Presumably you don't want to run afoul of the fair labor act. There are numerous "exemptions" such as executive, administrative and professional workers; salespeople; computer specialists, apprentices, and some others, but I doubt installers would qualify. If your installer is an independent contractors they may be exempt (unless all of their income comes from one source).

The Fair Labor Act does allow for pay on a piece-rate. As long as you end up paying the minumum wage (including time and a half for overtime), you may be ok on that basis.