I've read the docs for the geocoder gem which state you can set a key, client and channel when using Google Premier.
According to some other posts I've read here, it's now possible to use an API key and still not pay as long as you're below the free threshold. We need to do this as we host with Heroku and we keep hitting our daily limit. We're not ourselves, but without any sort of other identification, we're probably reaching a limit identified by IP shared with other Heroku sites. Using a key will help identify us and therefore keep us from hitting a limit.
However, when I look at the sign up pages for the Google API, there are a baffling array of client ids, api keys and secrets, for installed apps, web apps and so on. Which combination is the one required to make geocoder burst into life?