Power, Voltage & Current

To understand charging rates, it helps to know these basic electrical concepts:

Voltage (V, volts)

The “pressure” pushing electricity through a wire.
The UK and Europe use 230V mains; the US uses 110V.

Current (A, amps)

The amount of electricity flowing.
A standard domestic socket delivers up to ~13A in the UK, ~16A in most of Europe, and ~15A in the US.

Power (W, watts)

The rate at which energy is delivered. Power = Voltage × Current.
Since watts are a small unit, it’s common to talk in terms of kilowatts (1 kW = 1000 W). Power measures the rate of energy transfer — see kW vs kWh below for the relationship between power and energy.

AC vs DC

Two different types of electrical current (alternating current vs direct current).
Your mains supply and home chargers use AC, whereas EV batteries store and use DC.
The car’s on-board charger converts AC to DC to charge the battery — this conversion is typically what limits AC charge speed.
Public rapid chargers bypass the on-board charger entirely and supply DC directly to the battery, which is why they can charge at much higher rates (50–350 kW).

kW vs kWh — what’s the difference?

This is one of the most common sources of confusion with EVs:

kW (kilowatt)

A measure of power — how fast energy flows. Think of it like the width of a hosepipe: a wider pipe fills a bucket faster.

kWh (kilowatt-hour)

A measure of energy — how much is stored or used. Energy = Power × Time, so a kilowatt-hour is the energy delivered by 1 kW over 1 hour. Think of it like the volume of water in the bucket.

A 7 kW charger running for 1 hour delivers 7 kWh. The same charger running for 10 hours delivers 70 kWh — roughly enough to fill a large EV battery.

Economy & Consumption in EVs

In the UK and US, petrol and diesel cars measure economy (aka 'efficiency') — how far you go on a unit of fuel in mpg (miles per gallon).
In Europe it’s more common to talk about consumption — how much fuel or energy you use in L/100km (litres per 100 km).

For EVs you'll typically see one or more of the following when describing how efficiently they use electricity:

mi/kWh (UK & US)

Miles driven per kilowatt-hour of electricity. The EV equivalent of mpg — higher is better. A typical modern EV achieves 3–4 mi/kWh.

km/kWh (EU)

Kilometres driven per kilowatt-hour. Same concept in metric — a typical EV achieves 4–6 km/kWh.

kWh/100km (EU)

Kilowatt-hours consumed per 100 kilometres. The EV equivalent of L/100km — lower is better. An EV doing 5 km/kWh consumes 20 kWh/100km.

Battery Capacity

Measured in kWh. A bigger battery stores more energy and gives more range. Typical sizes:

• BEV (full electric): 40–100+ kWh. A 64 kWh battery with a 3.5 mi/kWh (5.6 km/kWh) efficiency motor gives about 224 mi (360 km) range.
• PHEV (plug-in hybrid): 8–20 kWh. Much smaller — designed for short electric-only trips (typically 15–35 mi / 25–60 km), with a fuel engine for longer journeys.

Battery Degradation & Charge Limits

Like all lithium-ion batteries, EV batteries slowly lose capacity over time. Regularly charging to 100% or letting the battery drain very low can accelerate this degradation. For this reason, many EV manufacturers recommend only charging to 80% for everyday use, reserving a full charge for longer trips where you need the extra range.

Recommendations vary between manufacturers and even between models, so always follow your manufacturer’s guidance. Most EVs let you set a charge limit directly in the car or via a companion app.

Charging Rates & Home Chargers

The charge rate (kW) determines how quickly your battery fills. Common home charging options:

• Domestic power outlet: In the UK/Europe at 230V, a standard plug delivers about 2.3 kW (230V × 10A). In the US at 110V, a regular outlet delivers only about 1.4 kW (110V × 12A). Very slow — fine for topping up a PHEV overnight, but fully charging a BEV could take 24–48 hours.
• Dedicated home wallbox: In the UK/Europe, typically 7.4 kW (single-phase 230V × 32A) or up to 22 kW (three-phase). In the US, a 240V circuit (like a dryer outlet) with a Level 2 charger delivers around 7.7 kW (240V × 32A). A 7.4 kW charger fills a 64 kWh battery in about 8–9 hours — perfect for overnight charging.
• Public rapid chargers: 50–350 kW DC. Great for long trips but usually much more expensive per kWh than home charging.

Important: the car’s on-board charger limits the AC charge rate, not your wallbox. Many PHEVs have a 3.6 kW on-board charger, so even if your wallbox can supply 7.4 kW, the car will only draw 3.6 kW. Full BEVs typically accept 7.4–11 kW AC. Always pick the lowest of your car’s maximum AC charge rate and your wallbox’s maximum power output — that’s the number to enter in the calculator above.

Off-Peak / Economy Tariffs

Many energy suppliers offer cheaper overnight electricity rates specifically aimed at EV owners (e.g., Octopus Go, Economy 7). You set a timer on your car or charger to only draw power during the cheap window — typically a block of 4–6 hours overnight. This can cut your electricity cost by 50% or more compared to the daytime rate, making EVs dramatically cheaper to run.

Plug-in Hybrid (PHEV) — how costs work

A PHEV uses its electric battery first, then switches to fuel for the remainder of a journey. Short daily commutes might be entirely electric, while longer trips use a blend of both. The trip distance you enter determines the EV/fuel split for the trip comparison. For annual estimates, use the EV usage % slider to reflect your real-world driving mix.

Glossary

ICE

Internal Combustion Engine. A traditional petrol or diesel engine that burns fuel to produce motion. Often used to refer to non-electric cars in general.

BEV

Battery Electric Vehicle. A car powered entirely by an electric battery — no fuel engine at all. Examples: Tesla Model 3, Nissan Leaf, Hyundai Ioniq 5.

HEV

Hybrid Electric Vehicle. Has a small battery and electric motor alongside a fuel engine, but cannot be plugged in to charge. The battery is charged only by regenerative braking and the engine. HEVs improve fuel economy but can’t drive on electricity alone for meaningful distances. Examples: Toyota Prius (non-plug-in), Honda Jazz Hybrid.

PHEV

Plug-in Hybrid Electric Vehicle. Has both a small battery and a traditional fuel engine. Unlike a HEV, a PHEV can be charged directly from an EV charge point. Can drive short distances on electricity alone, then seamlessly switches to fuel. Examples: Mitsubishi Outlander PHEV, BMW 330e.

EVCP

Electric Vehicle Charge Point. The device (wallbox, public charger, etc.) that supplies electricity to the car. Also known as an EVSE (Electric Vehicle Supply Equipment). The EVCP sets the maximum power available, but the car’s on-board charger decides how much to actually draw.