This is probably because macOS Big Sur is doing some background work to set itself up – and after 2 hours or more, things should have settled down and your battery life should be back to normal.
But if its not working, then make sure all your apps are updated. You can also click the battery icon on the top menu bar, which will show you what apps and processes are taking up a lot of your battery life. Close those and see if the battery improves.
If it is working, then make sure the apps in question have been updated, and check any of their settings to see why they would be taking up so much battery life in macOS Big Sur.