A common criticism of MPGe is that it gives electric vehicles (EVs) an inherent advantage because it does not account for the low efficiency of the current electric grid, an advantage exacerbated by the high efficiency of electric motors. There’s validity to this criticism (which I’ll get to), but it’s specious to base it on the assertion that – unlike other fuels – electricity is just an energy carrier resulting from burning fuel (e.g., coal) upstream.
Yes, electricity is an energy carrier. But so is every other fuel, by definition; that’s just physics. Fuels of any type provide potential energy that is converted to kinetic energy by a vehicle’s engine, be it an electric motor, an internal combustion engine, a steam engine, etc.
Furthermore, every practical vehicle fuel today results from upstream energy conversions that involve burning some other fuel. Gasoline, for example, is produced from crude oil in refineries that burn and pollute (obvious to anyone who has driven by a refinery); additional fuel is burned during the extraction of crude oil, during its transportation to a refinery, and during the transportation of the gasoline to a station near you. Producing ethanol from corn seeds involves burning fuel during farming, during the production and distribution of fertilizer, etc.
So, in terms of upstream energy conversions, there is no fundamental difference between electricity and gasoline – there’s just a difference of degree. As mentioned above, the difference is substantial for the current electric grid, so from this single viewpoint electricity has a MPGe advantage. But there are balancing factors.
As many have pointed out, one could avoid the issue by including estimates of upstream energy conversion losses in the definition of MPGe – i.e., defining it on a wells-to-wheels basis rather than a pump-to-wheels basis. We decided against this for several reasons.
For one thing, consumers understand pump-to-wheels measures – nobody does a wells-to-wheels calculation when they refuel. For another – from a technical viewpoint – we wanted to have an unambiguous, soundly-based, figure-of-merit that depends only on things the vehicle designer can control. This is because a main focus of the Progressive Automotive X PRIZE is to stimulate innovation in vehicles, and vehicle designers have little control over wells-to-pump efficiencies. Furthermore, upstream fuel efficiencies are ambiguous as they involve controversial and changing-in-time assumptions and predictions on which proponents of different fuels rarely agree. So, a wells-to-wheels definition of MPGe would result in arguable estimates rather than objective measured values.
Finally, we do not ignore wells-to-pump efficiencies and upstream fuel burning – these are addressed directly and indirectly by the second X PRIZE figure of merit, a cap of 200 g/mi on total wells-to-wheels greenhouse gas (GHG) emissions. Relative to gasoline vehicles, this GHG requirement is a significant disadvantage for EVs. To meet the GHG requirement, EVs will have to exceed our 100 MPGe threshold by about 20%. Critics of EVs point out that a 100 MPGe gasoline vehicle has even lower WTW GHG emissions (roughly 115 g/mi). However, a 3.75 mi/kWh (128 MPGe) EV running off the estimated 2014 California grid can match this (see the energy and emissions spreadsheet, described here).
While electric motors are more efficient than internal combustion engines(roughly twice as efficient), EV battery-packs are heavy (especially for longer range vehicles), which reduces overall efficiency. And EVs have a variety of other disadvantages (range, recharge time, and battery cost) that make the Mainstream Class requirements a stretch goal (including 200 mile range, production-capable at reasonable costs for 10,000 units per year).
It’s difficult to be certain how these advantages and disadvantages balance out, and EV critics may believe that EVs retain an overall advantage. On the other hand, EVs have great value in displacing petroleum (one of the our goals) - indeed, the wells-to-wheels use of petroleum by EVs is roughly 90% less than that of gasoline.