How Green are Electric Cars?


To fully understand the true environmental impact of electric cars, one has to examine the ‘generation-to-wheel’ CO2 emission cycle. Generation-to-wheel emission is zero if the electricity used to charge the car originates only from renewable energy such as wind. In Ireland, the share of renewable energy in the electricity mix is targeted to be 40% by 2020. Thus electric cars provide an opportunity to reduce CO2 in the transport sector through the electricity system. The graphic below illustrates that even with the current mix of electricity electric cars emit less than half of the emissions of a conventional car i.e. 70 grams of CO2 for an electric car compared with 150 grams for a conventional car.

Generation to wheel emissions

This chart shows the emissions in grams/kWh and grams/km for electricity generated by wind (0g/kWh and 0g/km), gas (350g/kWh and 50g/km) and coal (900g/kWh and 130g/km). The end of chain emissions in 2010 is 153g/km and in 2015 130g/km. The grid average in 2010 is 474g/kWh and 70g/km. The grid average in 2020 will hopefully be 393g/kWh and 55g/km.

More energy efficient

Conventional cars with internal combustion engines are extremely inefficient with 80% or more of the fuel energy lost through wasted heat. Decades of cheap oil has led to a disincentive to improve car engine efficiency. However oil price hikes in recent years combined with emissions policies have driven car manufacturers to focus on engine efficiencies and the revival of electric cars.

Electric cars provide a leap in energy efficiencies, even accounting for all stages of energy conversion, ecars are a clear winner, as illustrated below.

Cycle efficiency of ecars versus conventional cars

This diagram shows the cycle of efficiency of electric cars from generation to wheel. Tank-to-Wheel efficiency is 15% from petrol and 20% from Diesel. 

Electricity from gas generates 55% efficiency, transmission gives 90% efficiency and an ecar is 75% efficient, giving a 37% overall efficiency rate. 

Power generated from wind is 100% efficient (renewable source), transmission gives 90% efficiency and an ecar is 75% efficient, giving a 68% overall efficiency rate.