hi
i just built my greenhouse, i use a cheapo wall-mart electric heater, and just on cold nights, and i actually had it set to "LOW"
I used the home depot clear polycarb stuff, its not a big greenhouse, and i have insulation 4ft + down

it was 27 degrees the other night, when i walked into the greenhouse in the morning, it was about 55-60
cool, but not enough to hurt anything

If you need a lot more than that, i would get a 220v heater, if you can get 220v into the greenhouse. thats probably the most efficient/ cheapest way

To me, the most important thing is air leaks.
if its closed in well, you dont need a lot of heat.

Here is a link that might be useful: heater 220v

This post was edited by greenman62 on Sat, Jan 18, 14 at 4:39

Water is efficient at storing heat in the day and releasing it at night which saves on heating. I have one 55 gallon barrel in my greenhouse for that reason. It is blue but if painted black would be even better. Also a cheapo Wal Mart electric heater to protect from freezing. I also have a remote thermometer which I can monitor in the comfort of my home. Ideally maybe I get another barrel and use them on both ends to hold up shelving dual purpose style.

You might want to consider Phase Change Materials. They are passive (like water) but store a lot more heat in much less space. You can pair PCMs with a fan to maximize gain. It is passive (without the fan) so it would work whether or not you had a power outage. With a fan system, it would be more efficient.

http://greenhousefashions.blogspot.com/

i wonder if adding rock salt
to a 55 gallon drum of water would help ?

Here in New Orleans it doesnt normally get very cold. a couple of weeks ago it was abou 30 and my little cheapo heater was on LOW setting 750watts and it worked just fine.
Yesterday it was 32 and for some reason it wasnt keeping up. i think it was the humidity which was much higher

it was raining, it felt much colder in there.
maybe i will have to blow that little heater on a rock or something ?