Wait 'til the farmers see me in my Prius!

I just bought a 2004 Toyota Prius. It has little dings around the edges, but it still runs well and gets over 50mpg. Replacing my long-serving 1988 Honda Accord wasn't easy, but the Prius looks to be up to the task!

When the warranty expires in about six months, I'd like to try converting the Prius to a plug-in electric vehicle (PHEV). Unfortunately, battery packs like the Hymotion cost about $10,000. That price requires some serious reflection on my long-term driving plans. If I drive the PHEV Prius into the ground, perhaps putting another 170,000 miles on it over the next eleven years, I might look back from the year 2020 and say the conversion was worth it....let's do some calculations and try to find out.

First, the basics on PHEVs: they make a lot of sense for shorter trips, either allowing all-electric driving for 10-20 miles at lower speeds or doubling the already high gas mileage of a Prius for faster 30-40 mile drives. Automakers keep talking about selling new plug-in hybrids, but since cars like the Chevy Volt remain in the uncertain future, current examples almost all feature battery conversions of a Prius. These cars must be plugged in all night for a full charge, but this can be done at any outlet and having the gas engine for backup means PHEVs won't leave their drivers stranded by the side of the road when the batteries run dry.

Much of my driving consists of 10-20 mile trips; I only get on the freeway to leave the county about once a week. I'm going to assume that over half of my miles would be battery-powered, a total of 8,000 miles a year (for example, a round trip from my house to the Rexville digester site is about sixteen miles and can be done on back roads driving less than 35mph--if I'm the one who has to visit the project every day, I would put on almost 6,000 all-electric miles a year). Going battery-only on some trips (and battery-assist on many more) will save about three gallons of gasoline a week compared to just driving an unmodified Prius while adding roughly 30kWh to my power bill. At current prices, I'd save about six dollars a week in fuel costs.

I expect the price of both gasoline and electricity to be higher in 2020. Some of the inflation will likely be due to greenhouse-gas regulation, which makes exact comparison difficult. But let's just assume that after subtracting any carbon tax, gasoline costs $5.00/gallon and retail electricity costs $0.13/kWh. In 2020, I'd be saving about eleven dollars a week in fuel costs; over the course of eleven years, I would have saved a total of $5,000 dollars. That's about half the cost of the PHEV conversion.

I subtracted the fuel carbon cost above so I could fairly estimate the real greenhouse-gas savings from driving a PHEV; it looks like I'll have to consider the other half of the plug-in cost as a contribution to slowing climate change. By 2020, my gasoline reduction would be responsible for avoiding fourteen metric tons of carbon-dioxide emissions. Unfortunately, the calculations don't stop there. Since roughly half of Puget Sound Energy's electricity comes from burning fossil fuels, the new electricity I'll use for driving each year will add hundreds of pounds of CO2 emissions. My total net reduction in carbon-dioxide emissions is unlikely to exceed ten metric tons. Those are expensive tons; I'd pay about $500 per avoided ton of CO2!

PHEVs aren't going to save the world. The increased efficiency of electric driving pales in comparison to serious transporation solutions like commuter trains and better bus systems, while larger projects (like anaerobic manure digesters) can cut greenhouse-gas emissions much more quickly and economically. I hope the battery conversion kits come down in price; I may still get one if they don't. It's a subject that deserves plenty of reflection.