The max DPS increase you can recieve from Dark Ritual is 20%. This is done through 4 easy steps.
1. Activate Energize- You must make sure you do not use any abilities after this, or else energize will be lost
2. Once Energize has cooldowned, make sure that the golden outline is around the symbol, if not than energize has not been activated or has been used already.
Now activate Powersurge
3. Activate Energize again
4. Activate Dark Ritual


This will altogether increase the DPS by 20%, 10% from powersurge(i.e 300% dps) then 10% from the energized dark ritual.