My 180 square foot sunshed is heated by a natural gas, ceiling-mounted heater. These are heaters commonly used in garages. The heater is over 20 years old and has suffered from being in that greenhouse through changes each of those years. A couple of years ago, the repair guy said that it should be replaced rather than repaired with replacement parts. However, the heater has worked quite well since he said that.
It has been repaired a few times but I have no idea if it will work in 2021. I suppose that the real problem for it is that it only runs for 8 or 10 weeks, each year. The last few years, I have backup, electric heaters in there and they would do the job, if necessary. I would really rather not heat with electricity, too expensive. I'd really rather not replace the gas heater, too expensive!
I can "get by" well enough with 2 bathroom, electric heaters - each burning about 1000 watts of power. Artichoke Lover, knowing your heat lamp requirements should give you some idea on that. I imagine that when you are trying to protect plant starts over several weeks, outdoor temperatures will not be all that different from most locations. After all, those plants will soon be in the outdoors. Your outdoors, my outdoors,
@flowerbug 's outdoors,
@Marie2020 's outdoors - regardless. Similar plants have similar temperature tolerances. Even if you go with only one heat source, a backup is a good idea. (Also, a remote thermometer makes life easier.)
Steve