I think 5, 10 and probably even 2 energy in a purchasable capacity would probably be somewhat frowned on, but the idea of +1 energy per level has some interesting implications (and probably would also be frowned on) but it is fun to think about. I would love to give more reasons for players to level up.

The original suggestion is way too OP but I do like the suggestion that vaelinn made about enticing a player to level up. The gain would have to be small though otherwise you're also entering that realm. Something like 2e every 5 levels? Still gives you an extra 40e at level 100.

It should increase like 

+5 E AFTER LVL 10

+10 E AFTER 20

+20 E AFTER 30

+30 E AFTER 40

+40 E AFTER 50

+50 E AFTEE 60

Not like you get +5 and +10 as 15 on lvl 2.

Like the total 200 + 10 E