From a safety point of view the kind of electric heater is very important.
Make sure it is designed for use in very humid environments or you could risk being electrocuted :)

Well that's exactly what I was wondering - are there specific types, brands, etc. I should look for?

And what about christmas lights (I read about someone using those). Can those be used just for very localized heat on some things when it gets warmer?

How do you like that...an answer just leads to more questions!

Isn`t that always the case? :)
Heaters specifically designed for the greenhouse are usually more expensive simply because they have better protection against dust and water ingress.
You can normally judge how safe an appliance is by checking its IP rating (ingress protection rating).
The higher the IP rating the better, but at least IP54 for peace of mind.

The first digit relates to dust and the second digit relates to water.

http://en.wikipedia.org/wiki/Ingress_protection_rating

Not to disagree with hex2006 in any way...in general we have tried to pay close attention to safety issues with our greenhouse, including the proper types of GFI outlets, outlet covers, conduit, etc. And, I did purchase my exhaust fan from a real live greenhouse supply company (with a real live price tag.)

However, for heaters, I just bought two Honeywell 1500w electric heaters from Walmart, about $40 each. I have them on thermostats (purchased from www.kkontrols.com) so I they come on at the temps I want.

I read many comments here from others who used inexpensive electric heaters purchased from box stores, and decided I wasn't assuming an unwise risk by doing so (of course, I use common sense and don't douse them with the watering can.) ;-) My heaters do have a feature that causes them to shut off if they are accidentally tipped over.

I'll add though...my greenhouse (desert environment) is far from humid at any point in the year.

Jamie, I found this heater calculator helpful in determining the number of heaters I'd need to maintain the temps I wanted in my zone. I estimated roughly 5000 btu per 1500 watt heater. Maybe it will be helpful:
Heater Calculator

The greenhouse heater in the link below is almost identical to the Wal-mart $18 milkhouse heater. I purchased a greenhouse heater years ago for $69. It only lasted one season. The Wal-mart heaters last about 2 winters or more depending on use. But a lot cheaper to replace. I have had an oiled filled one for 4 years and its still going.
To me the greenhouse heater was not any different. Not anymore water proof.
Just use common sense and don't get it wet and keep away from flamables.
My $18. Wal-mart heater at least 2 years old.
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My oil filled heater.
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Here is a link that might be useful: greenhouse heater???

I have an oil filled one that I use in the house and I love it. Tip it over, no problem. It is the ONLY heater I feel safe in using when I go to bed. No dangerous fumes.

If and when I get my GH finished that is what I will use in it if heat is needed.

Just my .02

1eyedJack and the Dawg

The design is important to safety, oil filled and tubular type heaters have enclosed electrics.
Compare the IP rating of a normal fan heater and one designed for use in a bathroom.. there will be a difference.
Using a hair dryer in the bathroom probably carries a similar risk as a non-rated fan heater in a greenhouse.
Neither heater has any protection against Murphys Law :)

I agree with hex2000. If it's not rated as safe for use in a bathroom- don't use it in a GH.

BTW: Freezer weight plastic bags can make extension cords a lot safer in a GH too. Open the bottom of the bag, slip it over the extension cord and after making the connection, slip the bag over it and seal off both ends with twist ties or plastic tape.

I have one small inexpensive heater in my 6x8' HFGH and insulation on the north wall and ceiling. Before you get your bubble wrap call a local Packaging Co. They have the same thing for 2/3's less! They even have it in 4' rolls.

Also Kkontrols.com has the same type thermostats for cheapo heaters Charlie sells, for half the price.

Why not use GFCI in your GHs? We have them in both GHs and on the circuits that go to our ponds to run water pumps?

Installing a GFCI (RCD etc) to cover any outside equipment is essential.
Notice how they never include any sort of guarantee stating it will protect you TOTALLY from electrocution?

Think of it as extra protection rather than a failsafe.
A very small amount of current can kill and a sweating body in a hot, humid greenhouse makes a pretty good electrical conductor :)

Thanks, all! I appreciate the extra info. For now, my power will be coming from a contractor's extension cord, as there's really no cost-efficient way to run electrical outlets out there at the moment (it's all the way at the back of our yard, quite a ways from the house). It will be the same cord structure that we use for our Christmas lights, so it's rated for wet, winter weather (and the end will be inside the greenhouse, of course). It plugs into a covered GFCI outlet on the side of our house, so we're good as far as that goes. Thanks for the tip on the plastic around the connection - I'll definately do that, just as a preventative.

I anticipate needing the heater from March through late April...then it will be stored in the house, so not too long. I have one of those Lakewood heaters at my office desk, and it wasn't expensive at all. That seems like a good temporary solution, for the amount of time I'll be using it. I don't really need to extend the growing season in the fall, but I really need to get a jump on things in the spring, so that's my goal.

Thanks again for all the great information...I can't wait to get things growing around here! :-)

I just bought an oil filled radiator style heater like the one in cactusfreak's pictures at Lowe's for $12 and a utility type one for $9....should probably have picked up an extra one...In fact, I may just go now and try to get one...for $12 it can sit in the box in my garage for a year or more if it needs too...

So whats the difference between an oil filled one and the fan style heaters I'm looking at? Does an oil-filled heater take less energy, or is it just safer to use? For $12, I'd certainly consider that, assuming it runs on any normal household electrical current. I assume it's safer due to the electrical being on the inside, as hex said earlier...

I thought I might run out this weekend to look at what's available. If our Lowes has the oil-filled, and it would be safer and either just as or more efficient to run than the fan style, I'd consider picking one up now...

Oil filled heaters have oil or something like antifreeze in the chambers. When it is heated it retains the heat in the liquid and releases it back out slowly. Whereas an electric heater cools off very quickly ocne it cycles off. Oil filled stays warms for quite some time. I much prefer them when the price is right.
Also there are not any coils that might overheat and shoot out if one breaks and no direct element to catch something on fire. Also seem more water resistant.
I had one last several years and one only lasted one year. Same as with the milkhouse heaters. You just never know one you will get a dud. I just don't buy that name brand again.

Thanks - I appreciate the info. Maybe I'll try one of the oil-filled if I can find them cheap here, and see how that works!