An Australian-based tech firm, Solar Sailor, is working on just that—massive sails embedded with solar panels. A computer pivots the sails toward the sun (it's always sunny in the French Riviera!) and the energy collected is used to power the yacht. CEO Robert Dane thinks a whopping 10 percent of the fuel consumption could be offset by the panels. Ten percent? Wonder how much energy you'd save by turning down the jets on your diamond-encrusted hot tub.