*Star Wars VII: The Force Awakens*. What struck me was the details they showed:

The weapon is powered by siphoning off energy from a star, and then blasting it at a distant target. It occurred to me this week that it's actually pretty easy to figure out the energy needed to blow up a planet, and so I could assess the feasibility of doing it with power from a star. (Marika is sure I'll be put on government watch lists for talking about this.)

Planets (and stars) are held together with gravitational potential energy, which depends on the mass and the density. You can calculate it for a uniform sphere by adding up a series of concentric shells, or you can look up the answer on Wikipedia:

It's negative because gravity is an attractive force, and this represents the energy that must be overcome to scatter the planet far enough that it won't reform. For the Earth, this comes to 2.2 x 10^33 Joules, so now we need to know how much energy we can get from a star.

The Sun radiates energy at a rate of 3.846 x 10^26 Watts, or Joules/second. If the base collected the light passively, it would take about 66 days to charge up, which isn't ideal. It needs even more energy than that, given that it's able to target multiple planets at once:

Let's suppose it can absorb all the energy the star would produce over it's entire lifetime. For our sun, the estimated lifespan is 10^10 years. It probably won't have the same luminosity over its full life, but I couldn't find a simple model for how it would vary, so we'll suppose it's constant. Multiplying by the power from earlier, this gives 1.21 x 10^44 Joules, or enough to destroy 55 billion planets! Neither situation seems very practical, which may be why the Empire/First Order has so much trouble holding on to power.