Your guide to understanding where EV charging losses happen.
Summary.
Why energy is lost when charging your EV (10–25% average).
AC/DC conversion. On-board chargers generate heat when converting alternating current (AC) from the grid into the direct current (DC) needed by the battery.
Thermal management. Battery conditioning uses power to warm or cool the battery before and during charging, especially in winter.
System overhead. Internal vehicle computers and safety systems stay active throughout the entire charging session.
Electrical resistance. Long, thin, or low-quality cables lose energy by dissipating it as waste heat.
Your charging optimisation checklist.
Upgrade hardware. Use a dedicated 7kW home wallbox rather than a slower, less efficient 3-pin granny cable.
Time your charge. Plug in straight after driving while the battery is already warm to avoid using extra energy on heating.
Pre-condition early. Warm the battery and cabin using grid power while the car is still plugged in before you leave.
Cap at 80%. Avoid the slow, heat-heavy tapering phase at the end of a charge by capping daily charging at 80%.
Check cables. Keep charging sessions efficient by using shorter, high-quality cables with clean contacts.
Tracking your true energy cost.
What to ignore. Your car dashboard or trip computer, as it only measures the energy successfully stored in the battery cells.
What to use. Your home smart meter, In-Home Display (IHD), or smart charger app to track the true energy drawn at the wall.
Every electric vehicle (EV) owner has experienced it. When you pull up to your home charger and plug in only to discover that your EV isn’t as fully charged as expected. Where does that extra energy go? Believe it or not, it’s not a malfunction – it’s science.
From electrical resistance to chemical conversion, energy loss is an invisible passenger on every charging journey. This guide uncovers how EV charging losses happen, how it impacts your bill, and practical steps to minimise them.
Why some charges are less efficient than others.
Did you know that not all charging sessions are created equally? You might lose 10% of your energy on a warm day with a high quality wallbox, but lose 25% on a chilly night using a 3-pin plug. Let’s find out what drives those numbers.
Why the charger inside your car matters.
The big box on your wall isn't technically a charger. Your actual charger is buried deep inside your car, also known as the on-board charger.
When you use a home wallbox or a slow 3-pin plug, you’re feeding your EV Alternating Current (AC). However, batteries can only store Direct Current (DC), so your on-board charger has to do the heavy lifting of converting AC to DC.
This conversion process generates heat and uses energy. If you use a slow charger (like a 3-pin plug), the car’s computer systems have to stay active for longer to manage the charge. Think of it like leaving your laptop running just to charge your phone. It’s overhead energy use like this that really adds up, making slow charging notoriously inefficient.
How cable quality impacts your bill.
Your choice of cable also plays an important part in charging efficiency. Electrical resistance is the main issue here. The longer and thinner your cable, the harder the electricity works to push through. As a result, heat is generated.
For home charging, using an unnecessarily long cable (e.g. 10 metres or more) could increase resistance losses. Then there’s cheap aftermarket cables that may use thinner copper wiring than recommended, turning your electricity into waste heat before it even reaches your car.
How bad weather can drain charging energy.
Batteries don’t like being too hot or too cold (a bit like us). In the UK, winter can be a challenge. When your battery’s cold, the internal fluid becomes thicker, increasing internal resistance. As a result, your EV has to use energy to warm the battery up to a safe charging temperature.
So, if you plug in a cold car to charge, a chunk of your electricity bill is effectively being used to run a radiator for your battery.
What you pay for in your bill when EV charging losses occur.
Here, we’ll help you understand why billed energy is usually higher than the kilowatt-hour (kWh) shown on your EV’s dashboard, and just how systems account for normal charging losses.
How EV charging is metered for billing.
Whether you’re at home or in a public place, you’re always billed at the meter. This is the point where the electricity leaves the grid and enters your hardware.
Think of it like a petrol pump with a slightly leaky nozzle. You pay for all the fuel that leaves the pump, and not just the fuel that lands in your petrol tank. Roughly 10–25% of energy lost to heat, cable resistance, and conversion, is energy that’s passed through your meter. You have to pay for it.
Why smart meters show higher usage than your EV app.
At home, the billing point is your domestic electricity meter. The journey from that meter includes the fuse box, the wiring to the garage, the charging unit, the charging cable, and of course, your car's on-board charger.
Your car's dashboard usually only reports the energy that successfully settled into the chemical cells of the battery. It is completely oblivious to the energy lost as heat. This is why standard domestic smart meters and EV tariffs always show a higher figure than your EV does.
Public charging: AC vs DC billing differences.
Now, let’s weigh up AC (Alternating Current) and DC (Direct Current) charging.
Public AC.
Just like at home, you usually pay for the AC electricity flowing from the post. Your car then converts it, incurring the usual losses.
Public DC (rapid/ultra-rapid).
Here, the huge charging unit converts AC to DC before sending it to your car. While you’re generally billed for the DC energy delivered to your car, the price per kWh is higher. This premium covers the expensive infrastructure and energy losses by the operator during the high speed conversion process.
Basically, you’re still paying for the inefficiency, but it’s added into the unit price rather than the unit count.
Tools to track your true charging costs.
To understand what your real charging costs are, stop looking at the car's trip computer for charging data and turn to the following instead:
Smart charger apps.
An app such as the E.ON Next Home app will measure energy at the wall, giving you a more accurate figure to your actual bill.
Home smart meters.
An in-home display (IHD) shows exactly what the house is drawing in real-time.
Public charging receipts.
These receipts show the billable kWh, which is the only number your wallet wants to know.
Your check list: how to minimise EV charging losses.
Check out how you can tighten up charging efficiency yourself with some easy everyday practices.
Ditch the ‘Granny Cable’. Avoid using the 3-pin plug ‘Granny Cable’ for daily charging because it’s the least efficient. Installing a dedicated 7kW home charger is safer and more efficient.
Charge up while it’s warm. The best time to charge is just when you get home. This is because the battery is already warm from driving, so your car doesn't need to waste grid energy heating up to accept a charge.
Use pre-conditioning. If you have a smart charger, set the car to ‘pre-condition’ (warm the cabin and battery) while it’s still plugged in, specifically just before you leave in the morning. This uses grid power rather than battery power – generally more efficient for range.
Review your cable choices. Ensure contacts are clean and the cable isn't excessively long. If the cable feels unusually hot to the touch, it might be time for a higher quality replacement.
Don't brim the battery daily. Charging the final 10-20% takes longer and generates more heat as the battery cells balance out. So, unless you need the range for a big trip, stopping earlier is more efficient.
Upgrade your EV charger today.
Time for an upgrade? Explore our fully installed home EV chargers, then match it with a smart energy tariff to help save money by charging at off-peak times.
Get an EV charger quoteFrequently asked questions
Why do EVs lose energy during charging?
Energy is lost during several stages, including grid transmission, the charging unit, cables, AC/DC conversion, and the battery's thermal management. These little inefficiencies all add up, meaning the electricity drawn from the wall is higher than what reaches your battery.
Do home chargers have fewer losses than public chargers?
Typically, yes. Home AC charging generally has lower losses because it delivers lower power, meaning less heat and less wasted energy. However, the on-board charger’s efficiency varies between EV models.
Can I reduce energy loss when charging my EV?
Yes, you can. Try charging during mild temperatures, use good quality cables, and avoid charging from very low states of charge. You should also pre-condition your battery and charge at moderate speeds instead of using rapid chargers.
Why does my EV seem to use more electricity in winter when charging?
It’s all to do with the weather. Cold temperatures force your car to warm the battery before and during charging, and the chemical efficiency of the battery drops. This increases thermal management losses, making winter charging less efficient.
Why is charging to 80% recommended for efficiency?
This is because as a battery fills up, internal resistance increases (creating more heat) and the charging speed slows down to balance the cells. This tapering phase is less efficient. So, stopping at 80% keeps the battery in its most efficient operating window, helping to prolong battery life.
Fast or slow charging – which is more efficient?
While fast charging generates more heat and needs active cooling, the sheer speed means your EV’s ancillary systems are running for minutes rather than hours. However, for battery health and cost, slow AC charging through a wallbox is usually the best option for daily use.