Turns out there is a public api, not offered by efergy directly, but by the company who are hosting their measurement data, energyhive.
I found this site after noticing some unknown outgoing hosts on my monitoring server, login.lightapp.com. After doing a reverse dns lookup and checking for other domains on the IP, which included engage.efergy.com, I ended up on energyhive.com. Seems that efergy partnered up with hildebrand to do their software backend.
This top banner on the site looked very similar to the efergy portal, so I tried logging in and it worked!
Browsing a bit further on the website led me to the Developers page, which contains the basic info needed to get started.
So there is a semi official API to read data from the efergy devices.