As Dan Reicher (Director, Climate Change and Energy Initiatives, Google.org) summarized in testimony early this year to the Senate Finance Committee, many people see energy efficiency as the
“fastest, cheapest and cleanest opportunity to address our energy challenges….Duke Energy CEO James Rogers has termed energy efficiency our ‘fifth fuel’ and energy efficiency guru Amory Lovins measures it in ‘Negawatts’.”
In a nutshell, that’s why a main AXP figure of merit is miles-per-gallon equivalent (MPGe), a measure that expresses fuel economy in terms of the energy content of a gallon of petroleum-based gasoline.
Basically we ask: how much energy was delivered to the vehicle, and how far did it go? We convert the energy to the number of gallons of gasoline containing equivalent energy, and we express the result as miles per gallon.
Different fuels have different pros-and-cons, but in all cases it’s valuable to increase efficiency (increase MPGe), which conserves energy. This is even true, for example, if the fuel is electricity generated from alternative energy sources. Alternative energy is not infinite energy. Increasing electric vehicle efficiency will result in more energy available for other purposes.
MPGe is an attractive figure of merit because it’s a direct measure of overall “pump-to-wheels” efficiency, because it’s technology-neutral, and because it relates nicely to consumer intuition – i.e., it reduces to the familiar MPG if the fuel is in fact gasoline.
MPGe is also attractive because it applies if a vehicle is powered by more than one fuel, such as plug-in hybrid electric vehicles (PHEVs), which typically use electricity plus a liquid fuel (often, but not necessarily, gasoline). Here’s how to compute MPGe for this important case:
MPGe = EG / (g*EF + e*EW)
where
m = miles per gallon of liquid fuel used (MPG)
g = 1/m = gallons of liquid fuel used per mile (GPM)
e = plug-to-wheels electrical energy used per mile (Wh/mi)
EF = BTU per gallon of liquid fuel used (not necessarily gasoline)
EG = BTU per gallon of gasoline = 116,090
EW = BTU per Watt-hour (Wh) of electricity = 3.412
The formula above can also be used for pure battery electric vehicles (BEVs) and for pure liquid fuel vehicle – for BEVs, set g =0, and for pure liquid fuel vehicles, set e = 0.
Informal published values for BEV and PHEV fuel economy abound, but they can be inadvertently misleading. For example, some results report gasoline usage but not electricity usage (electricity usage is harder to measure). Also, not all results are well-documented, so accurate comparisons can be difficult.
One problem in applying the MPGe conversion formula above is that by definition MPGe is “pump/plug to wheels”, whereas the electricity usage (Wh/mi) data reported may be battery-to-wheels, which ignores the conversion loss that results from charging the battery via an AC (grid-connected) outlet. It’s not always obvious from test results whether or not the Wh/mi are measured from the plug or from the battery.
Bearing these issues in mind, I have computed MPGe for some real examples based on published data. I include example results below. You can see the details and other examples in the spreadsheet available here, which can also be used to explore additional examples. To account for cases where the Wh/mi measurement is battery-to-wheels, the spreadsheet includes an option for applying a plug-to-battery conversion factor.
Of the PHEV results, the main thing that stands out is that the Google.org data yield a significantly lower MPGe than other test data. This likely reflects the fact that the Google.org data are collected from actual daily driving by multiple drivers, whereas the other data are from fixed test cycles. Also noteworthy is the significant variation in the MPGe results from fixed test cycles, even for the same PHEV conversion tested over a similar range.
Overall, the results reflect a basic underlying problem – the difficulty of establishing test procedures that are not only well-documented and repeatable, but that reliably predict the fuel economy that consumers would experience. Indeed, that’s why DOE and EPA are intensively developing next-generation test procedures.
I welcome comments on these examples as well as pointers to other well-documented data. If appropriate, I will republish the spreadsheet and this blog post (note that the table below includes a publication date). Here are the examples:
Vehicle | Type of Test | Source | Range (miles) | Miles-per-gallon of Gasoline used (MPG) | Plug-to-Wheels electricity used (Wh/mi) | MPGe |
Prius PHEV conversion | Real-world driving | Google.org Recharge-IT (1/11/2008) | N.A. | 66.2 | 110.6 | 54.5 |
Prius PHEV Conversion (Energy CS) | UDDS Charge-Depleting | DOE ANL | 50 | 212 | 160.3 | 106 |
Prius PHEV Conversion (Energy CS) | UDDS Charge-Depleting (California Mode) | DOE ANL | 90 | 113.9 | 86 | 88.4 |
Prius PHEV Conversion (Hymotion) | UDDS | DOE INL | 100 | 83.7 | 35.8 | 76.9
|
Prius PHEV Conversion (Hymotion) | HWVET | DOE INL | 100 | 67.4 | 39.2 | 62.5
|
Prius PHEV Conversion (Energy CS) | UDDS | DOE INL | 100 | 89.2
| 56.5
| 77.7
|
Prius PHEV Conversion (Energy CS) | HWVET | DOE INL | 100 | 75.2 | 59.3
| 66.5
|
Tesla Roadster EV | EPA Recharge | Tesla | 245 | 0 | 310 | 109.8
|
2000 Nissan Altra EV (Li-Ion) | EPA Recharge | DOE | 129 | 0 | 276.5 | 123.1 |
2003 Toyota RAV-4 EV (NiMH) | EPA Recharge | DOE | 136 | 0 | 301.5 | 112.8 |
