Friday, October 21, 2011

Making Plug-in Hybrid Comparisons

Published October 21, 2011

Last year, General Motors officially launched the Chevy Volt, a range-extended electric vehicle (REEV) with an electric range of 35 miles and an EPA-rated electric-only equivalent fuel economy of 93 MPGe. This past summer, Fisker began deliveries of its REEV, the Fisker Karma luxury sports sedan, which will have an official range of 32 miles in all-electric mode—during which time it will get an equivalent efficiency of 52 MPGe. In a few months, Toyota will release its Prius Plug-in hybrid (PHEV,) which will be able to run gas free for its first 15 miles after charging, and is expected to receive an 87 MPGe rating.

All three cars run on both gas and battery power but there are key distinctions, stemming mostly from the differences between PHEVs and REEVs. Where the Volt and Karma run strictly on electricity until their battery range is depleted, the Prius optimizes use of its electric and gas power even when it's battery is at or close to full charge. In other words, you can floor the accelerator while merging onto a freeway in a Volt or Karma and still operate on electricity alone, but a Prius Plug-in will likely use its gas engine to assist in that process.

What does this mean for drivers in terms of their emissions footprint and daily fuel costs? As Pike Research's John Gartner pointed out this week, that all depends on how far you drive between charges. In comparing the Prius Plug-in to the Chevy Volt, Gartner calculated fuel costs for both vehicles as they relate to trip distance. (The Fisker Karma wasn't included on the study, but it has the same electric range as the Volt and follows the same principals even though its gas-only fuel economy is lower.)

Gartner's results, seen in the graph below, indicate that drivers who travel within their REEV's electric-only range can expect significant fuel savings compared to a Prius Plug-in. Beyond that point the cost comparison begins to narrow:

“Assuming a gas price of $3.50 and electricity at 11 cents per kilowatt-hour, the Volt’s bigger battery makes it cheaper to operate as long as you drive 70 miles or less between charges.  At distances of greater than 70 miles, the Prius PHEVs’ greater fuel economy as a hybrid makes it cheaper to operate.”

It should be noted that a PHEV's occasional use of its gas engine before its battery runs out contributes very little to total fuel use. A commuter driving less than 15 miles between charges won't spend much on gas or contribute a significant emissions toll whether she's in a Fisker Karma, a Chevy Volt, or a Prius Plug-in. Still, for some drivers who rarely travel more than 35 miles at a time, the allure of going totally gas free could prove to be a very attractive selling point for REEVs.


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