> the most efficient way to convert the power of the sun into forward movement

Very inefficient, and the difference is several orders of magnitude.

A horse needs 2-40 acres of land: https://horses.extension.org/how-much-land-do-i-need-for-a-h...

Photovoltaic panels can generate 500 kilowatts on 2 acres. Tesla's V3 supersharger is 250 kilowatt, adds 170 miles in 15 minutes of charging.

If the sun is shining for 6 hours, photovoltaic panels on 2 acres charge Teslas to drive 8000 miles. For a horse, the figure is like 40 miles per day.