Can you extend the house heating system into the sunroom? In the long run, that may be the most cost efficient.

Thanks, Vladimir. That is an excellent idea, someday if/when I can afford it! Actually, I'd love to enclose the whole sunroom to make it functional. But in the meantime I have two old cats to keep warm this winter.

There are such things as ventless wall or floor mounted propane heaters. They do rooms. If you have the electrical capacity, you can to a wall heater or baseboard. The better ones are 220v and hard-wired.

Just a few other notes: The energy use for electrical heaters is the same per btu (heat output) and watts supplied, whether radiant, oil-filled, etc. There may be a few less often on/off cycles with oil-filled, but the end electricity bill would be the same in a defined space. Jim Mat's calculations would seem to reduce the desire for these in any case.

Vent-less propane heaters are illegal in homes in my state, for good reason. Any small problem with them and they can produce carbon monoxide gas.

Thank you all. So if I understand correctly, any kind of space heater would be inadequate and just run up the electric bill?

I think it would be more cost effective to provide the cats with a “cat house” or enclosure with heat. It would be expensive to heat the entire sun room.

A sun lamp or heat lamp with a house or two. I looked at fosters and smith, they sell heated beds, houses, heat lamps.

Heat the cats, not the sunroom!

Wow. Thank you. I would have wasted money on a space heater and high electric bills for nothing! Maybe I could get a cage and drape a heating blanket over it.

Another option is a ceiling hung, 220v circulating heater. Previously, I heated the 10x16 greenhouse with two 1,500 watt portable heaters - with radiant propane as backup. (Other than the south-facing glass roof, it's well insulated.) Northern Tool had several units: originally $249, marked down to $149 only to go on sale for $99 two years ago. I couldn't get to the store quick enough....

The have several 'sizes' or total watts. Each has three wattage settings in addition to a thermostat. Mine is a 7,500 watt unit, but I've only ever used the lowest 3,500 watt setting to maintain temps above 56F....

But yeah, heating the cats and not the space would be cheaper. If you have cats, keep the jugs the litter comes in. These can be filled with water (with a few drops of bleach) and placed in the sun for thermal mass.

apg4.........what interesting ideas! Are you saying that water containers can store daytime heat?

Yup...thermal mass - be that water barrels, masonry floors or a trombe wall - or anything else that can passively 'store' the suns warmth, is one of the keys to greenhouse management. It will even out or moderate big temperature swings. 'Passive' systems require no other energy inputs; 'active' systems have pumps or fans to move the energy about from collection to storage. There's volumes of data out there....