Solar panels are somewhere around 20% efficient under optimal conditions (directly facing sun, at noon, on a cloudless day, not behind glass). The process of decomposing water into hydrogen is something like 33% efficient. That alone makes your 100W panel output only 33W equivalent power in hydrogen. Let's say the combustion process is 25% efficient at extracting heat energy (combustion) and making kinetic energy. That means 8.25W of power provided by the 100W solar panel, or about 1% of 1 horsepower.
You may as well attach a hamster wheel to the crank, or add half a PSI to all 4 tires.
Efficiency means making as few conversions of energy as possible. If you're capturing sunlight and turning it into electrical power, keep it as electrical power to maximize efficiency rather than subjecting it to these conversion losses:
sunlight > electricity > hydrogen (chemical energy) > heat > kinetic
Exploiting energy in a Rube Goldberg way is not efficient, just as his contraptions for flipping a switch are not efficient.
