Don't pay for energy that turns into heat—calculate your EV charging cable power loss and choose the right gauge to maximize your charging efficiency.
Home charging feels simple: plug in, kWh rise, bill arrives. But resistance in the cable path turns part of your payment into warmth you never drive. That is pure physics—Joule heating in copper—and it scales with current squared. A marginal extension cord on a 32 A Level 2 session is not marginal on your wallet.
Why cable losses matter (the “why”)
Your utility meter measures energy at the wall. The pack only stores what survives:
- Charger conversion loss (AC → DC)
- I²R loss in premises wiring and the EV cable
- Contact resistance at plugs
This calculator isolates the cord + run I²R slice so you can compare gauges and lengths before buying hardware.
The engineering in plain language
For copper near room temperature:
Round-trip Ω ≈ 2 × (0.0175 × length ÷ mm²)
Loss (W) = I² × Ω
Wasted kWh = W × hours ÷ 1000
Double the amps → four times the heat. Double the length → twice the ohms. Halve the mm² → twice the ohms. That is why undersized long cords feel warm and why fleets spec heavy gauge for permanent installs.
Reference loss %
We compare loss watts to I × 230 V charge power—a planning snapshot for EU-style single-phase supplies. Adjust mentally for 208/240 V split-phase sites.
What to do with the results
| Output | Action |
|---|---|
| > 50 W steady loss | Upsize mm² or shorten run |
| > 1 kWh wasted / session | Re-price against EV Charging Cost |
| Warm plug faces | Stop using cord; inspect contacts |
Pair with Residential Voltage Drop when the whole circuit length—not just the EV jumper—is long.
Charging efficiency starts at the copper. Measure I²R before you blame the car, the charger, or winter—sometimes the electrons never made it past the cable jacket.