = energy (kWh) × price per kWh.
- A lamp labelled “12 V, 36 W” takes 3 A and has a resistance of 4 Ω when working normally.
- Energy saved by swapping to LEDs = (old power − new power) × time.