The most common question we hear at EV Cable Hub is some version of this: “I’ve got a 7kW wallbox. Why is my car only charging at 3.7kW?” Or the reverse: “Will a 22kW cable charge my car faster?” The honest answer to both questions comes down to a component most people have never heard of: the onboard charger inside your car. Once you understand that, everything else clicks into place.
The number that actually controls your charging speed
What is an onboard charger?
Every electric vehicle contains a component called the onboard charger, or OBC. It sits between the cable socket and the battery, and its job is to convert alternating current (AC) from your wall or wallbox into direct current (DC) that the battery can actually store. Think of it as a gatekeeper with a fixed capacity: it will never let power in faster than its rated maximum, no matter what is on the other side of the cable.
That rated maximum varies significantly by model. According to manufacturer specification sheets, the Tesla Model 3 and Model Y both carry an 11kW onboard charger. The Kia EV6 and EV9, both built on the E-GMP platform, also accept up to 11kW AC. The Renault Zoe in its later versions takes this further, with a 22kW onboard charger that can genuinely exploit a three-phase supply. At the other end of the scale, older Nissan Leaf models were fitted with either a 3.3kW or 6.6kW OBC depending on the specification. The vast majority of other UK electric cars land at 7.4kW: the Volkswagen ID.3 and ID.4, the BMW i3, the Leaf on a standard UK spec.
That 7.4kW figure comes from the maths of single-phase AC power: 230V multiplied by 32A gives you approximately 7.36kW. It is the natural ceiling of a single-phase 32A supply, and it is the ceiling built into most UK EVs.
To understand what cable amperage means for charging speed in more detail, our cable amps explained article walks through the full calculation.
Why your cable is never the speed limit
This is the most persistent myth we encounter, and it is worth being direct: a cable cannot make your car charge faster. A cable is a conductor. It carries whatever current the system asks it to carry, up to its rated maximum. It does not generate power, negotiate charging rates, or influence how the onboard charger behaves.
Here is why this matters practically. A 32A Type 2 cable connected to a 7.4kW wallbox will deliver 7.4kW to a Tesla Model 3. A 32A Type 2 cable connected to a 22kW three-phase wallbox will also deliver 7.4kW to that same Tesla, because the car’s 11kW OBC is the ceiling on single-phase, and the single-phase supply limits things further to 7.4kW. The cable is identical in both scenarios. It is irrelevant to the outcome.
What is relevant is the combination of your supply, your wallbox output, and your car’s OBC rating. Charging speed settles at whichever of these three is lowest.
Miles per hour of charge: a real-world table
Numbers in kilowatts are abstract. What most drivers actually want to know is: how many miles of range will I recover in an hour? The table below uses an average real-world efficiency of approximately 3.5 miles per kWh (a reasonable figure for a typical UK EV in mixed driving) to translate charger output into something tangible.
| Charger output | What the car accepts | Miles added per hour | Example cars at this level |
|---|---|---|---|
| 2.3kW (10A granny cable) | Up to 2.3kW | ~8 miles/hr | Any EV (granny cable limits it) |
| 3.6-3.7kW (16A single-phase) | Up to 3.7kW | ~13 miles/hr | Most PHEVs (Outlander PHEV, many others) |
| 7.4kW (32A single-phase) | Up to 7.4kW | ~26 miles/hr | Most UK EVs: Leaf, VW ID.3/4, BMW i3 |
| 11kW (32A three-phase) | Up to 11kW | ~38 miles/hr | Tesla Model 3/Y, Kia EV6/EV9 |
| 22kW (32A three-phase) | Up to 22kW | ~77 miles/hr | Renault Zoe (later), some public AC posts |
Based on approximately 3.5 miles/kWh average efficiency. Your car’s actual efficiency will vary by model, speed, weather, and load.
- label: 2.3kW (granny / 10A), value: 8, unit: miles/hr
- label: 3.7kW (16A single-phase), value: 13, unit: miles/hr
- label: 7.4kW (32A single-phase), value: 26, unit: miles/hr
- label: 11kW (32A three-phase), value: 38, unit: miles/hr
- label: 22kW (32A three-phase), value: 77, unit: miles/hr
How to read this table
Two things are worth noting. First, the “miles added per hour” figure assumes your car can actually accept the power listed in column one. A Nissan Leaf with a 6.6kW OBC on a 22kW three-phase wallbox will not appear in the 22kW row. It will sit in the 7.4kW row, because the OBC is the ceiling. Second, these are approximate figures derived from average efficiency. A large SUV at motorway speeds uses more than 3.5 miles/kWh; a compact city car in urban traffic may exceed it considerably.
For a closer look at how the Tesla Model 3 performs specifically on UK charge points and which cables are compatible, see our Tesla Model 3 charging cable guide. For Kia EV6 and EV9 owners wondering about Kia EV6 charging cable options, we cover those in a dedicated guide as well.
The three-way bottleneck explained
Charging speed always settles at the lowest value among three components: your supply and wallbox output, your cable rating, and your car’s OBC. Almost without exception, the cable is not the constraint. The real contest is between the wallbox and the OBC. Here are three scenarios that cover most situations UK drivers encounter.
- Three nodes in series:
- 1. Wallbox / Supply: delivers up to X kW
- 2. Cable: carries whatever you give it (never the limit)
- 3. Onboard Charger (OBC): accepts up to Y kW
- Arrow between each, labelled power flows →
- Final outcome box: Actual charge speed = lowest of wallbox OR OBC
- Callout: highlight OBC node as the common ceiling; grey out Cable node as not the limit
Scenario A: 11kW car on a 7.4kW wallbox
A Tesla Model 3 (11kW OBC) is plugged into a standard 7.4kW single-phase wallbox with a 32A cable. The wallbox is the constraint. The car gets 7.4kW, roughly 26 miles per hour of range. The cable is not the issue. The car is capable of more, but the wallbox cannot deliver it on a single-phase supply.
To reach 11kW on this car, you would need a three-phase supply and a wallbox rated for 11kW or above, not a different cable. We explain the single-phase vs three-phase supply question in detail separately, covering when a three-phase supply is worth pursuing for UK homeowners.
Scenario B: 7.4kW car on a 22kW wallbox
A BMW i3 (7.4kW OBC, Type 2 connector) is plugged into a 22kW three-phase wallbox. The car accepts 7.4kW and no more. The wallbox delivers only what the car asks for. The driver is paying for a 22kW-capable installation but receiving the same 7.4kW they would get from a standard home wallbox. This is not a fault. It is physics.
The difference between a 7kW and 22kW wallbox matters only if your car can accept more than 7.4kW. Our comparison of 7kW vs 22kW charging cables covers when the investment in a 22kW setup actually pays off.
Scenario C: Everything matched
A later Renault Zoe (22kW OBC) is connected to a 22kW three-phase wallbox via a 32A Type 2 cable. All three components are matched. The car charges at approximately 22kW, adding roughly 77 miles of range per hour. This is the best-case scenario for AC home charging, and it requires all three elements to be correct, not just the cable.
Real-world factors that reduce charging speed
Even when wallbox, cable, and OBC are all matched correctly, the speed you see in practice may fall short of the theoretical maximum. Two factors account for most of the gap.
Cold weather and battery thermal management
In temperatures below approximately 5°C, battery management systems deliberately throttle how quickly cells accept charge. Lithium-ion chemistry becomes less efficient in the cold; pushing current into a cold battery too quickly can accelerate degradation and, in extreme cases, cause lithium plating. The BMS prioritises cell longevity over charging speed.
In practice, this means cold-weather charge rates may be 10-20% lower than you would see in mild conditions. A car that normally charges at 7.4kW might settle at 6-6.5kW on a cold January morning. Plugging in as soon as you arrive home, while the battery still retains heat from the drive, often produces faster charging than leaving it until the car and battery have fully cooled overnight.
State of charge and the 80% slowdown
Above approximately 80% state of charge, virtually every EV slows its charging rate significantly. This is not a fault or a software quirk; it is the fundamental physics of lithium-ion charging, which operates in two phases. During the first phase (constant current, roughly 0-80%), the charger pushes a steady current and power remains high. During the second phase (constant voltage, roughly 80-100%), current tapers off to avoid overcharging individual cells. The last 20% of a charge session can take nearly as long as the first 80%.
For daily home charging, the practical implication is straightforward: scheduling your car to charge to 80% overnight is both faster and better for long-term battery health than routinely charging to 100%. Most EVs allow you to set a charge limit in the car’s settings or companion app.
What overnight charging actually looks like
At 7.4kW, the output of a standard 32A home wallbox, most UK EVs will go from near-empty to 80% in roughly four to six hours. A car with a 64kWh usable battery (such as the Nissan Leaf 62kWh or the VW ID.4) takes about seven hours for a full charge at this rate, or roughly five hours to 80%. A Tesla Model Y Long Range, with a larger 75kWh pack, takes nine to ten hours to fully charge at 7.4kW. But from 20% to 80%, you are looking at five to six hours, which fits comfortably into a typical overnight window.
PHEVs carry smaller batteries and lower OBC ratings of 3.3-3.7kW, and they are faster in absolute terms. A plug-in hybrid with a 14kWh battery and a 3.7kW OBC will fully charge in under four hours even from a 16A supply.
The practical upshot for most UK EV owners is this: plug in when you get home, set an overnight charging schedule if your tariff has off-peak hours, and set your charge limit to 80% for daily use. You will wake up with a full battery and will likely never notice how long it took.
For Tesla owners setting up home charging, our Tesla Model 3 charging cable guide covers the specific cable and wallbox combinations that work best in UK homes.
Frequently asked questions
How fast does a 7kW charger charge an EV?
A 7.4kW wallbox adds roughly 26 miles of range per hour for a typical UK EV. From near-empty to 80% takes around four to six hours for most models with a 40-60kWh battery, which makes it ideal for overnight charging. Most UK domestic installations deliver 7.4kW on a single-phase 32A supply.
Does the charging cable affect how fast my EV charges?
No. The cable is a conductor: it carries the current but does not set the rate. Charging speed is determined by the lower of two limits: your car’s onboard charger rating and your wallbox or supply output. A cable with a higher current rating will not increase charging speed if neither the wallbox nor the OBC is the constraint.
Why is my EV charging slower than expected?
The most common reasons are: (1) your car’s onboard charger caps the rate below your wallbox output; (2) cold weather is causing the battery management system to throttle charge acceptance; (3) the battery is above 80% state of charge and has entered the slower constant-voltage phase; or (4) there is a phase mismatch: an 11kW car on a single-phase wallbox will only receive 7.4kW.
What is an onboard charger in an EV?
The onboard charger (OBC) is a component inside the car that converts alternating current (AC) from a wall socket or wallbox into direct current (DC) that the battery can store. Its kW rating is the absolute ceiling for AC charging speed. No cable or wallbox can push power in faster than the OBC allows.
How long does it take to charge an EV from empty to full at home?
On a 7.4kW wallbox, most UK EVs with 40-77kWh batteries take six to twelve hours for a full charge, or four to six hours to reach 80%. PHEVs with smaller batteries and lower OBC ratings (typically 3.3-3.7kW) usually charge fully in two to four hours even from a modest 16A supply.
Can a 22kW cable charge my car at 22kW?
Only if three conditions are met: your car’s onboard charger accepts 22kW (as in later Renault Zoe models), you have a three-phase supply at home or at the charge point, and your wallbox is rated for 22kW output. Most UK EVs have a 7.4kW OBC and will charge at 7.4kW regardless of the cable rating or wallbox size.
Does cold weather slow down EV charging?
Yes. Below approximately 5°C, the battery management system restricts how quickly the cells accept charge to prevent degradation and potential lithium plating. In our experience, this thermal throttling commonly reduces charge acceptance by 10-20% until the battery warms up, typically within the first few minutes of a charge session, especially if the car was recently driven.
What charging speed should I expect from a granny charger?
A standard 3-pin granny cable delivers approximately 2.3kW (10A), adding around eight miles of range per hour. It is adequate for PHEVs with small batteries and as a travel backup, but for daily full-EV use, a dedicated 7.4kW wallbox on a 32A circuit is strongly recommended.