What are amps, volts, watts and kWh?

Glossary · Wiring, circuits and faults

How the four units fit together

Electrical Safety First explains the first three with a hosepipe.^{[1]Electrical Safety First, Amps, Watts and Volts Calculator} Amps measure the flow of electricity as an electric current, like the amount of water flowing. Volts measure the force that makes the current flow, like water pressure. Watts measure the power, like the amount of water released. Amps multiplied by volts equals watts.^{[1]Electrical Safety First, Amps, Watts and Volts Calculator}

The same page gives the reverse: watts divided by volts equals amps.^{[1]Electrical Safety First, Amps, Watts and Volts Calculator} For single-phase home chargepoints, the government’s technical specification describes a 230 V supply.^{[5]Office for Zero Emission Vehicles, Residential chargepoints: minimum technical specification} The UK mains is also alternating current, which changes direction 50 times a second.^{[1]Electrical Safety First, Amps, Watts and Volts Calculator}

The fourth unit is about time. Energy from electricity is generally measured in kilowatt hours, and an Ofgem factsheet explains that a kWh equates to the use of 1,000 watts of electricity for a full hour.^{[3]Ofgem, Electricity generation: facts and figures (Ofgem factsheet)} Ten 100 watt light bulbs all lit for a full hour use one kWh.^{[3]Ofgem, Electricity generation: facts and figures (Ofgem factsheet)}

kW compared with kWh

The kettle figures come from Electrical Safety First’s appliance table.^{[2]Electrical Safety First, Ratings of electrical appliances} Think of kW as the speed shown on a car’s dial and kWh as the distance travelled. A 7 kW EV charger is a rate. The kWh is how much energy has gone into the car by the end of the session. Government specifications describe standard AC home chargepoints by their kW output, between 3.5kW and 7kW.^{[5]Office for Zero Emission Vehicles, Residential chargepoints: minimum technical specification} See home EV charging time for how the two combine.

Why the units matter at home

The practical use of watts and amps is to avoid overload. Electrical Safety First says every circuit in your home has a maximum current it can handle safely, and that plugging high-wattage items into low-amp extension leads generates dangerous heat.^{[1]Electrical Safety First, Amps, Watts and Volts Calculator} Its appliance table lists the highest figures it could find for popular items. For example, it gives a hair dryer as 10 amps and 2,200 watts, a washing machine as 10 amps and 2,200 watts, and an iron as 13 amps and 3,000 watts.^{[2]Electrical Safety First, Ratings of electrical appliances} It describes those figures as indicative only.^{[2]Electrical Safety First, Ratings of electrical appliances}

The kWh matters for your bill. Ofgem says each unit of electricity is charged at the unit rate, calculated in kilowatt hours, and that you also pay a standing charge every day.^{[4]Ofgem, Energy price cap and standing charges explained} See off-peak EV tariff for tariffs that vary the unit rate by time of day.

Working out what a circuit can safely carry is a job for a registered electrician. See overload and cable sizes, and our general electrical maintenance page.

Example

Sana in Meanwood has a 3,000 watt heater. Electrical Safety First’s table puts a heater of that rating at up to 13 amps.^{[2]Electrical Safety First, Ratings of electrical appliances} She leaves it on for 2 hours in the evening. At 3 kW for 2 hours it uses 6 kWh of energy. The amps tell her what her socket and lead must carry at any moment, while the kWh tell her what she will be charged for.

What people get wrong

The most common mix-up is treating kW and kWh as the same thing. A kW is how fast something uses power. A kWh is how much it has used. A 3 kW heater uses 3 kWh in an hour, 6 kWh in two hours and still draws 3 kW the whole time.

A second mistake is assuming amps alone tell you the risk. A high current through a lead rated for less will heat it, which is why the rating marked on the lead matters as much as the appliance.^{[1]Electrical Safety First, Amps, Watts and Volts Calculator}

The bottom line

Amps are flow, volts are push, watts are power and kWh are energy over time. Multiply amps by volts to get watts, and divide watts by volts to get amps. Your bill is in kWh, while the safety of a circuit or lead depends on amps. If you are unsure whether a circuit can carry a new appliance, ask a registered electrician.