'Towel radiator' is really a bit of a misnomer, they are now often more correctly referred to as 'towel warmers'. As a rule of thumb, a 'towel warmer' will emit about one third of the heat emitted by a radiator of the same size. But that assumes it is uncovered. Once covered with fluffy insulating towels the heat output is reduced to not very much at all.
Having said that I have known larger electric towel warmers 250 watts or more, if left on permanently keep a small well insulated bathroom reasonably warm, but would be hopeless at heating the room up from scratch first thing in the morning.
My first house (super insulated timber frame construction) with a small bathroom had an electric towel warmer and also a wall mounted fan heater which gives instant heat for a fast top up as required. The towel warmer kept the chill off, and I just switched the fan heater on whilst I was in there showering.