If you’re considering using a 100W system for an outdoor party, it will be loud enough for a smaller gathering of around 25-50 people. Any more than that, and you’ll have to upgrade to more robust, loud outdoor Bluetooth speakers.
If it’s an indoor party of 50-75 people, 100 watts will more than suffice. In fact, it will probably be an overkill with fewer people.
Bear in mind the volume level will depend on a combination of factors like sensitivity, speaker position, acoustics, and more.
To see how sensitivity interplays with the wattage, check out the Loudness section in our guide to buying the best Bluetooth speaker for an outdoor party!