The energy from the star isn't needed to destroy a planet, the Death Star could do that. I'm pretty sure it's needed to shoot the planet destruction laser thing (PDLT) into hyperspace, because reasons.
Just a bit of in-universe information. The energy output of the Death Star 1's superlaser cannon was calculated by a Star Wars fan with a scientific background as being about the same as the output of a sun-type star for the same duration (about 1/2 of a second). This was based on an assumption that Alderaan was about the same size as Earth and, based on that, calculating the average outward velocity of the debris and using that to calculate the energy added to the planet's body to break it apart in such a way.
Given its range and the fact it could destroy several bodies in one discharge, we must assume that the Starkiller's weapon was many times more powerful.
Well, there ya go. If the output of a Death Star was the output of an average sun for 0.5 seconds, then consider the fact that a sun maintains such an output for several billion years. With that in mind, it's logical to conclude that you could literally destroy several billion planets with the entire energy output of one star. They could hypthetically use some justification like having to use EXTREME amounts of energy to fire the weapon through hyperspace, but this doesn't really fall in line with hyperspace being a relatively common feature in this universe, and even small, private operators (like Solo) having access to ships with hyperspace. It would also make firing it from hyperspace highly inefficient as opposed to firing it within normal ranges.
TL;DR: Starkiller base is a poorly conceived macguffin.
