OK, I used the online calculator here:

http://www.littlegreenhouse.com/heat-calc.shtml

I assumed:

Surface Area: 200 ft2
Outside Temp: 15F
Inside Temp: 35F
Heat Loss Val: 0.7 (based on twinwall 4mm)

This comes out to about 2940 BTU, or 861 Watts.

Now, my next question is: could I put 861 Watts of heat tape in the floor and use that for heating? I like the idea of heating up the floor, since it might improve the soil temperature in my pots a bit.

Cheers,

Ed.

Ed,
If you have calculated you need 861 watts, then that amount of heat tape on the floor will work well. As the heat rises, it will help to warm the pots.

Make sure you insulate the floor and perimeter well, otherwise the 861w will also be trying to heat a few thousand tons of the surrounding soil ;)

I got a little different wall space, using the HFGH dimensions of 6'W x 8'L x 8'Peak x 6'6"Wall Height. Total is 255 square feet. Now, if you orient the gh properly, you can eliminate the north-facing wall by heavily insulating it with pink polystyrene or covered styrofoam. So, that's 255 minus (6.5' x 8') = 203.

Insulating the floor may not be such a good idea, as you lose the benefit of solar energy depending upon how you do it. Any heat energy you add during the night hours will rise, so you won't be heating the ground much anyway. (I've found that as the winter wears on, I lose the benefit of residual warmth in the ground from summertime warming, but there's never a cooling liability from an earth floor. I know this because I have a thermometer probe in my floor.) If you insulate below a layer of brick, rock, gravel, whatever you use -- that will certainly be better. However, your only choice is pink polystyrene because moisture will ruin any other type of insulating material. Wet earth actually has the potential of storing quite a bit of solar energy.

Using the figure of 203 square feet, minimum outside temperature of 20F, desired interior temp of 35F, and 4mm panels, your BTU requirement is 2238 BTUs or 656 watts.

Easily achieved...