Charging losses are the gap between the energy your charger delivers and the energy your battery stores. Every session delivers more kilowatt-hours than end up in the pack: some becomes heat in AC-to-DC conversion, and some runs the car’s heating, cooling and electronics during the session. You pay for the delivered figure, so the loss is a real, if modest, part of your charging cost.
Key Takeaways
- Charging losses are the gap between delivered and stored energy; you pay for the delivered figure.
- The loss comes from AC-to-DC conversion, resistance heating, and the car’s thermal management and awake electronics.
- Level 2 is on average about 10% more efficient than Level 1, because shorter sessions waste less on fixed draws.
- Measure your own loss from one session: charger-delivered kWh minus the car’s percentage gain times usable capacity.
Where the Missing Energy Goes
No single part wastes the energy; the loss is a sum of small ones, in the car as much as in the charger:
| Where | What happens to the energy |
|---|---|
| AC-to-DC conversion | AC chargers supply alternating current; the battery stores direct current. The car’s onboard charger converts it, and conversion always sheds some energy as heat. |
| Cable, connectors and wiring | Every conductor has resistance; current flowing for hours warms cable and connections slightly, and that warmth never reaches the pack. |
| Battery heating and cooling during the session | The car may run pumps, fans or a battery heater to hold the pack’s temperature — energy from the wall, not the battery. |
| The car’s systems while it sits plugged in | Computers, sensors and the charge controller stay awake all session, drawing a small amount throughout — why session length matters. |
One thing losses are not: a charger’s standby draw between sessions — a separate question, and not part of what a session loses.
Why Level 1 Loses a Bigger Share Than Level 2
ENERGY STAR puts Level 2 charging at on average about 10% more efficient than Level 1. The reason is time: the fixed draws in the table — awake electronics, pumps and fans — run for as long as the session lasts. A Level 1 session crawls, so they run for many more hours to deliver the same energy, and the lost share grows. Level 2 finishes in a fraction of the time, so the fixed draws take a smaller bite.
How to Measure Your Own Loss
You need two numbers from one session: the energy the charger delivered, and the energy the battery gained. A session display or app reports the first — our guide to what home charging costs builds its worked examples on it. The second comes from the car: percentage added, multiplied by usable capacity.
An invented example: the car reports a 40% gain on a 60 kWh usable pack, so about 24 kWh was stored; the charger’s counter says 27 kWh delivered. The difference — about 3 kWh, roughly a tenth delivered — is that session’s loss. Your figures will vary with temperature, session length and charge rate; your own equipment gives you both numbers.
What Losses Do to the Cost Maths
Losses are already inside cost calculations, because your meter counts delivered energy. Storing 40 kWh at about a tenth lost means drawing roughly 44 kWh from the wall; at the EIA average residential price of 18.31 cents per kWh, that is about $8.10 instead of the $7.32 stored energy alone would suggest — the same tenth our home-charging cost guide assumes. Two consequences follow. Cost per mile already includes losses, because it starts from wall energy — one reason home charging still beats gasoline comfortably with losses counted. And the way to shrink the loss is to shrink session time: charge on Level 2 rather than Level 1 where you can, and let scheduling and time-of-use tariffs keep it in cheap hours instead of stretching it across the day.
The delivered figure is the one you measure from, so a charger that reports it earns its place: the EV-ACS4005T 40 A NACS portable charger shows the energy delivered per session.
Frequently Asked Questions
Do charging losses mean my charger is faulty?
No. Some loss is unavoidable: conversion and temperature management always cost energy. A loss suddenly much larger than usual, on the same equipment, is worth asking the manufacturer about.
Are losses worse in cold weather?
They can be. A cold battery is often warmed during charging — energy from the wall on top of what is stored — and sessions run longer in the cold, extending the awake-time draw.
Can I get charging losses to zero?
No. You can shrink the share — Level 2 instead of Level 1, fewer very short sessions, no long idle time after charging — but conversion and temperature management always take some of the delivered energy.
Related Reading
- How Much Does It Cost to Charge an Electric Car at Home?
- Is Charging an Electric Car Cheaper Than Gas?
- Smart Charging and Time-of-Use Tariffs: How Scheduling Cuts EV Charging Costs
- Solar Panels and EV Chargers: How to Charge Your Car With Sunshine
