Ok, so here’s a disclaimer: my works are actually a fan spin on the mainstream entries of the Ultraman Series by Tsuburaya Productions. That said, there is worldbuilding involved, as I’ve incorporated many of my own characters and settings into my stories.
Anyway, a species known collectively as Ultras, who are giant humanoids that serve as the driving force and protagonists of many of my works (and are the namesake of the Ultraman Series), has the power to fire highly powerful energy blasts from their arms by putting their arms together in a variety of different formations.
While most Ultras have their own versions of these energy blasts, which are collectively known as Ultra Beams, the most basic and most widely-used version is called the “Specium Ray”.
Now, seeing as this is a science-based question, I had to come up with what the Specium Ray actually is. But first, do note that understanding what the Specium Ray is is fundamental to understanding all other versions of the Ultra Beam.
With that out of the way, the Specium Ray and all other Ultra Beams are a hybrid beam. In the case of the Specium Ray specifically, it is a typical laser electrically connected to plasma particles. Additionally, the photons in the laser part of the Specium Ray are constantly shifting between different frequencies, similar to a phaser from Star Trek. These frequencies adhere to the Inner Light, a unique matrix of frequencies, of the individual Ultra who fires the attack.
As such, the reason the Specium Ray is the most basic Ultra Beam is less because of the signature and more because of the application. Essentially, all Ultra Beams are based on Specium, but some Ultra Beams would be less focused on the laser part of the attack and more focused on the plasma part, or vice versa, etc. but the beams themselves are still mostly composed of Specium. In other words, the Specium Ray is the most basic application/combination of the parts of an Ultra Beam.
Edit #1: The question, as said in the title, is what units could be used to measure the power output of an Ultra Beam. In other words, what units would be most fitting for how destructive an Ultra Beam?