Why a 150kW Charger Rarely Gives You 150kW

Charge point listings advertise a headline number: 7kW, 50kW, 150kW, 350kW. Drivers reasonably assume that number is what they will get. It almost never is, and the gap between the sticker and reality is the single biggest source of disappointment for new EV owners.

Nothing is being misrepresented. The headline is the charger's maximum output, and a charging session is a negotiation between three parties, of which the charger is only one.

The four speed bands, and what each is actually for

Slow — under 7kW. Three-pin granny cables and older 3.7kW posts. Roughly 8 to 15 miles of range per hour. This is overnight charging and nothing else. On PlugMap these are the grey markers.

Fast — 7kW to 22kW. The workhorse band, and the majority of public charge points in the UK. A 7.4kW post adds around 25 miles per hour, so a typical family EV goes from nearly empty to full in seven or eight hours. This is destination charging: you leave the car while you do something else. Supermarket car parks, leisure centres, hotels, workplaces, on-street residential bays.

Rapid — 50kW to 99kW. DC charging. Around 100 miles of range in half an hour on a typical car. This is the band that makes longer journeys practical, and 50kW units are the backbone of the existing motorway and A-road network.

Ultra-rapid — 100kW and above. 150kW and 350kW units, concentrated at motorway services and purpose-built hubs. On a car that can take it, 10 to 80% in around 20 minutes. Not every car can take it, which is the crux of the whole thing.

Three ceilings, and you get the lowest one

Your actual charging speed is the minimum of three separate limits.

The charger's rating. The advertised figure. Note that on many multi-bay ultra-rapid units the total output is shared: two cars on one 350kW cabinet may get around 175kW each. Some sites handle this gracefully and some do not.

Your car's maximum accepted rate. Every EV has a peak DC charging rate written into its spec sheet, and the range across cars on sale is enormous — roughly 50kW at the modest end to over 250kW at the top. Plug a car with a 50kW peak into a 350kW charger and you get 50kW. The charger is not underperforming; your car asked for 50 and got it. This is worth looking up for your own car once, because it tells you whether hunting for ultra-rapid sites is worth the detour or a waste of a mile.

What the battery will accept right now. This is the variable one, and it is where the charging curve comes in.

The charging curve

A lithium-ion battery cannot accept its maximum current across the whole state of charge. It takes power greedily when fairly empty, then progressively tapers as it fills, because forcing current into a nearly-full cell generates heat and degrades it. The battery management system protects the pack by throttling back, and it is right to.

A typical DC session looks like this. From 10% you get close to the car's peak rate. Somewhere between 50 and 70% the rate starts stepping down noticeably. By 80% you may be at half your peak. From 80 to 100% the curve flattens badly, and those last twenty percentage points can take as long as the first sixty.

This is why experienced drivers talk about 10 to 80% rather than "a full charge". On a long journey, two stops from 10 to 80% are usually faster overall than one stop to 100%, because you spend the whole of that second stop in the fast part of the curve instead of crawling through the slow part. Charging to 100% on a rapid unit is almost always the wrong call unless the next leg genuinely requires it.

Cold batteries charge slowly

The other big variable is temperature. A cold battery has higher internal resistance and the management system will limit DC charging hard to protect it. Arrive at a rapid charger on a January morning after a short drive from a cold driveway and you may see a fraction of the rate you would get in July.

Many modern EVs can precondition the battery — warming it on the move so it arrives ready to take full power. On most cars this triggers automatically when you set a rapid charger as your navigation destination, which is a genuinely good reason to route through the car's own sat-nav rather than your phone on a winter trip. If your car has the feature, use it; the difference can be ten or fifteen minutes on a single stop.

Why you pay in kWh, not minutes

UK public charging is priced per kilowatt-hour — per unit of energy delivered — not per minute. This matters, because it means a slow session is not a more expensive session. If your car limits you to 50kW at a 150kW charger, you pay for the energy you received and nothing more. You lose time, not money.

The exception is overstay fees, which several networks apply once your session ends or once you have been in the bay beyond a set period. These are charged per minute and they are not trivial. They exist to keep bays turning over rather than to make money, but they will still catch you if you wander off to a long lunch.

What this means for how you actually charge

Look up your car's peak DC rate once and remember it. If it is 50kW, ultra-rapid hubs buy you nothing and any working rapid unit is as good as the best one in the county.

Aim to arrive at a rapid charger with a fairly low state of charge. Turning up at 60% and charging to 90% is the slowest, least efficient use of a rapid bay there is.

Stop at 80% unless you have a specific reason not to, and do your topping-up to 100% overnight on AC where the taper costs you nothing but hours you were asleep for anyway.

Precondition in winter if your car supports it.

And for everyday use, remember that the fast band — a 7kW post while you do the weekly shop — quietly handles most charging for most drivers. Rapid charging is for journeys, not for Tuesdays.