This article is a short explanation of how your electric heater functions based on your plug size. Important to determine your plug size compatibility and then choose your unit according to your needs.
Each unit has a different heater size which determines the efficiency of the room being heated. Other environmental factors like insulation, room size, etc. do play a factor in the unit's behaviour. However, below we have noted plug sizes in comparison to the electric heater size (KW). All are in 208/230 volts.
For example, if your receptacle is a 15 amp power cord the heater will lean towards the 2.5 KW voltage from the heater. However, if you had a 20 amp power cord, the unit will send voltage to turn on both heaters out of 2.5 + 1.0 KW which will give you 3.5 KW.