A home EV wallbox might be rated at:
- 7 kW
- 11 kW
- 22 kW
while the car itself might advertise:
- 150 kW
- 200 kW
- 250 kW rapid charging
Yet the same vehicle may only charge at 7 kW or 11 kW at home.
That is because AC home charging and DC rapid charging use different parts of the charging system.
At home, the vehicle’s onboard charger plays a central role.
The wallbox offers AC power, but the onboard charger sets the vehicle’s maximum AC conversion capability — and the car may still choose to accept less.
And the most important rule is:
The lowest relevant limit in the charging chain sets the actual charging power.
What Is an EV Onboard Charger?
The onboard charger is power-electronic equipment built into the vehicle.
Its job during AC charging is to convert AC electricity → DC electricity.
The home’s electrical supply provides AC electricity.
The traction battery stores energy as DC.
The onboard charger sits between those two systems.
The onboard charger is the power converter that makes AC home charging possible.
Why Does the Battery Need DC?
The electricity supplied to a normal home is AC.
The vehicle battery cannot simply store that AC electricity directly.
During AC charging, the power path is:
Grid AC → Wallbox → Onboard Charger → Battery DC
The onboard charger performs the conversion inside the vehicle.
During home AC charging, the electricity remains AC until it reaches the vehicle’s onboard charger.
Is the Wallbox Actually the Charger?
Homeowners normally call the wall-mounted unit the EV charger.
That is perfectly understandable.
But during AC charging, the wallbox is not usually doing the main AC-to-DC conversion.
The wallbox mainly:
- provides the controlled AC supply
- communicates available current to the vehicle
- manages charging control
- provides relevant safety functions
The vehicle then converts the AC electricity using its onboard charger.
The wallbox controls the supply. The onboard charger converts the electricity.
AC and DC Charging Use Different Paths
- AC Home Charging
- Grid AC → Wallbox → Vehicle Onboard Charger → Battery DC. The onboard charger is part of the charging power path.
- DC Rapid Charging
- Grid → Rapid Charger AC/DC Conversion → Vehicle Battery. The large AC-to-DC conversion happens inside the rapid-charging station, and the vehicle's normal onboard AC charger is largely bypassed.
This is one of the most important things to understand about EV charging.
DC rapid charging moves the large AC-to-DC converter out of the car and into the charging station.
This is why AC and DC charging specifications can be dramatically different.
Why Can a 150 kW Car Only Charge at 11 kW at Home?
Example
A vehicle specification says maximum AC charging 11 kW and maximum DC rapid charging 150 kW.
Both figures can be completely correct.
At home, AC electricity passes through the vehicle's onboard charger, which may be limited to 11 kW.
At a compatible DC rapid charger, the external charger performs the high-power AC-to-DC conversion instead, so the vehicle may accept much higher DC charging power.
AC charging capability and DC rapid-charging capability are separate vehicle specifications.
And:
A car can accept 150 kW from a DC rapid charger while still being limited to 11 kW from an AC home charger.
The Complete AC Charging Chain
- Grid
- Electrical Installation
- Wallbox
- Charging Cable
- Onboard Charger
- Battery Management System
- Battery
Actual charging power depends on more than one component.
Any part of that chain can impose a lower limit.
Possible constraints include:
- available property supply
- charger configuration
- dynamic load balancing
- charging cable capability
- onboard charger rating
- vehicle battery controls
The lowest relevant limit in the charging chain sets the actual charging power.
The Onboard Charger Sets the AC Ceiling
Imagine the wallbox is capable of 22 kW. But the vehicle’s onboard charger is capable of 11 kW.
The car cannot suddenly charge at 22 kW AC. Its maximum AC charging capability remains around 11 kW.
Likewise, if the vehicle only accepts 7.4 kW AC, a 22 kW wallbox cannot force 22 kW into the car.
A wallbox cannot make the car accept more AC power than its onboard charger is designed for.
The onboard charger therefore creates an upper limit.
The onboard charger sets the AC ceiling. The actual charging rate can still sit below it.
Offered Power, Vehicle Capability and Accepted Power Are Different
- Offered Power
- What the wallbox is currently able or allowed to provide.
- Vehicle Capability
- The maximum AC power the onboard charger can convert.
- Accepted Power
- What the vehicle is actually taking at that moment.
There are three useful numbers to separate.
For example, the wallbox offers 11 kW, the vehicle onboard charger capability is 11 kW, and the vehicle currently accepts 8 kW.
The lower actual rate may be caused by vehicle conditions or control strategy.
Available power, vehicle capability and actual accepted power are three different numbers.
What Is the Battery Management System?
- Onboard Charger
- Converts AC electricity into DC.
- Battery Management System — BMS
- Monitors and protects the battery. It may influence charging based on battery temperature, state of charge, cell conditions, protection limits and thermal-management requirements.
The onboard charger and the battery-management system are not the same thing.
A simple way to think about the hierarchy is:
Wallbox offers → onboard charger converts → BMS permits
The onboard charger converts the power. The battery-management system decides what the battery is prepared to accept.
Why Might a Car Charge Below Its Onboard Charger Rating?
Imagine a vehicle has 11 kW maximum AC charging.
That does not mean it will draw exactly 11 kW for every minute of every charging session.
Actual power may be lower because of:
- limited property headroom
- dynamic load balancing
- charger configuration
- battery temperature
- battery protection
- state of charge
- thermal conditioning
- charging schedule
The onboard charger may be capable of 11 kW while the vehicle deliberately accepts less.
The rating is a maximum capability. Not a guaranteed constant power.
Common EV Onboard Charger Ratings
| Vehicle AC Specification | Approximate Power |
|---|---|
| 1 × 16 A | 3.7 kW |
| 1 × 32 A | 7.4 kW |
| 3 × 16 A | 11 kW |
| 3 × 32 A | 22 kW |
EV AC charging capabilities vary by vehicle.
The important point is that manufacturers may state AC capability using kW, number of phases, or current per phase.
If the specification is shown in phases and amps instead of kW, it is still describing the vehicle’s AC charging capability.
What Does 1 × 16 A Mean?
This means the vehicle is using one phase at approximately 16 A.
At nominal UK voltage: 230 V × 16 A ≈ 3.7 kW.
What Does 1 × 32 A Mean?
This means one phase at 32 A.
Approximate power: 230 V × 32 A ≈ 7.4 kW.
This is a common domestic single-phase charging level.
What Does 3 × 16 A Mean?
This means the onboard charger can use three phases at approximately 16 A per phase.
That gives roughly 11 kW of three-phase AC charging power.
What Does 3 × 32 A Mean?
This means three phases at approximately 32 A per phase.
That gives roughly 22 kW AC charging power.
Only vehicles designed to accept that level of AC charging can use the full capability.
Why 11 kW Is an Important Charging Level
EV charging is often discussed as 7 kW vs 22 kW.
But 11 kW is an important middle step.
Many three-phase-capable EVs support around 11 kW AC even when connected to a 22 kW wallbox.
That makes 11 kW a very relevant home-charging specification.
11 kW is a common practical step above normal single-phase home charging, but the exact vehicle specification still matters.
Does a 22 kW Wallbox Always Charge Faster?
No.
Suppose the vehicle has 11 kW maximum AC charging.
Connected to an 11 kW wallbox, maximum vehicle charging is about 11 kW.
Connected to a 22 kW wallbox, maximum vehicle charging is still about 11 kW.
The wallbox has additional capability. But the vehicle cannot use it.
A larger wallbox only makes charging faster if the car can accept the additional AC power.
Three Phase at the House Is Not Enough
Imagine a property already has three-phase electricity and the homeowner installs a 22 kW wallbox.
But the EV has 7.4 kW single-phase onboard charging capability.
The vehicle does not become a 22 kW AC car because the property has three phase.
Three phase at the property only creates faster charging if the vehicle can use the relevant three-phase AC supply.
Does an 11 kW Car Always Charge at 7.4 kW on Single Phase?
Not necessarily.
A vehicle may advertise 11 kW three-phase AC charging, but its maximum charging rate when connected to single phase can vary by vehicle design.
Do not assume that every 11 kW three-phase car will automatically use the full 32 A single-phase capability.
A vehicle’s three-phase AC rating does not automatically tell you its maximum single-phase charging rate.
The exact single-phase specification should be checked for the specific vehicle.
Type 2 Does Not Mean 22 kW
Another common misconception is that a Type 2 socket tells you the charging power.
It does not.
Type 2 describes the electrical connection interface.
A Type 2-equipped car might support 3.6 kW AC, 7.4 kW AC, 11 kW AC or 22 kW AC depending on the vehicle.
A Type 2 socket tells you how the vehicle connects. It does not tell you how much AC power the vehicle can accept.
The Charging Cable Can Also Limit Power
With an untethered wallbox, the removable charging cable also needs to support the intended charging arrangement.
The cable may have limits relating to current, number of phases and design rating.
This means a complete high-power charging session requires compatible property supply, wallbox, cable and vehicle.
The car, wallbox and cable all need to support the intended charging power.
Why Can a Plug-In Hybrid Charge Slowly on a 7 kW Wallbox?
Plug-in hybrids often have smaller batteries. Some also have lower onboard AC charging limits.
Imagine a wallbox at 7 kW and a PHEV onboard charger at 3.6 kW.
The car may only draw around 3.6 kW even though the wallbox can offer more.
A powerful wallbox does not make a lower-power plug-in hybrid charge faster than the vehicle allows.
Battery Capacity and Onboard Charger Power Are Different
- 80 kWh
- Describes approximately how much energy the battery can store.
- 11 kW
- Describes how quickly the vehicle can convert AC charging power under suitable conditions.
Suppose an EV specification shows a battery of 80 kWh and an onboard AC charger of 11 kW.
Those two numbers answer completely different questions.
Battery capacity is measured in kWh. Onboard charger power is measured in kW.
The Charging Job Is Not Usually the Whole Battery
Imagine a battery capacity of 80 kWh where the day’s driving used 25 kWh.
The car does not normally need another 80 kWh overnight. It needs to replace roughly 25 kWh plus charging losses.
The onboard charger sets the maximum rate. The energy used since the last charge sets the size of the charging job.
This is a much better way to think about home charging.
How Long Will AC Charging Take?
Example
A simple estimate is charging time ≈ required energy ÷ actual charging power.
Energy required 33 kWh. At 7 kW: 33 ÷ 7 ≈ 4.7 hours. At 11 kW: 33 ÷ 11 ≈ 3 hours. At 22 kW: 33 ÷ 22 ≈ 1.5 hours.
These are simplified theoretical figures before charging losses and vehicle controls.
But now suppose the vehicle onboard charger is limited to 11 kW. Connecting it to a 22 kW wallbox does not create a 1.5-hour charge. Approximate time remains 3 hours.
Energy at the Wall Is Not the Same as Energy Stored in the Battery
Charging is not perfectly lossless.
Some electricity can be used by onboard power electronics, cooling systems, pumps, battery conditioning and vehicle control electronics.
If the wallbox reports 20 kWh delivered, the traction battery may gain slightly less than 20 kWh.
The wallbox measures energy delivered to the car; the traction battery stores slightly less because charging itself uses energy.
Power Entering the Car Is Not Always Power Entering the Battery
Imagine the car is plugged in during winter.
The vehicle may be heating the battery, warming the cabin, running pumps and operating control systems.
Some incoming power may be used by those systems instead of increasing battery state of charge.
Power entering the car is not always the same as power entering the battery.
This is one reason observed charging speeds can differ from simple calculations.
Can Very Slow AC Charging Be Less Efficient?
Potentially.
The vehicle may consume a certain amount of power simply because the charging session is active.
At very low charging rates, those fixed electrical loads can represent a larger percentage of the incoming power.
The effect varies between vehicles, temperatures and charging conditions. So there is no universal rule.
Very low charging power can increase the proportion of energy used by the vehicle’s own charging systems, although the effect varies by model and conditions.
How Does Dynamic Load Balancing Affect the Onboard Charger?
Imagine a vehicle onboard charger of 11 kW and a wallbox of 11 kW. But the home currently only has 6 kW of available electrical headroom.
Dynamic load balancing may tell the wallbox to offer around 6 kW. The vehicle cannot take 11 kW because the house is not currently offering it.
Later, household demand falls. Available headroom increases to 11 kW. The wallbox can now offer more, and the car can potentially reach its full 11 kW AC capability.
The onboard charger sets what the vehicle could accept. Dynamic load balancing sets what the home can offer right now.
Smart Charging Can Also Reduce the Offered Power
Imagine vehicle capability 11 kW and property capability 11 kW. But the electricity tariff becomes cheap at 00:30.
The charger may deliberately wait. Before 00:30: 0 kW. After 00:30: up to the available charging power.
The vehicle’s capability has not changed. The charging schedule has.
Charging capability, electrical availability and charging schedule are separate constraints.
Solar Charging Can Also Be Below the Onboard Charger Limit
Imagine a vehicle onboard charger of 11 kW, a wallbox of 11 kW and a solar surplus of 2.5 kW.
If the charger is configured to follow surplus solar and the charging equipment and vehicle can operate appropriately at that level, the car may charge at around the available solar power rather than its 11 kW maximum.
A large onboard charger does not force the car to charge at maximum power. It simply defines the upper AC limit.
Why Can DC Rapid Chargers Use So Much More Power?
High-power DC charging stations contain substantial electrical hardware.
This can include large power converters, cooling systems and heavy electrical infrastructure.
The charger performs the AC-to-DC conversion outside the vehicle.
That means the car does not have to carry equivalent high-power conversion equipment everywhere it goes.
One reason DC rapid charging can use far higher power is that the large conversion equipment stays at the charging station instead of travelling around inside the car.
A Larger Onboard Charger Does Not Mean Faster DC Charging
Example
Car A — AC charging 7.4 kW, DC charging 200 kW.
Car B — AC charging 22 kW, DC charging 130 kW.
Car B has the larger onboard AC charger. But Car A may charge much faster at a suitable DC rapid charger.
A larger onboard charger does not automatically mean faster DC rapid charging.
The AC and DC specifications should be compared separately.
The Onboard Charger Controls kW, Not Miles
EV charging speeds are sometimes described as miles added per hour.
That can be useful as a rough illustration. But it depends heavily on vehicle efficiency.
An efficient car can travel further on each kWh than a less efficient car.
The onboard charger itself is rated in kW, not miles.
The onboard charger controls kW, not miles. Range added depends on how efficiently the vehicle uses each kWh.
How Do I Find My Car's Onboard Charger Rating?
The most useful specification to look for is maximum AC charging power.
It may also appear as:
- AC charging rate
- onboard charger
- OBC
- AC input
- phases and amps
Do not confuse this with battery capacity, maximum DC charging power or rapid-charging headline figures.
If you want to know how fast the car can charge from a home wallbox, look for its maximum AC charging power.
Check the Exact Vehicle Variant
Charging specifications can vary between model years, trim levels, battery versions, option packages and different markets.
So do not rely only on the model name.
Check the exact model year and variant, not just the badge on the boot.
Useful sources include the manufacturer technical specification, the owner’s manual and official vehicle data.
Third-party databases can be useful for comparison, but the exact manufacturer specification should be checked before designing the charging installation.
How to Read an EV Charging Specification
- 82 kWh
- The approximate energy capacity of the battery.
- 11 kW
- The vehicle's maximum AC charging capability.
- 3 × 16 A
- The approximate phase/current arrangement that produces around 11 kW AC.
- 205 kW
- The vehicle's maximum quoted DC rapid-charging capability under suitable conditions.
Imagine a vehicle specification says battery 82 kWh, maximum AC 11 kW, AC arrangement 3 phase / 16 A, maximum DC 205 kW.
These numbers describe different parts of the charging system.
Worked Example: 11 kW Car on Different Wallboxes
| Wallbox Capability | Vehicle AC Limit | Approximate Maximum |
|---|---|---|
| 7 kW | 11 kW | 7 kW |
| 11 kW | 11 kW | 11 kW |
| 22 kW | 11 kW | 11 kW |
The 22 kW wallbox has more capability. But the car cannot use it.
The larger wallbox stops helping once the vehicle becomes the limiting factor.
Worked Example: 7.4 kW Car on a Three-Phase Property
Example
Property: three phase. Wallbox: 22 kW. Vehicle: 7.4 kW single-phase AC capability.
The property can provide more. The wallbox can offer more. But the vehicle cannot use three-phase 22 kW charging.
Actual maximum remains around 7.4 kW.
Three phase at the house only helps if the vehicle can use three-phase AC charging.
Worked Example: Dynamic Load Balancing Limits an 11 kW Car
Example
Vehicle capability 11 kW. Wallbox capability 11 kW. Available home headroom 6 kW.
The charger offers around 6 kW, so the vehicle charges at around 6 kW.
Later, household demand falls and available headroom rises to 11 kW. The wallbox can now offer the full 11 kW.
Vehicle capability tells you the ceiling. Site headroom tells you what can be delivered now.
Worked Example: Plug-In Hybrid
Example
Wallbox 7 kW. PHEV onboard charger 3.6 kW. Energy required 9 kWh.
Maximum AC rate: approximately 3.6 kW. Simplified charging time: 9 ÷ 3.6 ≈ 2.5 hours before losses.
Installing a larger wallbox would not make that vehicle charge faster unless the vehicle itself supported a higher AC rate.
Worked Example: AC vs DC Charging
Example
A vehicle specification says maximum AC 11 kW and maximum DC 180 kW.
At home on suitable AC equipment: maximum around 11 kW, subject to electrical headroom and vehicle conditions.
At a compatible DC rapid charger: charging power can potentially be much higher. Actual DC power still depends on charger capability, battery temperature, state of charge and battery-management strategy.
Different charging path, different limit.
Should I Buy a 22 kW Wallbox for Future-Proofing?
Maybe.
But future-proofing should consider more than the wallbox.
Ask:
- what can the current car accept?
- what might the next car accept?
- does the property have three phase?
- can the electrical installation support the future power?
- is faster AC charging actually useful?
- will the charging window make the extra power valuable?
A current car may only accept 7.4 kW while a future vehicle might accept 11 kW. Preparing some infrastructure now may therefore be sensible.
But:
Future charging capability depends on the vehicle and the property, not just the wallbox.
What About Vehicle-to-Home and Vehicle-to-Grid?
Bidirectional charging allows electricity to move into the vehicle and potentially back out again.
Depending on the system, power conversion may take place using different onboard or off-board architectures.
So normal AC onboard-charger capability does not automatically describe V2H capability, V2G capability or backup capability.
Bidirectional capability should be checked separately from the vehicle’s normal AC onboard-charger rating.
What Should You Check Before Buying a Wallbox?
Before choosing a charger, check:
- current vehicle maximum AC charging power
- maximum single-phase charging rate
- maximum three-phase charging rate
- property supply type
- wallbox capability
- cable capability
- dynamic load-balancing requirements
- solar charging requirements
- typical daily kWh requirement
- normal charging window
- likely next vehicle
- whether faster AC charging solves a real problem
Choose the wallbox around the car, the property and the charging job — not simply the largest number on the specification sheet.
Common EV Onboard Charger Questions
What is an EV onboard charger?
It is power-electronic equipment inside the vehicle that converts AC electricity into DC during AC charging.
Does the onboard charger convert AC to DC?
Yes. That is its main role during normal AC charging.
Is the wallbox the actual charger?
In everyday language, yes, people commonly call it the charger. Technically, during AC charging the main AC-to-DC conversion takes place in the vehicle’s onboard charger.
Why does my 22 kW wallbox only charge my car at 11 kW?
The vehicle may have an 11 kW maximum AC onboard charging capability.
Why does my car only charge at 7 kW at home?
Possible reasons include the vehicle AC limit, property supply, wallbox setting, cable, dynamic load balancing or vehicle conditions.
Why can my car rapid charge at 150 kW but only charge at 11 kW at home?
The 150 kW figure refers to DC rapid charging. Home AC charging uses the vehicle’s onboard charger, which may be limited to 11 kW.
Is 7.4 kW an onboard charger rating?
Yes, around 7.4 kW is a common nominal single-phase AC charging capability.
Is 11 kW three phase?
Around 11 kW AC charging is commonly achieved using three phase at approximately 16 A per phase.
Is 22 kW three phase?
Around 22 kW AC charging normally uses three phase at approximately 32 A per phase.
What does 3 × 16 A mean?
It usually means three-phase charging at approximately 16 A on each phase, giving around 11 kW nominal AC power.
Does an 11 kW car always charge at 7.4 kW on single phase?
No. The maximum single-phase charging rate depends on the vehicle’s onboard charger architecture and specification.
Does Type 2 mean the car can charge at 22 kW?
No. Type 2 describes the connector system, not the vehicle’s maximum AC charging capability.
Can the charging cable reduce charging speed?
Potentially. The cable must be suitable for the intended current and number of phases.
Why does my plug-in hybrid only charge at 3.6 kW on a 7 kW wallbox?
The vehicle may have a 3.6 kW onboard AC charging limit.
Is battery capacity the same as onboard charger power?
No. Battery capacity is measured in kWh. Charger power is measured in kW.
Does battery temperature affect AC charging?
Potentially. Vehicle battery-management and thermal systems can reduce accepted charging power.
Does state of charge affect charging power?
It can. The onboard charger rating is a maximum capability, not a guarantee of constant power throughout every charging session.
Can preconditioning use some of the wallbox power?
Yes. Some incoming power may be used for cabin or battery conditioning rather than increasing battery state of charge.
Can dynamic load balancing reduce charging below the onboard charger rating?
Yes. The onboard charger defines what the vehicle can accept, while load balancing determines what the home can currently offer.
Does solar charging change the onboard charger limit?
No. The onboard charger ceiling remains the same, but available solar power may cause the wallbox to offer less.
Does a bigger onboard charger make DC rapid charging faster?
Not necessarily. AC and DC charging capabilities are separate specifications.
How do I find my vehicle's onboard charger rating?
Look for the maximum AC charging power in the manufacturer’s technical specification or owner’s manual.
Should I buy a 22 kW wallbox for future-proofing?
Only if the likely future vehicle, electrical supply and charging requirements make that capability useful.
Does V2H or V2G use the onboard charger?
It depends on the bidirectional charging architecture. Normal AC onboard-charger ratings should not be assumed to describe V2H or V2G capability.
Know the Car's Limit Before Choosing the Charger
The wallbox rating is only one part of the charging system.
To understand how fast the vehicle can actually charge at home, you need to know:
- what the property can provide
- what the wallbox can offer
- what the cable supports
- what the onboard charger can convert
- what the battery-management system is willing to accept
The wallbox controls the supply. The onboard charger converts the electricity.
The onboard charger is the power converter that makes AC home charging possible.
The lowest relevant limit in the charging chain sets the actual charging power.
The onboard charger sets the AC ceiling. The actual charging rate can still sit below it.
Available power, vehicle capability and actual accepted power are three different numbers.
The onboard charger converts the power. The battery-management system decides what the battery is prepared to accept.
Battery capacity is measured in kWh. Onboard charger power is measured in kW.
The onboard charger sets what the vehicle could accept. Dynamic load balancing sets what the home can offer right now.
AC charging capability and DC rapid-charging capability are separate vehicle specifications.
And the key principle remains:
The wallbox offers AC power, but the onboard charger sets the vehicle’s maximum AC conversion capability — and the car may still choose to accept less.