Yes, you can use electricity to free hydrogen from water and then burn that hydrogen. But with current technologies, electricity is much easier than hydrogen to store and to transform into motion. Why not directly use the batteries to run electric motors and avoid the two efficiency-draining steps of electrolysis and the internal combustion cycle?To answer your question, I would recommend a gasoline engine for your backup. As a fuel for an internal combustion engine, gasoline has a much higher energy density than hydrogen. On a more practical note, you are much more likely to find a mechanic for your gasoline engine when your car breaks down in the middle of nowhere.