How much energy does EV charging lose as heat?

The physics: current squared times resistance

Every metre of charging cable is a small resistor. Current flowing through it dissipates heat at I² × R per conductor — the calculator sums that over both conductors of the run (three for three-phase), using its wire data: 8 AWG copper resists 0.002103 Ω per metre, 10 AWG 0.003346, 6 AWG 0.001322. Because the current is fixed by the charger, the share of power lost equals the share of voltage dropped along the run — the calculator's loss percentage and its voltage-drop percentage are the same number seen from two ends.

The squared term is the story. Double the current and the heat per metre quadruples; that is why a Level 2 session sheds a hundred-plus watts along the cable while a Level 1 session at 16 A sheds almost none per hour — and why shortening the run is the one fix that always works, since every metre removed removes its resistance entirely. It is also why the car's own charging limit belongs in the arithmetic before the wire's rating: the Level 2 standard allows 80 A, but the Model 3 and Mach-E below accept at most about 11 kW on AC, so their cables never carry the standard's headline current.

Worked charges, from the calculator's wire data

Each row is one 20–80% charge run through the calculator — 49.2 kWh delivered to the Model 3, 52.8 kWh to the Mach-E — indoors, with the current set by the lowest of three limits: the charging standard's, the wire's, and the car's own AC charging limit. US runs are entered in the calculator in feet, exactly as its form asks, so the 15 m runs below are typed as 49.2 ft:

Three patterns in the numbers

The table is small, but the rules it shows are general:

• Length is linear. The same 10 AWG run at 30 m loses exactly twice the share it loses at 15 m (4.01% vs 2.00%) — every metre of cable is another resistor in series.
• The car sets the current; the gauge only sets the resistance. Every Level 2 row above runs the same 48 A, because the Model 3's 11.5 kW AC limit binds before the Level 2 standard's 80 A and before any of these wires' ratings. At that fixed current, gauge is a pure trade: at 15 m, 10 AWG loses 2.00%, 8 AWG 1.26%, 6 AWG 0.79%.
• Level 1's percentage is a small-current artifact, not thrift. At 16 A the cable sheds just 13.7 W — but the same charge takes 25.6 hours, so the session still loses 0.35 kWh to heat, as much as the shorter Level 2 runs lose in a fraction of the time. The percentage is low because the current is, not because the setup is somehow thriftier per mile charged.
• Per-charge losses compound quietly. 0.63 kWh over 100 charges is 63 kWh — more than one full 20–80% session (49.2 kWh) — spent warming a cable instead of a battery.

What this number leaves out

Be clear about the scope: this calculator isolates the cable run. It does not model the car's onboard charger conversion or losses inside the battery, which are separate effects with their own physics — the figure here is the installer's share of the total, the part decided by where the charger sits and what wire feeds it.

The run's environment matters too — and this is the one thing the calculator does not price in. Switch its installation setting from Indoor to Outdoor Covered or Outdoor Exposed and the loss, heat, and time figures do not move at all; the calculator only appends a note that losses may run about 10% (covered) or 20% (exposed) higher. Read those percentages as unmodelled margin on top of the indoor numbers above, not as something already included. The calculator does warn once cable heat passes 50 W — a line every Level 2 row above crosses — because a cable shedding a hundred-plus watts in a hot garage or direct sun has to get rid of that heat somewhere. A short, thick, indoor run is the whole prescription; the calculator exists to price the compromises when the panel is not where the car is.