Charging efficiency and losses
Why the meter always reads more than the pack received.
You'll learn
- Explain charging efficiency and losses clearly and accurately
- Read the numbers that define the behaviour
- Apply the idea to a real charging session
Overview
Between five and fifteen percent of the energy you pay for never reaches the cells. Knowing where it goes helps you choose how to charge.
In depth
| Loss | Typical share | Notes |
|---|---|---|
| Rectification / on-board charger | 4–8% | Worse at low power |
| Cable and connector resistance | 1–2% | Rises with current and poor contact |
| Thermal management | 1–5% | Much higher in extreme weather |
| BMS and vehicle electronics | ~1% | Constant overhead per session |
Calculator
Charging Cost Calculator
What a session costs at the wall, including charging losses.
kWh
% SOC
%
per kWh
Energy into pack
36.0kWh
Energy billed
40.0kWh
Session cost
4.80
Formula
cost = capacity × ΔSOC ÷ 100 ÷ efficiency × priceYou pay for energy measured at the meter, not the energy that reaches the cells. Dividing by efficiency accounts for the difference.
Example: 60 kWh × 60% ÷ 0.9 = 40 kWh billed. At 0.12/kWh overnight that is 4.80.
- AC home charging is typically 85–92% efficient end to end; DC rapid is 92–96%.
- Some public networks bill by time, which penalises cars that taper early.
- Compare against fuel using cost per 100 km, never cost per fill.
Slow trickle charging is not the most efficient
At very low power the fixed overhead of keeping the car awake dominates. A 7 kW session is usually more efficient overall than a 2 kW one.
Key takeaways
Remember this
- Expect 85–95% wall-to-pack efficiency.
- Overheads dominate at very low power.
- Extreme weather adds thermal losses.
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