The biggest value that WattzOn is offering through its WattzOn API is access to its “Embodied Energy Database” (EED). This database seeks to detail the amount of energy required to manufacture, transport, use, and dispose of all our stuff. The idea is that if we knew the embodied energy in all the items we use, we could make better consumption choices, avoiding those items with unexpectedly high embodied energy.
The WattzOn API is RESTful and returns data in JSON format. It has been stable for just about a year now according to the changelog. To use the API you’ll need to sign for an API key up through the WattzOn website. Most of the API can be accessed with an API key, but to upload new data or read user data that has been marked as private a username and password must be provided over basic authentication. Of course, after completing this authentication you are only granted access to read your own private data or change your own public usage data.
Here's an example of entry from the EED for the iPhone: