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hi there
I use a small electic heater available from B AND Q and Homebase quite cheaply but make sure you get one with a thermostat to maintain no more than 10 degrees otherwise your bill will be hefty~ if no power
you might want to think about bubblewrap insulation and maybe a small paraffin heater but that will put out quite a bit of moisture too!
good luck Arlene
if you check you will find that we have covered this quite recently?.

To be absolutely safe, I think it's wiser to get an electric heater designed for greenhouse use. I am not sure of the difference, but they are sold specifically as "greenhouse heaters" while indoor fan heaters are marked as indoor heaters. It might be to do with the water used in greenhouses, I suppose.

Arlene is right - you do need one with a thermostat, so that you can set it to come on when it's frosty outside, or higher if you have really tender precious plants.

Paraffin and gas heaters do have disadvantages to do with condensation and possible grey mould on your plants without daily ventilation and wiping away the moisture.

I used to use a paraffin heater but changed to a small thermostatic fan heater which is much easier to control until a power cut! I was out of elec for nearly 24 hrs and was pleased I had a paraffin heater as standby. But if you are only buying one go for a fan heater.

Rather than a fan heater I would choose a tubular electric heater designed for greenhouse use. These are waterproof, to a degree, and much safer. Also, be careful how you run the cable to the greenhouse, you don't want to be damaging it.

just to add I have armoured cabling to my greenhouse put in years ago~well worth it! I have used my heaters for quite a few years without any problems but obviously be careful where it is sited~don't stand plants in a blast of hot air ~and when watering!

I changed from parafin to an electric fan last winter. It has a thermostat setting that comes on to keep the gh at 5 degrees. It's also on a plug timer to come on overnight only (saves on the leccy bill!). The atmosphere in the gh has been much, much better since ditching the parafin heater. Used it the other day to warm the chickens up, though!

Also section your green house off with bubblewrap or similar so you are not heating the whole place, just the area you want.