Amps, Volts, kW & kWh Explained
We work in the industry, so terms like 32A, 7kW, or a battery listed as 60kWh make sense to us. But we speak to customers every day who don’t fully understand this new terminology around EVs and EV charging. We get it, it’s a completely new world if you’re new to driving an EV.
Amps, volts, watts and kilowatt hours, they all sound like technical terms, but once you understand how they relate to each other, EV charging becomes much clearer.
In this guide, we will run through what these terms mean and the difference between kilowatts (kW) and kilowatt-hours (kWh). We’ll also show how these electrical units apply to EV charging, walk through some practical charging calculations and explain why understanding them matters for both electricity billing and charger installation. So you’ll have a clearer understanding of how EV charging works in simple, practical terms.
The Simple Water Analogy
One of the easiest ways to understand electricity is to compare it to water flowing through a pipe.
Think of:
- Voltage in volts as water pressure
- Current (amps) is the amount of water flowing
- Power (watts) is the total amount of work being done
If you increase water pressure, more water can move.
If you widen the pipe, more water flows through it.
Electricity flows in a similar way, through wires instead of pipes.
What Is a Volt?
A volt (V) measures electrical pressure, the force that drives electric current through a circuit. In the UK, most homes operate at around 240 volts (officially standardised as 230V, though it is still commonly referred to as 240V). Voltage works similarly to water pressure in plumbing: just as higher water pressure pushes water through pipes more strongly, higher voltage pushes electrical current through wires. A higher voltage can allow more electrical power to be delivered when the current increases. When you see the voltage rating on an electrical device, it indicates the electrical pressure available to move current through the system.
What Is an Amp?
An amp (A) measures electrical current, the amount of electricity flowing through a circuit. In the water analogy, amps are like the amount of water flowing through a pipe. The higher the amps, the greater the electrical power flowing through the system. For example, a typical 7kW EV charger draws around 32 amps, while a 3.6kW charger draws roughly 16 amps. Because EV charging usually runs for extended periods, this current flows continuously, which is why correct installation, appropriate cabling and properly rated circuit breakers are essential for safety and reliable operation.
What Is a Watt?
A watt (W) measures electrical power, essentially how much work electricity is doing. Electrical power is calculated using a simple formula: Watts = Volts × Amps, which links voltage, current, and power. For example, if your home voltage is around 240 volts and your EV charger draws 32 amps, the calculation would be 240 × 32 = 7,680 watts, which equals 7.68 kilowatts (kW). While power is technically measured in watts, EV charging is usually discussed in kilowatts because the power levels involved are much higher, making kW a more practical unit.
What Is a Kilowatt (kW)?
A kilowatt (kW) equals 1,000 watts. For example, 1 kW equals 1,000 watts, 7 kW equals 7,000 watts, and 150 kW equals 150,000 watts. Kilowatts measure the amount of electrical power being delivered at any given moment. In the context of EV charging, the kilowatt rating determines the charging speed; the higher the kilowatt output, the more power is delivered to the vehicle, and the faster the battery can charge.
What Is a Kilowatt Hour (kWh)?
This is where much of the confusion often occurs. A kilowatt hour (kWh) measures energy, not power. It represents how much electrical energy has been used over a period of time. One helpful way to think about it is to compare it to driving: kilowatts (kW) are like speed, while kilowatt hours (kWh) are like the distance travelled. For example, if your EV charger runs at 7 kW for one hour, it will use 7 kilowatt hours (kWh) of electricity. Electricity billing is based on kilowatt-hours rather than kilowatts, meaning your energy provider charges you for the total amount of energy consumed over time.
kW vs kWh
|
kW (kilowatts) |
kWh (kilowatt hours) |
|
Power |
Energy |
|
Charging speed |
Battery capacity |
|
Instant output |
Total consumption |
|
Measured in kW |
Measured in kWh |
A 7kW charger doesn’t mean it uses 7kWh instantly; it means it can deliver up to 7kW of power per hour.
What Is a Watt-Hour (Wh)?
A watt-hour (Wh) is simply a smaller unit of energy than a kilowatt-hour. In fact, 1,000 watt hours (Wh) equals 1 kilowatt hour (kWh). For example, 500Wh is the same as 0.5kWh. Smaller batteries, such as those used in power banks or portable electronics, are often measured in watt-hours because their energy capacity is relatively small. Electric vehicle batteries, however, store much more energy, which is why their capacity is typically measured in kilowatt-hours (kWh).
EV Charging Calculations
Let’s apply this to real-world EV charging.
Example 1: 7kW Home Charger
UK home voltage = 240 volts
Charger current = 32 amps
Using the formula:
240 volts × 32 amps = 7,680 watts
≈ 7.7 kilowatts (kW)
If you charge for 3 hours:
7 kW × 3 hours = 21 kWh used
If electricity costs 30p per kWh:
21 × £0.30 = £6.30
That’s how electricity billing works for EV charging.
Example 2: 3.6kW Charger
Voltage = 240 volts
Current = 16 amps
240 × 16 = 3,840 watts
≈ 3.8kW
If charging for 6 hours:
3.6 kW × 6 hours = 21.6 kWh
Same energy as above, just slower delivery.
Example 3: EV Battery Capacity
If your EV battery is 60 kilowatt hours kWh:
That means it can store:
60 kW of power for 1 hour
OR 6 kW for 10 hours
If charging at 7kW:
60 ÷ 7 = approx. 8.5 hours (from empty)
Real-world charging may vary slightly due to efficiency losses.
Single Phase vs Three Phase Power
Most homes in the UK use single-phase electricity at around 240 volts. With a single-phase supply, home EV chargers are typically limited to around 7kW, which usually draws about 32 amps. In contrast, three-phase power, more commonly found in commercial or industrial settings, can support higher charging speeds such as 11kW or 22kW. Three-phase systems can deliver more electrical power because electricity is distributed across multiple supply lines, enabling a more balanced flow to the charger.
What About Higher Voltage DC Charging?
Rapid public EV chargers operate differently from home chargers. Instead of relying on standard household AC electricity, they use higher voltages, direct current (DC), and external power conversion. In a home charger, the car converts AC power from the grid into DC to charge the battery. Rapid chargers perform this conversion within the charging unit, delivering electricity directly to the battery. Rather than the typical 240V AC supply used in homes, rapid chargers often operate at 400V DC or higher. This higher voltage enables much greater power to be delivered in a short period, which is why rapid and ultra-rapid chargers can achieve charging speeds of 150kW or more.
How Circuit Breakers Protect Your System
Circuit breakers are designed to protect your electrical system by automatically stopping the flow of electricity if a fault occurs. If the electrical current exceeds safe levels, the breaker trips, cutting off the power and preventing potential hazards such as overheating or electrical fires. For example, a 32-amp home EV charger must be installed on a circuit with a correctly rated breaker to ensure it can handle the load safely. If a circuit is overloaded, the wiring can overheat, which is why proper installation and appropriate protection devices are essential to ensure your EV charger operates safely and efficiently.
Why These Units Matter for EV Owners
Understanding the relationship among these terms can help you make better decisions when charging an electric vehicle at home.
If you understand what all these terms mean, it will help you when you are choosing your charger. To make sure it suits your property and to understand how charging speed works, so you can easily estimate how long it will take to charge your electric vehicle, and calculate the electricity cost.
It also helps prevent overloading your home’s electrical supply and improves overall energy efficiency. For example, if your home has a 60-amp main fuse and your EV charger draws 32 amps, you need to consider the simultaneous electricity use of other appliances in the house to ensure the total load does not exceed your supply’s capacity. Proper planning ensures safe, reliable and efficient EV charging, but don’t worry, if you have any questions about anything at all, our installers will run through everything with you before installation.
Bringing It All Together
Here’s the simple summary:
- Voltage in volts = pressure
- Amps = current flow
- Watts = electrical power
- Kilowatts (kW) = charging speed
- Kilowatt hours (kWh) = energy used or stored
When electricity flows through your EV charger, it is measured in watts at that moment. Over time, the total electrical energy used is measured in kilowatt-hours (kWh).
Frequently Asked Questions
Why is my charger rated in kW but my bill in kWh?
Because kW measures power delivery speed, while kWh measures total energy used over time.
How many kilowatt hours kWh does an EV use?
It depends on battery size. Common UK EV batteries include 40, 60 and 77 kWh.
Why does my charger show amps?
Because amps measure current flow, important for safety and load management.
Is 240 volts standard in the UK?
Yes. UK homes typically operate at around 240 volts (officially 230V nominal).
What does 1000 watts mean?
1000 watts equals 1 kilowatt (1kW).
Still have questions?
