Couple of variables you need answered to determine ambient heat requirements:
1. How many MILs (thickness) is the tarp
2. Height (you will need to calculate cubic feet) ex: 5x5x?(height)

I try to maintain a soil tempature @ min. 50-55.

Johnny

I maintained 52-55 degrees average in my heated GH, and the Bhuts did fine. They like it warmer though.
I hooped some on the outside and they didn't make it through a cold Texas winter.
You know, if you could get some heat mats and set the pots on those the direct contact may keep the soil warmer, plus the generated heat could go into the structure. Gonna try that this year on my overwinter stuff.

I would be using a temperature controller, so as long as the heater can produce enough, it would regulate itself. I'd probably put the probe in the soil of one of the pots so it works towards keeping soil temps regulated.

Sounds like there are two votes in the 50-55 range so far. I'm not looking to have the plants grow, I'm just preventing them from dying.

I would take temp readings in multiple places and try to get the system working as efficient as possible. My guess is as long as you don't let anything freeze hey should make it provided they get sun and needed nutrients.

I wonder if you could just keep them vegetating and producing through? the winter? Higher electrical bill for sure to see if you could.

TomT, you said "average temp". To me that means it gets above 55 and below 52 regularly. Do you have min/max temps? I am also curious about air temp readings close to the plant stems.

dj

djoy,
Since the GH has a clear/opaque covering, the temp fluctuates as much as 20 degrees during the winter, depending on outside temp, wind, and cloud cover. On an average winter day here, say, a high of 50 degrees and a low of 30 degrees with broken clouds and a moderate wind of 10 MPH, the inside temp will be as much as 75 during the peak of the sunshine, but won't fall below 52-55, or whatever I have the thermostat set on. I use two ceiling fans in the structure to blow the hot air down from the top and keep it circulating 24/7. An exhaust fan with thermostat kicks on when the temp gets above 80 degrees or so. Found a neat little $9 battery operated thermometer/hydrometer that gives you hi/lo temp and humidity for a 24 hour period. Home Depot had it.
I placed one of these on the floor and found little difference as long as I didn't put it near a drafty area. The GH is on pier and beam, but the skirting is air tight and the decking seams have been plugged with foam. A ground level GH would stay warmer as the earth acts as a heat sink.

I would set the thermostat sensor for your heater near the leaves of the plants, not in the soil. Especially on a clear, cold night, you could have very significant radiant cooling of the air before the mass of soil decreased in temperature enough to trigger the heater on. The result might be that the air temperature goes below freezing, killing the tops, while the soil is still fairly warm. I winter peppers over in a greenhouse, and find that even at a set point temperature minimum of 45 F, that the peppers do well. During the day in the winter I allow the greenhouse to heat up to 90 F before venting. The plants don't mind the higher temperatures, and the soil mass gets much warmer, storing more heat for the night.
Renais

Thermostat is set on a wall at 6' or so, to give an average air temp. The air temp in the GH never approaches freezing even on the coldest nights...as long as I don't run out of propane. Seedlings are elevated off of the floor with bottom heat, and love the environment. You can see the thermostat, and propagation box in this pic. The heater is directly above the door. Thermostat is on the upper left of the door.

Funny the lengths we go to inprder to keep a plant alive. I can't say nothing myself either. I had a separate 50amp breaker installed on my house to run the heaters (3k BTU), a drip itrigation system, a/ fans last winter.
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Johnny

jtight,
Having a GH beats having to re-buy plants when they freeze. Plus, I gotta have a place to grow super-hots to torture DW with... ;-)

Sweet info thanks Tom and Renais. I really dig the green house Tom.... one if these days I will get me one. For now its going to be, how many clones can I stack in the sun room.
dj