I measured a charger efficiency of 86% in the Leaf and 95% in the Ioniq.

Electric cars display an efficiency figure which can range from 3 to 7 miles per kWh in my experience. This is the efficiency that the car measures, from the battery to the road. There are other losses in the system, not least the energy lost in battery charging. The AC power has to be converted to DC to charge the battery. There will be heat lost in cables, conversion losses in the inverter electronics and heat lost in charging the battery. As the battery ages so internal resistance rises, increasing this loss.

When I can, I charge from my own solar PV to the car. More often I charge overnight on a renewable tariff. What I can’t measure here are losses in getting the (renewable) energy from the source to my house, more important to me is that the energy is renewable and some loss in the grid is inevitable.

Of course petrol cars also have losses outside the car, the energy used to find oil deposits, extract oil, transport oil around the world, refine it, and transport the refined fuel to the forecourt are very hard to pin down, but could be large. Some estimates are that the energy used to refine oil into petrol is more than the energy usefully delivered when it is burned! What is more, this is a finite resource, causes a lot more carbon release than renewables, and is unsustainable.

So, to my two examples: the 5 year old Leaf and a 6 month old Ioniq Electric.

Leaf 24, 2013

50 miles of driving, at average 5.1miles/kWh reported by the car. 50/5.1 = 9.8kWh used.

Charged at 3.6kW, 11.4kWh used (measured by my Open Energy Monitor EmonPI).

Difference  = 1.6kWh, so 14% is lost in charging.

What does this mean for the 5.1 figure? 11.4kWh over 50 miles is actually 4.4 miles/kWh.

(In “petrol” terms with 10 kWh/litre, this would be 44 miles per litre, or 200mpg using Imperial gallons)

Ioniq electric, 2017

233 miles of motorway driving in one day, at 5.6miles/kWh reported. The car was charged during the trip.

To get back to 100% charge, 26kWh of electricity was used to add 88% charge (at 6.6kW). Assuming a real capacity of 28kWh, that’s 24.6kWh added.

Difference = 1.4kWh, so only 5.4% losses.

That would bring the overall efficiency from 5.6 to 5.3 miles/kWh. The Ioniq is still more efficient on the motorway than the Leaf is on slow country roads!

Reasons why there’s a large difference between the two cars might be-

  • newer battery so less “ohmic losses” due to a lower internal resistance
  • higher charging speed in the Ioniq (6.6 vs 3.6kW) so the system is running for less time per kWh delivered, resulting in a lower “overhead”
  • the Ioniq battery would probably have been warmer than the Leaf and so have an even lower internal resistance, due to a rapid charge and a long journey
  • newer more efficient charger technology (perhaps)


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