As Matt said, the difference is between them is formarity. For us, in casual conversation, げんきだ sounds a bit strong and we say げんき or げんきだよ with suffix よ.
By the way, this だ is used for NA-adjective when the sentence ends up with it, you can see it on articles, thesis, novels, comics, etc.
If you need more details, please wait for the more professional answers. I hope this was hrlpful.