They are full hybrids, able to run on battery power. They offer greater battery capacity and the ability to recharge from the grid. They can be either parallel or series designs. They are also called "gas-optional", or "griddable" hybrids. Their main benefit is that they can be gasoline-independent for significant distances, with the extended range of an ICE for longer trips. Electric Power Research Institute research found a lower total cost of ownership for PHEVs due to reduced service costs and gradually improving battery technology. The "well-to-wheel" efficiency and emissions of PHEVs compared to gasoline hybrids depends on the grid energy sources (the US grid is 30% coal; California's grid is primarily natural gas, hydroelectric power, and wind power).