You want to save on heating? Let's use a little science. Supposing your room is spherical, the heater is in the center, the thermal properties are homogeneous (doors and windows have the same properties of walls) and you are a reptile, this could be an equivalent thermal circuit.
Actually there are rules of thumb to calculate the amount of power required for to heat a room:
living room / office: 80W / m^3
bathroom: 90W / m^3
other rooms: 65W / m^3
BTW I'm not sold on the 'glass is a thermal insulator' argument. The surface area is huge (think why glazing is usually double and the trend is to go for triple). The argument is just as useless as (for example) comparing the conductivity of steel versus copper without taking the actual size of the conductor into account.
You're overthinking the problems with this thing.
We use this the whole time in electronics engineering. So for me this is second nature.
The thing is that what keeps me warm is not the energy dissipated by the heater, it is the temperature of the room. If I have a room with an incredibly high thermal resistance from its walls to the world, say, infinite, not considering the thermal capacity of the system, i.e., giving enough time, with a 1W heater, the temperature of the room will be infinite.
Conversely, if I have a room with a very low thermal resistance, say, zero, not even the most powerful heater will be capable of raising the temperature of the room, not even by one degree (Celsius, Fahrenheit or whatever).
Of course, if your heater has a poor thermal resistance, it will be hot, not the room.
So if you want an efficient heating system, make sure your house has a good thermal insulation, and that your heater is capable of producing the least temperature difference between the heating element and the room.
You can instantly destroy Solus's claim by showing that what makes heating efficient is the SYSTEM (heater+room) not the heater per se.
Any Inuit will tell you.