What Is a kW vs kWh at a Charge Point?

A kilowatt (kW) measures how fast electricity flows into your car's battery, and a kilowatt-hour (kWh) measures how much energy is actually stored. Think of it this way: kW is the rate of the tap, kWh is how many litres end up in the tank. When you pull up to a charging cable and charge point, the kW rating tells you the speed available; the kWh your battery holds tells you how much it needs. Understanding both makes it much easier to predict how long a session will take and to compare charge points meaningfully.

Why are charge points rated in kW?

Charge points advertise their output in kW because that tells you the maximum speed they can deliver electricity. A slow three-pin socket offers around 2.3 kW. A standard home wallbox typically offers 7 kW. Public rapid chargers are rated much higher, sometimes at several tens of kilowatts. The kW figure is the ceiling, not necessarily what your car will accept. The actual charging speed is always governed by whichever limit is lowest: the charge point's output, your car's onboard charger, or, with rapid DC charging, the car's maximum DC acceptance rate.

Why do you pay in kWh?

When you pay at a public charge point, the pricing is almost always per kWh rather than per minute or per session. That is because kWh is a direct measure of the energy you've actually received, in the same way your home electricity bill charges for the units you use. It means paying for energy in kWh is the fairest method: a car that charges quickly takes the same kWh as one that trickles slowly to reach the same state of charge. Always check current rates at a charge point before plugging in, as pricing varies between networks and changes over time.

How does the tap-and-tank analogy help you estimate a session?

If you know roughly how depleted your battery is and you know the charge point's output, you can get a ballpark sense of duration. A battery that needs, say, half its capacity refilled will take roughly half as long as a full charge at the same kW rate. Your car's onboard display or the charge point's app will give a live estimate once charging begins, and that is always more reliable than calculating in your head. The analogy breaks down slightly towards the end of a session, because most cars deliberately slow charging above around 80 percent of capacity to protect the battery: the tap reduces its flow as the tank fills up.

Why does your car's onboard charger limit AC speed?

All AC charging, whether from a home wallbox or a public lamp-post unit, passes through your car's onboard charger before reaching the battery. That component converts AC mains power into the DC power batteries store, and it has its own maximum rating in kW. If your car's onboard charger is rated at 7 kW, plugging into an 11 kW or 22 kW AC charge point will still only deliver 7 kW. The extra capacity the charge point offers simply goes unused. This is why upgrading a charge point beyond your car's AC acceptance rate brings no speed benefit for AC sessions. DC rapid chargers bypass the onboard charger entirely, which is why they charge so much faster on supported vehicles.

Does UK mains power affect what speed you can get at home?

Yes. UK domestic electricity is single-phase, which limits most home wallboxes to around 7 kW. Three-phase supply, available in some commercial premises and occasionally in newer properties, can support wallboxes up to 22 kW on AC. If you see a charge point rated at 22 kW AC, it requires a three-phase connection, which most UK homes do not have. For most home charging situations, a 7 kW wallbox is the practical maximum, and for many drivers it is more than enough to top up overnight.

What about efficiency losses?

Not every kWh that leaves the charge point ends up stored in your battery. There are small losses during the conversion process: heat generated by the onboard charger, losses in the cable, and the electronics managing the charge. These losses are typically modest on modern EVs but they do mean you may use slightly more kWh from the network than your car reports as gained. When budgeting the cost of a charge, it is worth knowing that what you pay for and what arrives in the battery may not be exactly equal, though for most practical purposes the difference is small.

Frequently asked questions

What is the difference between kW and kWh in simple terms?

kW is the speed at which electricity is delivered, like the flow rate of a tap. kWh is the total amount of energy stored, like litres in a tank. A charge point's kW rating tells you how fast it charges; your car's battery capacity in kWh tells you how much energy it can hold.

Why does a high-kW charge point not always charge my car faster?

Your car's onboard charger has its own maximum rating. If a charge point offers more kW than your car's onboard charger can accept on AC, the car simply caps it at its own limit. The charge point's higher rating only benefits vehicles designed to take it. Always check your car's AC and DC acceptance rates in the owner's manual.

Why do some public charge points charge per minute rather than per kWh?

Some older charge point networks or slower units price by time rather than energy. This can be less fair if your car charges slowly. Per-kWh pricing is more transparent and increasingly common, especially on newer rapid charging networks. Always check how a network prices before starting a session.

Can I work out the cost of a charge session at home?

Roughly, yes. Multiply the kWh you need by your electricity unit rate in pence per kWh. If you need to add around 30 kWh and your tariff is 25 pence per kWh, that gives you 750 pence, or £7.50. Check your current tariff, as rates change and smart tariffs may offer cheaper overnight rates for EV charging.

Does DC rapid charging bypass the onboard charger?

Yes. DC rapid chargers deliver power directly to the battery, bypassing the onboard AC-to-DC converter entirely. That is why DC rapid charging is significantly faster than AC charging for vehicles that support it. Your car still controls the charge rate and will taper it above a certain state of charge to protect the battery.

What does kWh per 100 miles mean on an EV spec sheet?

It is an efficiency figure, like MPG for a petrol car but inverted. A lower kWh per 100 miles means the car uses less energy to cover the same distance. It helps you compare running costs and estimate how much a charge will take you. Figures on spec sheets are based on WLTP test conditions, which may differ from real-world driving.

Getting your head around kW and kWh is the first step to making confident decisions at the charge point. Once you know your car's onboard charger rating and your battery capacity, choosing the right public charger becomes straightforward. Browse our Type 2 charging cables to make sure you have the right lead for AC charge points, or explore our full portable EV charger range for flexible home and away charging options.