By using energy staff you gain 45 % more energy than without staff.
Is it worth it?
It depends on your tactic, deck and if there is energy boost.
If you use staff just purely for the energy generation, then you’ll lose 0.43 bars if used before energy boost. This can be a smart trick because, with energy staff you can “save” your energy for some seconds by placing the energy staff when you hit 9.5 bars. That way you force the fight to your side of the map which is preferable when playing mystical. Let the zap damage do its job. You can now push the enemy harder as your units will be more clumbed up.
If you use staff once you get energy boost you’ll gain a surplus of 1.25.
Conclusion:
If you use energy staff before energy boost you’ll lose 0.43 bars.
If you use energy staff after energy boost you’ll gain 1.25 extra bars.