The problem is that any heater is just converting electricity to heat via resistance in a wire (or actual resistor). If you know about electricity, energy loss via heat escaping is a suuuuuuuper inefficient form of energy conversion. Heat generated is literally something that is normally avoided and attempted to prevent due to how much power it wastes in any system that is not MEANT to put out heat. Horrible "bang for your buck".
It takes a lot of wattage to generate heat. Therefore it is going to be REALLY power intensive to run any sort of electric heater. Whether that is a fish tank heater, heat tape, a hairdryer, a forced air heater to heat your off grid cabin (to keep the humans warm), a red heat bulb, etc..
People have success with solar based lighting and solar chicken doors because it is not hard to power a 9 Watt CFL bulb, or a much smaller wattage LED bulb, but it is hard to power a 50 to 100 watt heater (that is on the low end of what you would need) for the entire time the temp is below freezing. So months on end, when the sun comes out the least (robbing your solar panels of any recharging juice). Same for a chicken door. Not super hard to have enough solar juice to run a small motor for less than 60 total seconds per day with the time between used to recharge the battery.