Let's try to solve this riddle by starting with efficiency. In thermodynamics, efficiency is a term that indicates how well energy is converted into useful work. So, when talking about space heaters, we want it to produce as much heat as possible with little effort and without wasting energy.
Efficiency can be measured by comparing the level of invested energy to the level of output energy. If a space heater isn't very energy efficient and wastes energy, it will produce less heat energy compared to electricity it consumed. Ideally, we want all of the energy used to be converted into heat energy without any waste.
How is the energy spent?
As an example, let's take two space heaters with equal power, a 2 kW portable fan heater and a 2 kW oil filled radiator.
Since electric heaters are all 100% efficient, they convert all consumed electrical energy into heat, which means there’s zero waste. If both of our heaters have a real power of 2 kW, exactly 2 kW of heat is created by electrical heating elements and dissipated into the environment.
This means that our fan heater and oil radiator should heat the room equally. But, that might not be the case.
The main difference between these two heating devices is that a fan heater consumes some energy on the motor that keeps the fan blades running, and this energy isn't producing any heat. Because of this, we might only get 1.7 kW of heat that is produced by the fan heater. Since an oil radiator doesn't have any moving parts and dissipates heat through radiation and convection, every invested watt is converted into heat. This comes down to 15% difference in heat produced, and we can't say it's a negligible difference.
Also, if two heaters are declared as 2 kW, this doesn't mean that they actually consume 2 kW of power at all times. Depending on the design and materials used in a heating element, it's effective value of current and real power decrease as the heating element gets hotter and it's ohmic resistance increases.
If we want to be completely accurate with our calculations, we would need to measure energy consumption of both heaters with a wattmeter. Just because a heater is labeled as 2000 W, it doesn't mean it has 2 kW of power in practice. If a heater labeled as 2 kW in reality consumes 1.8 kW of power, it may seem like we've saved 10% on our energy bills, but in fact - the heater produced 10% less heat.
Produced heat vs. effect on room temperature
Now that we've established the fact that both of our heaters are nearly 100% efficient, let's talk about how they affect the room temperature.
Equal production of heat per heater doesn't necessarily mean an equal effect on the average room temperature and the feeling of warmth. There are a lot of factors that play a role in how the heat is dissipated, and we'll name just a few.
Bigger heaters come with bigger exchange surfaces and heat up the air more easily. If we want to get the same temperature in every part of our room, it might be more efficient to strategically place two or more heaters at different places in the room. This would allow every unit to operate at lower temperatures and provide a more consistent temperature instead of one unit placed in one corner of the room. This concept is well known with underfloor heating which operates on low temperatures but covers a large (or whole) area of the heated room.
Feeling of warmth
The heat sensation is not the same with every type of heater, since the heat can be transferred by the three different vectors - radiation, convection and conduction.
A fan heater blows hot air around the room and warms the room more quicker compared to other heaters like oil radiators or convectors. The warm air circulates the room and reaches all parts of the room, thus producing consistent temperatures. But also produces noise and doesn't give that warm radiant feeling.
Oil filled radiator gradually heats up the air and the nearby objects by radiating heat. It doesn't blow hot air around the room, doesn't produce any noise and provides a gentle and comfortable feeling of warmth.
The infrared heater instantly heats objects and people and objects in its vicinity with infrared radiation, but it doesn't heat the air.
All space heaters except infrared generate and circulate warm air around the room to some degree either by convection, radiation or with the help of a fan that pushes the air around. This can play an important role in how we perceive the heat.
In either case, because of air stratification (thermal stratification), warm air immediately goes up and and it stays at the ceiling. As it cools down by the heat losses and heat dissipation to the structure of the house (ceiling, outer walls) it starts to come down again to be warmed again.
Cool and dense air sinks, warm and thin air rises.
Heater temperature control
Heaters with integrated thermostats usually aren't accurate enough and don't necessarily correlate with the room temperature but rather with the temperature of it's body and the temperature of the air around it. Primitive thermostats are too close to the heating elements, thus only sensing the temperature in it's vicinity.
Some heaters with more sensitive thermostats can detect slight changes in room temperature, while others detect change only when there's a few degrees drop in temperatures. This can affect the heater efficiency and ultimately the amount of energy it consumes.
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