I have an oil-filled radiator with three heat intensities:
Given the radiator has a thermostat, what is the most efficient heat setting to use?
My goal is to warm a conservatory on cooler morning, so it would probably run for a couple of hours.
Like most space heaters, an oil-filled radiator uses resistive heating, which is greater than 99% efficient at converting electric energy to heat. But there are a few other considerations to determine the best setting for your space.
The principal benefit of an oil-filled radiator compared to other space heater types is in the capacity of the oil to retain and continue radiating heat for an hour or two after it has stopped consuming power. This means that it may not be the best application for a space that only requires heat for an hour or two -- it takes time to heat up the oil, but once the oil is hot the heating element only needs to switch on periodically to keep it at temperature so that it can heat the space. You may find that the heater doesn't warm the space during the time you're there, and keeps warming it long after you leave -- an inefficient use of energy!
Another aspect to consider is the heating load. This is the amount of heat energy that needs to be added to a space in order to maintain a specific temperature. Heat load is a function of:
Imagine the room like a leaky tank of water with a faucet keeping it full. The heat load is the net loss of water through the holes, and the flow from the faucet must match this load to keep it full.
For your room, the heat load can be measured in watts, and your space heater setting must exceed this value in order to be able to bring the room up to temperature, and then maintain it.
The easiest way to determine the heat load is to test it with the heater. Start at the lowest setting (900 W) -- if this warms the room and keeps it warm, then the heat load is around 900 W. There's no need to use a higher setting, unless you want to more rapidly heat the room. This will also depend on how cold it is outside, of course.
As brilliantly pointed out by @LShaver, since all the radiator heat levels have the same efficiency and the same energy source, the question has little sense.
The parameters of this problem are others.
e.g. ask the people that has to stay in the conservatory if they are fine with lower temperatures among the one resulting by keeping your radiator on, for the same amount of time, on level 2 or level 1, rather than on level 3.
