Here is a good way to find out.
Do you have a smart phone? Go download a free sound meter app - it won't be super accurate since it's not calibrated to your phone, but it will give you an idea of what you need.
Once you have the app, setup your current system to "listening level" and note down the DB, then setup for your "party" level, and note the DB.
Let's say your listening level is 107db, your speakers are 104db, so you'd need 1 watt for 104, and double that (2 watts) for 107db.
Lets say your "party" level is 120db, (just throwing a number out there, personally I don't think party level has anything to do with high end gear but whatever) Again, your speakers are 104db at 1watt,
104db 1 watt
107db 2 watt
110db 4 watt
113db 8 wat
116db 18 watt
119db 36 watt
Again, without a proper calibrated sound meter (check Amazon for real ones), this will be all ballpark, but it will give you an idea of what *you* need. Your idea of "party level" is probably completely different than mine.