So, the easiest way for me to explain the relationship of power level to abilities is this, and I hope it's clear:
Each "power level" adds 10% effectiveness to each power level BELOW that current level, cumulatively.
So, for example, if your character has power level 8...
Level 7 abilities will be +10% effective
Level 6 abilities will be +20% effective
Level 5 abilities will be +30% effective
Level 4 abilities will be +40% effective
Level 3 abilities will be +50% effective
Level 2 abilities will be +60% effective
Level 1 abilities will be +70% effective
Now this is a general rule as some abilities have different scaling. (Minoletta's Minor Missiles, for instance, would only have a 35% bonus even though it's a level 1 ability).
This means that the most you can get in a single character is Power Level 10 providing a maximum bonus of +90% effectiveness to level 1 abilities and +10% to level 9 abilities. (Without equipment, which can raise this further).