If we assume 100 colonists on the ship, with 10 tons of mass per person, accelerating to 0.2c over 2 ly and then back down, the energy required is quite substan

If we assume 100 colonists on the ship, with 10 tons of mass per person, accelerating to 0.2c over 2 ly and then back down, the energy required is quite substantial. Multiple centuries of the total output of the whole planet.