In rocket science, the fuel efficiency of a rocket engine is measured by its specific impulse. Specific impulse refers to the units of thrust per units of propellant consumed over time. Because a solar-sail spacecraft carries no fuel, it has infinite specific impulse.
Future of Solar Sails
The major advantage of a solar-sail spacecraft is its ability to travel between the planets and to the stars without carrying fuel. Solar-sail spacecraft need only a conventional launch vehicle to get into Earth orbit, where the solar sails can be deployed and the spacecraft sent on its way. These spacecraft accelerate gradually, unlike conventional chemical rockets, which offer extremely quick acceleration. So for a fast trip to Mars, a solar-sail spacecraft offers no advantage over a conventional chemical rocket. However, if you need to carry a large payload to Mars and you're not in a hurry, a solar-sail spacecraft is ideal. As for traveling the greater distances necessary to reach the stars, solar-sail spacecraft, which have gradual but constant acceleration, can achieve greater velocities than conventional chemical rockets and so can span the distance in less time. Ultimately, solar-sail technology will make interstellar flights and shuttling between planets less expensive and therefore more practical than conventional chemical rockets.