I just received one of these DIN-rail units from GearBest – and the price isn’t bad at £7.73 (sterling)… inc free shipping to the UK. https://goo.gl/aQYcqU
At first I could not understand how they managed with two wires. Simple really. A pair of connections take in mains power (for the meter) and you feed neutral directly to the load and the live by a suitably thick cable through a round vertical hole and off to your load (or a breaker as required). The current measurement works like a standard clamp meter using magnetic induction.
This unit measures from 80-300v and from 1-99 amps and hence could be used for measuring power to one device or the whole house depending on your house. Only tackle this it you are comfortable handling mains voltages and have suitable experience. If you blow yourself up, no responsibility accepted.
Personally I plan to use it to measure the appliances in our kitchen as I’m convinced that’s where the bulk of our power goes. Of course you don’t HAVE to mount it on a DIN rail.