I just went through the specs on the Roland KC-220, draws just 500ma, which calculates out to 6-watts. The peak power is rated at 30-watts, so by and large, your keyboard's onboard sound system is twice as powerful and, obviously, balanced to the keyboard.

When I had some small, living-room and deck parties to play, I often used the onboard system with a couple homemade speaker covers that deflected the sound outward toward the audience. This was a great setup for these situations and never got a complaint that it was not loud enough. The speaker covers looked a lot like air scoops on a hot-rod car and were made from black cardboard and held in place with Velcro. Additionally, the scoops prevented any feedback yet allowed sufficient sound to reach the player for monitoring.

Once you get above 20 ppl in the audience, I think your standard sound system comes into play in order to provide the needed coverage.

Hope this helps,

Gary