Comparing output of hydronic plinth heaters

[Why Not Indeed]

Looking at heaters from Myson, Smiths and Thermix.
Thermix and Smiths specifications show heat output at flow temperatures from 40-80°C in 5°C steps. Myson specify theirs at two, "∆T 50°C" and "∆T 30°C". For physically similar models, output at ∆T 50 seems to correspond roughly with a flow temperature of 80°C in the others.
What's the difference between these two ways of quoting output?

 

[Notch7]

I rather doubt there’s much difference between them, after all the physical constraints of these type heaters are pretty much the same across different brands.

 

[AdrianUK]

∆T means the difference between TWO temperatures, so, for example, the temperature inside your house is 25 C & the temperature outside is 5C then the ∆T across the wall is 20C.

Myson are quoting the power emitted when the temperature difference between the hot water flowing in & the water flowing out is 50C and again at 30C. This is really the only correct way to do it - you cant calculate the heat output without knowing the temperature drop across the heater & the flowrate through it