Domestic Electricity

The kilowatt-hour — the energy transferred by a device of power 1 kW operating for 1 hour. It is a unit of energy, not power, despite the name.

energy (kWh) = power (kW) × time (h)

Converting to joules — 1 kWh = 1000 W × 3600 s = 3.6 × 10⁶ J = 3.6 MJ.

Example: a 2.5 kW heater used for 3.0 hours transfers 2.5 × 3.0 = 7.5 kWh. At 30p per kWh that costs 7.5 × 30 = 225p, or £2.25.

Why energy is billed in kWh rather than joules — a joule is inconveniently small for domestic use. The same 7.5 kWh is 2.7 × 10⁷ J, a number that means nothing on a bill.

Cost —

cost = energy (kWh) × price per kWh

The commonest error is to leave the time in minutes or the power in watts. Both must be converted first: kilowatts and hours, every time.

Mains supply — UK mains is about 230 V a.c. at 50 Hz. Alternating current reverses direction 50 times each second, which is why transformers work and why mains electricity can be distributed efficiently at high voltage and stepped down for use.

Live, neutral and earth — the live wire carries the alternating supply and is dangerous; the neutral completes the circuit and sits close to earth potential; the earth wire is a safety route that carries current to ground if a fault connects the live wire to an exposed metal case.

How a fuse and an earth work together — if the live wire touches a metal case, the earth wire provides a very low resistance path to ground. The resulting large current melts the fuse and disconnects the supply, making the appliance safe. The earth alone would not make it safe; it is the large current through the earth that blows the fuse.

Double insulation — an appliance with a plastic case and no exposed metal needs no earth wire, because no accessible part can become live. Such appliances carry a double-square symbol and a two-core lead.