If your concern is temps for the plants, why not consider a heat mat or two for your bench. They come with or without temp controllers. Excellent for seed starting, I leave mine on even when the plants have moved off them, and it provides plenty of economical heat for my small greenhouse.

Electricity = Economical heat?

Lost me there. On a small scale perhaps :)

Composting is a useful and cheap way to provide heat to a greenhouse. Very googlable and many interesting designs.

Composting isn't really feasible on a small scale either seeing as how it takes a large amount of space for the compost to work correctly. Plus I would rather smell the sweet scent of lemon blossoms and lillys than manure. Although expensive I had dreamt up another solution using compost in a tumber with a small water line pinned inside underneath the pile and hooked up to a pump with restrictor orifaces to slow the rate of return on the water to both give the water in the line time to heat up and to not cool down the heat build up within the compost. But when I really thought about it I'd be dropping close to 500 in equipment and supplies for a very inconsistent method of "free" heat. I like the idea of mats and plan to get some in the future. Anyone know if the heating mats pull a lot of amps. My heater already pulls 11-12 and it's plugged into a 15 amp socket. Seedlings are not the only plants I keep on my GH I have a lemon tree, lillys, strawberries in hanging baskets, gladioluses, and tomatoes all of which are not seedlings. So the heater stays and I'll add mats if I can run them both with out throwing the breaker. But does it not make since to only warm half the GH? I'm talking about splitting it horizontally under the benches and devise a contraption of sorts to roll it up towards the back when I need to work inside. Something like a retractable projection screen would be awesome.

You could have a large compost pile within ~20ft of the greenhouse, then run an underground insulated pipe to pump the heat into the structure.

The hydrofarm heat mats I use are rated at 107 watts. I don't understand electricity, so that's about all I can tell you. I do know that the two I use don't impact our electrical bill very much, and they are set to run continuously when it is under 72F in temp. They do keep the greenhouse from ever freezing.

Anything with a cord will tell you how many watts it draws. 100 watts is roughly equal to 1 amp. Most houses are built with 15-amp circuits.

Your power bill will tell you the kilowatt/hour price you pay for electricity. 12-20 cents is common. Whatever your price is, that is how much you pay to run either a 100 watt heating pad for ten hours, or a 1000-watt heater for one hour. Each would be one kw/h.

We have an unheated greenhouse in zone 2b. I didn't intend it this way. I have a natural gas furnace but no gas hookup yet.

We managed to start seedlings through -40 degree weather by using heat mats (as has been suggested above) placed on old blankets to prevent contact between the mat and cold table top. I used germination domes covered by several layers of floating Agribon row cover to keep the seeds at 70 degrees.

Once the plants were too big to use germination domes, we erected a row cover tent over the tables. With heat mats and two layers of heavy row cover, we could keep the temperature at plant level about 40 to 50 degrees F warmer than ambient.

I think almost all professional greenhouse growers myself included feel that ventilation is more important than heating. Looks to me like it would be about 150 degrees in your greenhouse and that the only way to deal with it is to remove the plastic. Plastic is sooo sucky. Our civilization has committed a major crime allowing it.

Well now that spring is here I don't have to worry about cold temps. It's about 65 at night which is what I'm looking for. Day time temps climb to only about 95 but I lose all humidity during the day because I have to open the window and door. I have a mister system installed along the top of the GH but I need away of setting it to come on when temps hit 100 to come on and turn a fan on so I can leave the door and window closed. I've seen solenoid water valves and have thought about getting one that I can plug into a thermostatic controller but surely there has to be a thermostat/ solenoid valve combo some where. I just don't know what it would be called or where to look. Everything I've come across is 2 separate pieces of equipment. Would love some input on how to control the temps using what I have already in place (misters and fans). When they are on it works really well so I don't need to replace them just get them to work together by themselves because I can't be home when they need to run. Timers on the misters doesn't really make since because I only need them to cool not to water.

This is 15 min after temps hit 115.