I've been using a 25m 1.5mm2 extension for about 9 months with the trickle charger. It drops 1 volt per amp at that length.

The EVSE allows the car to draw 7.1 amps.

The wall socket provides 240V at 7.1 amps, 1704 watts.

The EVSE receives 233V at 7.1 amps, 1654 watts.

The Kona gains exactly 2% per hour, so 0.02 x 64kWh/1h is 1280 watts.

So, the trickle charging itself is 1280/1654 = 77% efficient.

The extension wastes another 1704 - 1654 = 50 watts, or 2 watts per metre length.

With respect, please note that you can only derive the actual power in an AC circuit from the product of Volts and Amps if the load is a purely resistive one (aka non reactive or linear) when the power factor will be unity or 1.

So your power and charging efficiency calculations are likely to be incorrect (pessimistic). They are not taking into account the power factor for the reactive load (the cars on board charger will likely be highly reactive so the Volts and Amps will not be completely in phase with each other).

This means that the actual Watts being consumed at the 13A socket will likely be somewhat less than the 1704 that you have assumed here. Therefore the charging efficiency will likely be better than 77%.

Nevertheless, your loss calculation for the extension lead will be realistic as the lead itself is a non reactive linear resistance.

To measure the true Watts being consumed at the 13A socket requires the use of a proper AC power analyser instrument that can read true RMS power and will take into account the power factor of a reactive load and the relative phase difference between current and Voltage waveforms.

Peter