My envoy-s is mounted on the side of my house next to the switchboard. My Envoy-S has a CT clamp clamped around the mains wire coming out of my meter to measure consumption which then leads into the envoy-s which measures the consumption data coming from this clamp.
Envoy-S stating it does consumption
CT Clamp
The Envoy C appears to be a little more portable, only meant for indoors and I assume it only talks to the microinverters at the back of your solar panels.
Envoy-C not mentioning anything about the ability to read consumption.
Only the Envoy-S metered version has the Consumption Monitoring capabilty.
I found this link that confirms this.
https://www4.enphase.com/en-uk/products-and-services/envoy/family