Can you show a screenshot of what you're talking about here?  I think I know, but I'm not sure.

If the XP Bar the short answer is: just play the game. FWIW the best/quickest way to rack up XP is just campaign, especially when you get to 12-3 brutal.

This XP bar DTHORNE is talking about (and I'm assuming youa re as well) is your progress through your current level.  It's not your strength or energy.

Does this low % affect my warriors?

No...it is just an indication how much you have advanced to your next account level. It has nothing to do with power of your champs or anything else in the game mechanics...it is just a counter to show you how many XP you gained on your current level.

Thanks for the answer 

Thank you so much for the information.

what a really pointless bot necro of a 2 year odl thread

and we lock the silly necro