Every level 5 power boost will increase your player's power by 5.
This directly corresponds to increasing the chunks that your Faction can claim by 5!
Upon checkout, you can decide if you want to apply the boost to your player or your Faction.
If you apply the boost to your Faction, you will immediately be able to claim more plots, but if you leave the Faction the power will remain with the faction.
Should you choose to apply the power to yourself, you will need to let your power regenerate up to the new maximum by being online. Thus your power won't be available right away, but the power will stay with you if you leave a Faction.