Hybrids, Plug-in or Otherwise

Martin Eberhard, Co-founder of Tesla Motors, September 27, 2006

Many of you have asked me about hybrid technologies, forcing me to gather my thoughts on the subject. Let me start off by drawing a clean distinction between today’s hybrids (like the Prius), and “Plug-in Hybrids” as proposed by James Woolsey and others.

Today’s hybrids are purely gasoline-powered cars. Forget the hype from Toyota – the car is no more an electric car than my 1942 Jeep. The only way you can put energy into the car is through the gas tank. The hybrid system simply captures some of the energy that would otherwise heat up brake pads, and reuses some of this captured energy to offset acceleration.

My calculations show that this energy recycling should increase the urban driving cycle of an equivalent non-hybrid car by up to 15%. There will be no savings for highway driving because you don’t brake much on the highway.

Do I think hybrids of this type are a good idea? Sure. Who can be against a 15% fuel savings? In fact, I expect all gasoline cars to be some form of mild hybrid in the next decade. This feature will be like fuel injection. (Do you remember when cars used to sport a chrome “fuel injection” badge? Later this became just “EFI”, and now you never see such a badge. Obviously, fuel injection didn’t go away – it became the norm.)

As gasoline-powered cars, hybrids still force you to choose between performance and efficiency – pick one, or compromise both. (Toyota Prius: 55 MPG; 0-60 mph: 10.4 sec, Lexus RX400: 30 mpg, 0-60 mph: 7.2 sec)

A world of 100% hybrids is still 100% addicted to oil.

Several of you have suggested installing a gasoline (or other fuel) generator in the trunk of a Tesla car as a range extender. Such a feature would convert the Tesla car into a form of plug-in hybrid. There are outfits like Hymotion that sell Prius modifications, filling the trunk with batteries and providing plug-in capability.

Note that the gasoline engine for such a hybrid car has to be pretty much as big as a car engine, since it ultimately must power the car by itself when the batteries are exhausted. Don’t think about that 1-cylinder, 1-kW Honda generator you can buy at Home Depot; think Prius engine, at least.

AC Propulsion built an electric VW Jetta a few years back that had a VW Lupo engine in the trunk, powering a generator. As with all AC Propulsion conversions, this was a masterpiece of craftsmanship that looked almost like it rolled off a VW assembly line. But it carried two drivetrains – electric in the front, gas in the rear – and so had no trunk space at all. The electric range was pretty low (I ran it empty in about 40 miles), and the car was kind of weird to drive because the engine would start up on its own, and would run full-speed (loud) no matter how slowly you drove.

I’m a bit conflicted about whether or not I think plug-in hybrids are a good idea. Plug-in hybrids are really a stepping stone on the road to a pure EV. Most people view them as an intermediate step. At Tesla we’d prefer to jump to the finish line and build the best pure EV possible now. Let me run through the pros and cons.


  • A plug-in hybrid definitely allows you to drive on non-petroleum energy. Depending on the battery size, this could be a substantial amount of your daily driving.
  • A plug-in would allow you to take a long trip without waiting for an electric charge – it simply operates as a gasoline car (albeit with a heavy load of batteries onboard) once the electric range is exceeded.


  • A plug-in hybrid (with a small battery pack) is much more abusive to the batteries than a pure electric car is. This seems counter-intuitive, so bear with me. Let’s say your particular battery design is good for 500 full charge-discharge cycles. On a pure electric car that goes 250 miles per charge, the battery pack should last 500 X 250 = 125,000 miles. On the other hand, a hybrid with, say, a 50 miles range will cause the capacity of the batteries to drop much sooner: 500 X 50 = 25,000 miles.
  • Of course, you could put a full Tesla Roadster-sized battery pack into a plug-in hybrid – but then the car is even more expensive – a full electric drive train PLUS a full gasoline engine. (And the weight of the gasoline engine will reduce electric range.)
  • Any hybrid is still subject to the complexity of gasoline engine maintenance: oil changes, smog checks, tune-ups, etc.
  • From the perspective of a small manufacturer like Tesla Motors, a hybrid drive also means another nightmare of legal requirements in the form of EPA regulations, CAFE reporting, servicing, mandatory emissions component warranties, etc.
  • A hybrid has double the safety concerns: a pack full of charged batteries AND a tank full of highly flammable gasoline
  • While appealing at first glance, the whole idea of a gas-powered range extending trailer is not practical. Just think about the safety and emissions nightmare! And backing up a trailer is no fun! (Our CTO, JB Straubel, had a philosophical problem with a gas-powered generator in an EV’s trailer. There are just too many unneeded energy conversions from mechanical to electrical energy and back. So he built this amazing contraption and followed it up with a second version. When I read about it, I knew that he was either a wacko, or a mighty clever guy with a keen sense of humor. :-) )

In the end, I guess plug-in hybrids are okay as a stop-gap measure for people who really want one car that is a daily driver and also a road trip car. But it seems so much more practical to use a purely electric car for the 99% of our driving that is less than 250 miles per day, and just take our other car (or rent a car!) for the occasional road trip. That way, we don’t lug around a whole gasoline drivetrain every single day just to be ready for the rare long-distance trek.

Tesla Motors will remain focused on building the best electric cars for the foreseeable future. With each passing year, our driving range will get longer and the argument for plug-in hybrids will get weaker. To hell with gasoline :-)