Comment by leoedin | Hacker News Reader

Comment by leoedin | Hacker News Reader

Parent Full thread

The book says 50km/30 miles per day in a petrol car is 40kWh. For a Tesla that would be 7.5kWh of electricity. It also assumes 12 km/litre - which in UK terms is 33 mpg. A Tesla has regenerative braking, so it would be better to compare the fuel efficiency to an aerodynamic hybrid. A Toyota Prius gets 61 mpg (UK) - which would mean 21.6 kWh per day.

So really the comparison is 7.5kWh of electricity compared to 21.6kWh of petrol. The Prius gets 34% of the miles compared to the Tesla for a given energy input. In the olden days when renewables didn't do much, the chemical energy input for 7.5kW of electricity was probably 15 kWh. So the electric car is a little bit more efficient (taking advantage of the efficiency of large power plants).

These days, thanks to renewables the fossil fuel input into the electricity is lower.

> These days, thanks to renewables the fossil fuel input into the electricity is lower.

Also, natural gas to electricity in CC plants is ~60% efficient.

Yeah, I assumed 50% to account for various losses and other less efficient generators. I don't know what the actual number is - probably in the 40-50% range.

Interestingly thanks to renewables the "carbon intensity" of a kWh of electricity is already better than a kWh of fossil fuels in some countries. A kWh of petrol releases about 250g of CO2, and quite a few countries (predominantly developed ones) have lower carbon intensities per kWh of electricity. Considering round trip efficiencies the carbon released by an electric car in those countries is easily 1/3 to 1/4 of the carbon released by a petrol car.

In the USA it's not as good, but electric cars are still releasing about half the carbon.

The big question in my mind for efficiency figures is whether it's with respect to the Lower Heating Value (LHV) or Higher Heating Value (HHV) of the fuel. The latter includes the latent energy from condensing the water of combustion.

Reply on news.ycombinator.com