The hidden cost
Here's something most people don't realize: when you charge your EV, not all the electricity makes it to your battery. Some gets lost along the way. This "charging loss" typically adds 5-15% to what you actually pay, sometimes more.
Why does this happen?
Converting AC to DC
Your home outlet gives you AC power. Your car battery stores DC power. The car's onboard charger converts between them, and that conversion isn't 100% efficient. Some energy becomes heat.
Battery thermal management
EVs heat or cool their batteries while charging to keep them healthy and charge faster. That takes energy too, and it doesn't end up as stored charge. In cold weather especially, this can be significant.
Fixed overhead
Your car's electronics (control units, displays, cooling pumps) draw power the entire time you're plugged in. The longer you charge, the more this adds up.
Cable and wiring losses
Some energy is lost in the charging cable and your home's electrical wiring. This is usually small but adds to the total.
How much energy is lost?
Based on real-world testing by ADAC (a large European automobile club), here's what to expect:
- Regular outlet (~2-3 kW): 10-25% lost. Low power means longer charging time, so fixed overhead adds up. Some cars tested showed over 20% losses.
- Home wallbox (~7-11 kW): 5-10% lost. Higher power spreads fixed overhead over fewer hours. Most cars tested were in the 6-10% range.
- DC Fast Charging: 5-15% lost. Can be very efficient when the battery is already warm, but losses increase if the car needs to heat a cold battery or run heavy cooling.
The numbers vary quite a bit by vehicle and conditions. Temperature matters a lot.
Why faster AC charging is often more efficient
It's not that faster charging generates less heat. It's that your car's electronics run the whole time you're plugged in. If you charge at 2 kW for 10 hours, those systems draw power for 10 hours. If you charge at 11 kW for 2 hours, they only run for 2 hours.
For DC fast charging, it depends. Warm battery? Very efficient. Cold battery that needs heating? Losses go up.
What actually helps
1. Use a wallbox, not a regular outlet: Higher charging power means less time, which means less fixed overhead. Also safer and more reliable.
2. Charge in moderate temperatures when possible: Extreme cold or heat means the car works harder to manage the battery.
3. Precondition before fast charging: This helps charging *speed* and protects the battery. It doesn't necessarily save total energy (the heating energy is used during driving instead), but it makes fast charging work better.
4. Don't always charge to 100%: The last 20% (80-100%) is less efficient than the middle range. One study from ScienceDirect found losses in this range were almost double compared to 20-80%. Note: Some cars with LFP batteries have different guidance from the manufacturer, so check your owner's manual. This will also wear out your battery faster.
A note on EPA/WLTP figures
If you're using official EPA (US) or WLTP (EU) consumption figures, those typically already include charging losses (assuming Level 2 AC charging). So if you plug EPA kWh/100mi directly into a cost calculation, you may already be accounting for typical losses. Don't automatically add another 10% on top.
The calculator has a charging loss field. If you're using your car's trip meter (which shows energy from the battery), add 5-10%. If you're using EPA/WLTP figures, you can leave it at 0% or a small amount.
Does this change the EV vs petrol math?
Usually not dramatically. If losses are 10%, your electricity cost goes up by about 11%. That's rarely enough to flip the comparison.
But it does matter if:
- Your electricity is expensive
- You rely on pricey public charging
- You're trying to be precise about costs
For most people charging at home, it's a modest addition to the bill, not a game-changer.