Define electric power and derive P = VI. What is the kilowatt hour? Show that 1 kWh = 3.6 × 10⁶ J.

1. 1 · Define power. Electric power is the rate of using electrical energy: P = W/t. Unit watt (W).
2. 2 · Derive P = VI.
   • W = VQ and Q = It, so W = VIt.
   • P = W/t = VIt/t = VI.
   • With V = IR: P = I²R = V²/R.
3. 3 · Define the kilowatt hour. The energy used by a 1000 W appliance in 1 hour; the commercial unit, called 1 unit.
4. 4 · Convert. 1 kWh = 1000 W × 3600 s = 3.6 × 10⁶ J.
5. 5 · Real-life link. A 60 W fan running for 10 hours uses 600 Wh = 0.6 kWh, which is 0.6 unit on the bill.
6. 6 · Close. So the bill counts energy in units, where 1 unit = 1 kWh = 3.6 × 10⁶ J.

Scoring tip: In bill sums, change watt to kilowatt (divide by 1000) before multiplying by the hours.