SnapEnergyLab
EV charging

AC vs DC charging, and where the lost kWh go

You pay for energy at the wall. Your car reports energy in the pack. Those are never the same number.

Published 23 August 2026 · Published by SnapEnergyLab. Methodology and default assumptions are documented separately.

A home wall charger connected to a car in a garage, with a brightly lit public rapid-charging station visible outside

Quick answer

Your meter counts energy at the wall. Your car reports energy in the pack. Between them sit a rectifier, a battery management system, cooling fans and pumps, and the car’s own electronics staying awake for the whole session.

Typical AC home charging loses 8–12%; a domestic socket at 2.3 kW can lose 20%. Any cost-per-charge figure that ignores this understates your bill by roughly a tenth.

How to read the numbersYour inputCalculated resultSourced value
Energy into the pack
50 kWh
Your input
At 90% charging efficiency
55.6 kWh at the wall
Calculated result
Extra cost at 0.30/kWh
≈ 1.67
Calculated result

What AC and DC charging actually do

A battery stores direct current, so somewhere in every charging chain, AC from the grid has to be rectified to DC. The only question is where that happens.

AC charging — a domestic socket or a wallbox — supplies alternating current to the car, and the vehicle’s onboard charger does the conversion. That unit is sized for weight and cost, so it is typically limited to 7.4 or 11 kW and is not especially efficient at low power.

DC charging puts the conversion equipment in the roadside cabinet, where size and cooling are not constrained. The car’s onboard charger is bypassed entirely, which is both why DC charging is fast and why its conversion efficiency is generally better.

  1. energy at the wall (kWh) = energy into the pack (kWh) ÷ charging efficiency
  2. cost per charge = energy at the wall × price per kWh
  3. effective cost per pack kWh = price per kWh ÷ charging efficiency

Divide, do not multiply: adding 10% to the pack figure is not the same as dividing by 0.90, and the difference grows as efficiency falls.

Typical loss by charging mode

Typical charging losses by mode
ModeConversionTypical lossWhy
Domestic socket, 2.3 kWAC, onboard charger12 – 20%Fixed overheads run for far longer, so proportional loss is worst here.
Wallbox single-phase, 7.4 kWAC, onboard charger8 – 12%The common home case, and the basis of most cost estimates.
Wallbox three-phase, 11 kWAC, onboard charger7 – 10%Slightly better; overheads are spread over more power.
DC rapid, 50–350 kWDC, charger-side conversion4 – 8%Conversion happens in the cabinet; cooling and thermal management add their own draw.

Worked example: the same charge, two ways

  1. Energy needed in the pack40 kWhYour input
  2. Wallbox at 7.4 kW, 90% efficient40 ÷ 0.90 = 44.4 kWhCalculated result
  3. Cost at 0.30 per kWh44.4 × 0.30 = 13.33Calculated result
  4. Domestic socket, 83% efficient40 ÷ 0.83 = 48.2 kWhCalculated result
  5. Cost at 0.30 per kWh48.2 × 0.30 = 14.46Calculated result
Penalty for charging from a socket≈ 1.13 per 40 kWh

Cold weather makes it worse again

Below freezing, many vehicles heat the battery before and during charging. That energy is drawn through the same meter and never reaches the pack as stored charge, so measured charging efficiency in winter can fall into the 70s for a slow AC session on a cold night.

Scheduling charging to finish shortly before departure helps twice: the pack is warm when you leave, and less of the heating energy is wasted keeping a parked car warm.

The same applies to preconditioning the cabin while plugged in. It is genuinely cheaper than doing it on battery power while driving, but it is not free, and it shows up as charging “loss” if you only compare wall energy against pack energy.

Cost your charging with losses included

The calculator separates energy added to the pack from energy drawn at the wall, with charging efficiency as an editable input, so the cost it reports is the one your meter will show.

Open the EV charging cost calculator

Assumptions and limitations

  • Loss ranges are typical measured values across vehicles and equipment, not a specification for your car. Onboard charger efficiency varies significantly by model.
  • Manufacturer-quoted consumption figures usually describe energy from the pack, so comparing them against metered wall energy double-counts nothing only if you apply charging efficiency yourself.
  • Public DC charging is normally billed on energy delivered to the vehicle, so charger-side losses are the operator's, not yours — but idle fees and session fees change the effective price per kWh in ways this arithmetic does not capture.

Sources

  1. IEC 61851 — Electric vehicle conductive charging system — International Electrotechnical Commission

    Supports: Definition of AC charging via the vehicle's onboard charger versus DC charging with off-board conversion.

  2. Charging efficiency of electric vehicles — measurement results — ADAC (Allgemeiner Deutscher Automobil-Club)

    Supports: Measured differences between energy drawn at the wall and energy stored in the vehicle battery across charging modes.

Related guides