A genset trailer is a range extending device for use with battery electric vehicles consisting of an internal combustion engine and an electric generator (collectively called a genset). They run on traditional fuels such as gasoline or diesel and are sized to provide the continuous power requirements for the vehicle they are used with. Most small to midsized passenger vehicles would require 15 to 20 kW for unlimited freeway travel using fuel. Larger vehicles could require 30 kW or more of power depending on how heavy or un-aerodynamic they happen to be. This essentially converts an electric vehicle into a series-hybrid.
One such trailer is the AC Propulsion backtracking Long Ranger range extending gas fueled trailer, making it a gas-electric series plug-in hybrid electric vehicle. This trailer used a 500 cc Kawasaki engine with a 9.5 gallons (40 liter) fuel tank and achieved 30 to 35 mpg (6.72 to 7.84 litres/100 km). It is rated at 20 kW DC output and can maintain 60 to 80 mph (95 to 128 km/h).
The setbacks involved with this system are emissions from running the engine (CO2 and other gases), as well as the additional cost of gasoline. Another setback with this system is the fact that it has to be a trailer, as these are impractical for some drivers.