Solar power was the use of solar energy to generate electricity. While using Daniel Eldred's rocket, the Second Doctor, Zoe Heriot, and Jamie McCrimmon relied on a homing beam that functioned with solar power. When the solar power source was disabled, the rocket was left to float in space.
The Doctor adapted the system so that it could be held in the hands and aimed. He used this to kill an Ice Warrior at the Weather Control Station.
Before heading to the Moon by T-Mat, the Doctor again adapted the device so that it was fully portable. With this he attacked an Ice Warrior guard, killing it before he was forced to surrender. (TV: The Seeds of Death)