If you have any skills it's easy to run wiring to your greenhouse and, as roseguy says, for peanuts. Tap into a box, run PVC electrical conduit that you bury in a trench between the box you tap into and the greenhouse termination point, pull wet location wire (individual conductors, not Romex) through the conduit, and attach the wires to an exterior style box inside the greenhouse with an all-weather cover for the receptacle. Use a GFCI receptacle if the circuit is not already protected.

Now here is the more complicated part. You say you need 10K Btu/hr, which is almost 3000 watts. Unfortunately, standard house wiring is 14 gauge and can handle a maximum of about 1800 watts before tripping the breaker. If you really wanted all that power, you would need 10 gauge wire protected by a 30 amp breaker, with nothing else on the circuit. That probably means running a new circuit from your breaker box to the greenhouse. Did you discuss the heat load you want to support with the electricians who gave you the estimates?

I don't try to keep my greenhouse at growing temperatures during the winter. My greenhouse is connected to a 15 amp circuit and my $50 electric heater has a switch that allows me to use it at 750W or 1500 W (which would nearly max out the circuit). It also has a thermostat, which I set for about 40 degrees F, and run the heater at the 750 W level. This keeps everything in he greenhouse safe and alive, but doesn't allow me to grow tomatoes in winter. It meets my needs, doesn't cost an arm and a leg for energy, and doesn't require me to be tending a fuel-based heater all the time. I could keep the greenhouse warmer with this heater, but choose not to.

You may want to re-evaluate how much heat you actually need and are willing to pay for. Lowering your goal would also make wiring much simpler.

@kudzu9: You have given me more information that I received from either electrician. I did mention to them that, according to the formula supplied by the greenhouse manufacturer, I need to run two 1500W heaters. Neither mentioned the need to run a new line from the breaker box. They both observed the existing box, and the trench I dug from that box to the greenhouse. My hope is to keep the temperature above freezing (I'm in North Carolina, zip code 27312) so as to protect a 40+ year old split leaf philodendron that is too large to get into my house. But, your comments have succeeded in getting me to rethink the wiring, and I may have to find another solution. Thanks for your input!

Unless the place you are tapping into is already a 30 amp circuit with nothing much else on it (probably unlikely), it's inexcuseable that a licensed electrician wouldn't mention that. Well, better to know that now than spend $1000 for a circuit that won't do what you thought it would. I'm in Hardiness Zone 8a, and you are probably in Zone 7b, so you aren't that much cooler than me. I think you could keep that greenhouse warm enough with one heater with no problem, especially since it looks like you have double wall glazing...I have single pane glass in a 10X15 greenhouse.

I suggest one of the radiator style, oil-filled heaters; this is the current version of the one I have been using for the last nine years: DeLonghi heater

Michael,

Nice looking greenhouse. Care to share where your bought it? I am looking for another of about the same size for my off and on winter vegetable production.

Yours should protect your philodendron very well. The earlier comments are all good advice in my opinion, having gone through the same decisions for my 10 X 14 hobby greenhouse from cedar-built. ( I did the 120 volt wiring myself off of an adjacent storage shed with electricity, with much help from my electrical parts supply store). It handles my 1500 Watt heater without a problem, and keeps my plants above freezing temperatures year round. (zone 8b)

I also agree with Kudzu9 about how much easier the maintenance would seem to be for an electric heater for an 8 X 12 greenhouse vs. propane or kerosene.

In checking the BTU requirement for your zip code, which was shown as USDA zone 7a, http://planthardiness.ars.usda.gov and your greenhouse size (8x12) (assuming twin wall poly carbonate), and keeping the structure at 40 F , I came up with just shy of 13000 btu. http://www.littlegreenhouse.com/heat-calc.shtml    

So, your 10,000 BTU estimate would probably keep the structure at least above 32 deg. F, , which is the published tolerance for your plant: http://www.desert-tropicals.com/Plants/Araceae/Monstera_deliciosa.html

Since this plant is so special for you, please post a photo..........

@kudzu9 and oakhill: Thank you both for your comments and guidance. I am very grateful I stumbled across this board, and I wish I had wisdom to help others the way folks here have helped me. I am going to scale back the wiring part of the project, and work on a solution that I can put in place without help. Last winter (my first in North Carolina), I kept the philodendron alive surrounded by visquee on a covered porch. I used a radiator-type heater, and it did just fine. My current revised plan is to find room in the newly cleaned-out garage (space I didn't have last winter), which we keep at about 45*. I'll try the radiator heater in the greenhouse and closely monitor the temperature to see if the plant would be safe there before moving it. (The philodendron is important to me--my deceased spouse brought it home in a "dixie cup" sometime in the early- to mid-70s, and, although I definitely don't have a green thumb, I am committed to keeping it alive. In its pot, it is now about 6 ft. high, and 6.5 ft. in diameter.)

The greenhouse is this one: http://www.gardenhouse.com/greenhouse-kits/mt-rainier-group/ It has twin wall polycarbonate (but only 3mm) “glazing.” We picked it because we liked the wood frame, and because, unlike some kits, the glazing was already installed in the panels, and it was supposed to be easier to assemble. Assembly took 2 of us about 12 hours (not including the 6x6 foundation, which a carpenter built); the task wasn't simple (I took more than a few swear words out of escrow), and the result isn't perfect, but we did it, and we're proud of the result.

Again, I appreciate the "adult supervision" that all have given me. Mike

Quite the philodendron! Good luck. It sounds like you've thought this through and your plans are sound.

I agree with Kudzo9, the oil filled electric heaters are best for this application. Much safer with no exposed heating elements or fans. I use one on my smaller greenhouse but it's against the house, so I have an extension cord of sufficient capacity running through the window and into an outlet with a GFI in between.

At our coldest temperatures I only have it on low and keep it at about 40F.