I had a small 800 W oil filled rad ( smallest I could find ), used to use it in the caravan or the VW splity all the time, they used to say it used to cost 2p a hour to run, which I was more than happy with

I used in the conservatory in the milder evenings, it was no good in the depths of winter and it was freezing out there

the only downside is that it just heats the air around it, so if you are 10ft away, you might to feel the warmth for a few hours ( depending on the output of the heater ) , leave it on all the time and it'll keep a room warm though, its just slow as it has no fan