I also tried to read the BS number shown on the front of the device and then search for the relevant British Standard. Depending upon how I read it, I was vectored to documents relating to drainage!
My second guess found that too. My third guess was for BS6301, which defines the safety requirements for connection to the phone network ... whereas BS6305 seems to be the more usual "general" set of requirements, and may be quoted more often. I stopped looking after that.
My understanding may be somewhat askew but surely it is not correct to say that a device which has a REN number assigned to it actually consumes some of the "ringing" energy. My overall understanding of the REN concept is that it is a simple numerical approximation of the overall loading that the device presents to the circuit.
My understanding was that the electrical power on the line was enough to be detected by devices, provided a total REN was no more than 4. Above that, devices weren't guaranteed to be able to ring properly.
In the days of real bells, I took that to mean that the share of the power would then not be enough to energise the ringer's solenoid.
(Too many loads + fixed power input) => share of power drops below threshold => ringing not detected
So the REN does indeed represent an approximation of "loading" - but the ultimate consequence comes from the consumption of energy that is the natural effect of adding more loads.
(However ... the length of the line plays a significant part in the "load" too, so I guess a REN of 4 mostly applies to the longest line, which places the biggest restriction on energy available to devices. Shorter lines can presumably cope with higher REN values).
In terms of a generic circuit, I always thought the load was indeed something the consumed electrical power, and that we could equate that with consumption of electrical energy. Wiki
agrees in part, but perhaps you see it differently.