A plug-in hybrid electric vehicle (PHEV) can operate on electric power for short trips without needing the liquid-fuel combustion engine. It thus offsets the need for liquid fuel by relying partially on the electricity grid. While liquid-fuel savings can be significant, the climate benefits of PHEVs depend on the local grid characteristics (coal/gas/nuclear/ renewables mix), when charging occurs, and the driving/recharging profile. As of 2007, no commercially-produced PHEVs were available to the general public.
Plug-in hybrids differ from traditional hybrids by having additional battery capacity and the ability to be recharged from an external electrical outlet. Depending on the size of the battery pack and other factors that typically affect fuel efficiency, PHEVs have an electric-only range of anywhere from 10 to 60 miles. The internal combustion engine is not needed for moderate speeds within this range, but is available for longer trips or if additional power is needed. Battery types include nickel-metal hydride (NiMH), currently used in all conventional hybrids, and lithium-ion (Li-ion). Li-ion batteries are smaller and lighter than NiMH, though they cost more and are not as safe or durable. When operating on liquid-fuels, PHEVs carry a weight penalty due to the relatively large batteries they carry.
The fuel and climate benefits of PHEVs are largely dependent on the amount of time the vehicle is using electricity instead of liquid fuel, which is further dependent on the electric-only range of the vehicle, trip duration, and recharging method. Climate benefits are additionally determined by the fuels used to produce the electricity, and whether excess capacity in the grid can be used. Due to this wide range of factors, estimates of the potential for plug-in hybrids to reduce CO2 emissions compared to standard petroleum vehicles range from 10 to 60 percent. If users do not charge their cars during off-peak times, the climate benefits of PHEVs are reduced.
Operating costs in the electric-only mode are estimated to be roughly equivalent to around $1 per gallon of gasoline, much lower than liquid fuel vehicles. However, PHEVs have higher upfront costs (largely for the battery) of roughly $7,000-10,000 compared to traditional vehicles. Additional R&D is needed to develop long-lasting, efficient batteries.
PHEVs offer significant energy security benefits, as they offset oil consumption with mostly domestic sources of energy whenever battery power is used. Additionally, PHEVs can reduce air emissions in urban areas as they do not emit pollutants during the electric-only mode. Conversely, they can lead to increased mercury, sulfur, and other pollutants from coal-fired plants supplying additional electricity.
PHEVs are still an emerging technology today, primarily due to their high upfront costs and limited electric-only range. With advances in technology that increase this range and reduce battery costs, plug-in hybrid vehicles are likely to become more common.