At the bottom of the food chain are solar powered organisms. They use sunlight to make the compounds which make them, and the things that consume them worth consuming. Accumulate incredible amounts of those little critters' bodies over time, bury them, subject them to the heat and pressure of burial and some tectonics, and those little solar powered beasties are rendered into oil, and a host of hydrocarbon gases which either come bubbling out at the surface or are trapped in the formations of rock from which we extract them.
You want more plant based? look no farther than coal, which ultimately is the stored and unoxidized remains of enormous amounts of long dead plant matter, plants which took their energy from the sun.
There is your efficient means of storing solar energy, and the means of recovering it and utilizing it are well known and have been refined over the years since we started using them.
Ultimately, all of the energy required to mine/extract the materials, refine them, build the vehicle, and power it came from the sun. (Hydropower relies on the sun evaporating water to form rain to fill the reservoirs behind the dams, the very elements utilized in nuclear energy are formed in stars), and the sun warms air masses which shift that altered density in the form of wind.)
Ultimately, it's all solar, it just takes different forms.
