By Ray Iannuzzelli, PE

Note: This is a rewrite of two earlier articles comparing a 2012 Volt with a 2010 Prius V.

With the introduction of the new Prius PHV in the U.S. more data has become available and it is now possible to compare it with the Volt EREV. I was very curious to see how the new Prius would stack-up to the Volt, especially since it has a much smaller battery and price tag.



2012 Volt EREV                                                     2013 Prius PHV

So How Do They Compare?

Figure 1 shows the Volt’s gross and net specific energy history over the past three months or so. The difference between these is that the net specific energy is determined using the Volt instrument readout while the gross specific energy is the actual amount of energy (KWh) used. Measurement of the gross specific energy requires an external KWh meter. There appears to be a 0.032 KWh/mile overhead associated with the pumps, heaters, and electronics when charging the Volt’s batteries.


Figure 1 Volt Specific Energy History
Figure 3 compares the Volt and Prius based on the energy used (mpg e ) as well as on the cost of the energy (mpg $ ). The EPA rating for electric plug-ins is based on energy content of gasoline. Figure 3 shows the cross-over distance where the Volt mpg equals the Prius mpg. Also shown in figure 3 is the standard Prius mileage as well as the EPA ratings for both cars.


Volt                                                                      Prius

Figure 2 2012 Volt vs. Prius PHV EPA Window Stickers

 


Figure 3 Volt vs. Prius PHV ‘Average’ Comparison (mpg $ & mpg e ) at $3.60 per gallon
We notice the Prius starts at a higher value at ~116 mpg e while the Volt is close to its EPA value of 93 mpg e . My guess is that this difference is probably due to the Prius’ lighter weight. The short distance cross-over occurs at around 16 miles. Beyond this the Volt dominates until we reach the long distance cross-over point of 75 miles. Figure 4 shows a map of where Volt & Prius dominate the mileage comparison. My guess is that the Volt was designed to dominate in the middle region (16 to 75 miles) where most driving miles are accumulated.

A note about the EPA mileage rating (figure 2):

The EPA mpg e rating is a composite of five simulated driving cycles. Therefore, for the purposes of this article, it is difficult to compare the EPA mileage with those reported here.



Figure 4 Mileage Comparison Map
Figure 5 shows a comparison of the long distance cross-over as a function of gas price. At today’s gas price of roughly $3.60/gal there is a significant difference between the cost based & energy based long distance cross-over. However, as gas prices rise this gap narrows until at a price of $6.00/gal the energy & cost based comparisons are equivalent. This suggests the true cost of gasoline is higher than it is today.

 



Figure 5 Cross-over Distances vs. Cost of Gasoline
 

The assumptions used to generate figures 3 - 5 are listed below in table 1.

Table 1 Volt vs. Prius Comparison Assumptions

1)     Average EPA rated gasoline mileage

2)     EPA electric range

3)     Volt owner’s manual

4)     Battery Size/Electric Range

5)     Ssee figure 1 which is based on average driving data from my volt between 11/18/11 and 2/16/12

6)     Average Prius mileage between 9/18/10 and 3/5/12 ~ 49.5 mpg

7)     Reported by various media sources & Toyota

8)     Recent price of 89 octane gasoline in my locality

9)     Based on recent utility electric bills

10)  Data averaged from several websites

11)  ‘Best’ electric range (reported in HybridCars.com)

12)  ‘Best’ electric mileage = battery size/’best’ electric range = 4.4/15 = 0.293

13)  Electric mileage reported by EPA

Conclusions:

1. Based on this analysis and assumptions, there appear to be distinct regions where either the Volt or Prius will dominate the mileage comparison (figure 4).
2. The criterion upon which a comparison can be made between the performance of the Volt and Prius is technically straightforward. On average, if we use as a criterion the amount of energy consumed, the Volt shows a long distance cross-over of 75 miles. Whereas, if we use the cost of the energy consumed at today’s price, the long distance cross-over falls to about 55 miles (figure 3).
3. A clear implication here is that as gas prices rise relative to the electric rate, the dollar based long distance cross-over will increase approaching that of the energy based long distance cross-over.
4. The electric rate and gas price are both relative to the local market in which the comparison is made and are important parameters in the comparison.
5. The time-of-year and temperature are additional variables that will affect the comparisons.