The USA Environmental Agency
So, how much CO2 will the Chevy Volt actually produce (or save)? According to GM, to travel 40 miles the Chevy Volt uses 8kWhrs. Note that this is energy from the battery pack. The amount of energy needed to charge it will be more than this depending upon the efficiency pf the charger circuit/battery combination.
However, according to the US Environmental Agency, in America 1kWh of electricity typically generates 7.18 x 10-4 metric tonnes of CO2, so 8kWh obviously generates 5.75 x 10-3 metric tonnes of CO2. A gallon of petrol produces 8.81*10-3 metric tons. Given that the Chevy Volt travels 50mpg during its petrol phase, this means it would generate 0.8x8.81*10-3 metric tons or 7*10-3 metric tons travelling 40 miles in the petrol phase. So, although the Chevy Volt is less polluting during its electrical phase, this is not zero emission and it is not as significant a saving as it might first appear.
In terms of petrol equivalence for CO2 emissions, in reality during the electrical phase, the vehicle is doing the petrol equivalent of 60.9mpg. Effectively, what we are seeing here is the cleaner generation of energy from electricity in terms of CO2 than from directly burning petrol in a vehicle. This is well known and as more electricity is generated by renewables or nuclear, this benefit increases. However, once again, GM are being slightly economical with the truth in implying that the Chevy Volt is a zero-pollution solution to commuter travel below its electric 40 mile range (assuming even this is, in real driving conditions, actually achievable).
But what if however the range isn't 40 miles in real world conditions? What is the pivot point for range on electric where petrol actually takes the lead in terms of CO2? The answer is easy to arrive at. The Chevy Volt does 50mpg on petrol @ 8.81*10-3 metric tons, the electric phase generates 5.75 x 10-3 metric tonnes of CO2. So, the pivot point is 50*5.75 x 10-3 /8.81 x 10-3 or 36.5 miles. I find this is both surprisingly close to the 40 mile range and that it causes immediate alarm bells given the real world performance of this car in real driving with real loads and real drivers, not admen engineers. In reality, this vehicle, if it achieves less than 36.5 miles in the real world, could actually generate MORE co2 in the electric cycle section, not less. And these figures have not included the inefficiencies in the charging process.
Even people travelling only in the electric cycle totally may contribute to the overall CO2 increase if they continuously recharge the battery pack without fully discharging it to make sure it is topped up. I would suspect the energy losses become disproportionate the less you top it up, effectively the recharge cycle being better optimised for full charges although I have no figures for this and it may prove to be insignificant or not of any great issue.