A good way to identify what is might cause your power consumption is to use the Unity profiler and see what is going on under the hood. Making a game in Unity UI only, is suboptimal, as it has a lot of performance bottlenecks. You can find guides on optimization here: https://docs.unity3d.com/Manual/BestPracticeGuides.html especially Optimize UI!
Also, depending on your target platform (e.g. Android Samsung), you might also want to look into what Adaptive Performance does and you might get some ideas what do you can do: Adaptive Performance Package