Underfloor heating and room thermostats are controlled through a relay (on/off) signal. The relay is being activated with a varying duration when heat is requested.

At a high heat request level or high intensity heating, the relay is closed for a longer time than for a low heat request level or low intensity heating

This is data is summarised as Consumption data in the Themo webapp and displayed as graph

Consumption is calculated from how much time the relay has been activated and then multiplied by how much electrical power has been applied to the heating system. 

Themo has built in capabilities to estimate the power level of your direct electrical heating. For hydronic based systems the calculation of power consumption may be inaccurate.