Volts, Amps and Watts: Your Extension's Electrical Units Explained

The electrics on an extension are described in three different units, and a quote or a datasheet will switch between them without explaining why. The supply is 230V, a circuit is protected by a 32A breaker, an oven is rated at 7.2kW, and a plug carries a 13A fuse. Volts, amps and watts each measure a different thing, but they are tied together by one simple relationship. Once you can read all three, the wiring schedule on your consumer unit and the rating plate on an appliance stop being a mystery.

The three units at a glance

A handy way to keep them straight: volts is the pressure in the pipe, amps is how fast the water flows, and watts is the useful work that flow does at the other end.

How the three connect

The three units are not independent. Power equals pressure multiplied by flow, which for electricity means:

Because UK mains voltage is fixed at 230V, the formula does something useful: it lets you work out the current an appliance will draw straight from its power rating. Divide the watts by the volts to get the amps. A 3kW kettle is 3000W divided by 230V, which is about 13A. That is exactly why a standard plug fuse tops out at 13A, and why a kettle is about the most powerful thing you can run from an ordinary socket.

Volts: the pressure of the supply

Voltage is the electrical pressure your supply delivers. UK domestic mains is a nominal 230V. The figure was harmonised across Europe from the old British 240V, but the physical network never changed, so most installations still measure close to 240V on a meter. Both readings are normal and sit inside the permitted tolerance. For almost everything you do on an extension, the voltage is a fixed background figure you do not touch. You will see it printed on appliance rating plates as 230V or 230-240V, and it matters mainly because it is the number you divide into the wattage to get the current.

A few jobs run at higher pressure. The street feed before it reaches your home is much higher, and a small workshop machine might want a heavier supply. Inside the house, though, 230V is the constant you can rely on.

Amps: why circuits and breakers are rated in current

Current, measured in amps, is the unit that decides how an extension is wired. Cable can only carry so much current before it overheats, so every circuit is protected by a breaker, a miniature circuit breaker (MCB), rated in amps. If the current tries to exceed that rating, the breaker trips and cuts the circuit off.

Here is the part most guides skip, and the single idea that explains every circuit decision on your wiring schedule: the breaker protects the cable, not the appliance. Its job is to trip before the cable can overheat and start a fire, so its amp rating is matched to what that thickness of cable can safely carry. A thicker cable carries more current, so it gets a higher-rated breaker. The appliance looks after itself with the fuse in its own plug.

That is why circuits come in standard combinations of cable size and breaker rating:

A kitchen socket ring final circuit runs in 2.5mm cable at 32A, and that one circuit safely runs every socket in the room. A lighting circuit in thinner 1.5mm cable sits at 6A, which is plenty. A high-load appliance gets its own dedicated circuit on a thicker cable and a higher-rated breaker. This is why a wiring schedule lists circuits by amp rating: the breaker is policing the cable, and cable capacity is a current limit. The full set of circuit types is on the first fix electrics task.

Why a 7.2kW oven runs on a 32A circuit

At full power a 7.2kW oven and hob would pull about 31A, right at the ceiling of a 32A circuit, which seems to leave no room. In practice it never reaches that. You almost never run every ring and the oven flat out at the same moment, so the real continuous draw is far lower. Electricians size circuits using diversity, an allowance for the fact that not everything runs at once. After diversity a typical oven and hob comes out around 16A, sitting comfortably inside the 32A circuit. The same allowance is why a single ring final can feed a whole kitchen of appliances on one 32A breaker.

Watts and kilowatts: why a high-load appliance needs its own circuit

Power, in watts or kilowatts, is the headline figure on every appliance: how much energy it uses, and so how much heat or work it produces. The threshold that decides whether something can plug in or needs its own circuit is the socket itself. A 13A plug fuse is the largest made, so the most a single socket can deliver is 3,000W at 230V. That is why a 3kW kettle is about the most powerful thing you can run from a wall socket, and anything above it has to be wired in directly.

Run the bigger appliances through W = V x A and the reason for separate circuits is obvious:

• A 3kW kettle draws 13.04A, the exact plug-fuse limit, so it just squeaks onto a normal socket.
• A 10.5kW electric shower draws about 46A, far more than a socket circuit's 32A ceiling and more than its cable could carry. It needs heavy 10mm cable and a 45 A breaker on its own radial circuit.
• A 7.4kW EV charger draws about 32A, so it gets a dedicated circuit at 32A with 6mm cable.

The same logic covers a cooker and an immersion heater. Total up the power a circuit might be asked to deliver, convert it to amps, and the cable and breaker have to be big enough to carry it. A high-load appliance asks for more current than a shared circuit can safely give, so it is split off onto its own supply.

Plug fuses and the double-socket trap

The fuse inside a UK plug works on the same idea as the breaker, one level down: it protects the thin flex of that one appliance. The fuse is matched to the appliance, not fixed at 13A. A table lamp, laptop charger or anything under about 700W takes a 3A fuse; a kettle, heater, iron or anything near 3kW needs the full 13A. Fitting a 13A fuse to a lamp leaves its slim flex unprotected, so the right fuse matters.

A double socket is a common source of confusion: it is not double the power. Both outlets feed off the same ring final circuit and share its 32A ceiling, so plugging two 3kW heaters into one double socket tries to pull more than the circuit allows. The number of holes in the wall does not change how much the circuit behind them can deliver.

kW and kWh: power now versus energy over time

The one figure that trips up most homeowners is the difference between a kilowatt and a kilowatt-hour. A kilowatt (kW) is power: the rate an appliance uses energy at this instant. It is the number stamped on the rating plate. A kilowatt-hour (kWh) is energy: the total used over a stretch of time. It is what your meter counts and your supplier bills you for.

The link is simply power multiplied by time. A 1kW appliance left on for one hour uses one kWh. A 10.5kW shower sounds enormous, but run it for three minutes and it uses only about half a kWh, because it is on for such a short time. Despite how it sounds, a kilowatt-hour is not "kilowatts per hour"; it is kilowatts multiplied by hours. Your circuits and breakers are sized in amps and kW, never in kWh. The kWh figure belongs on the bill, not the wiring schedule, and keeping the two apart is the difference between understanding your quote and your energy costs.