There are three fuels that you can use to heat your greenhouse.
Electricity is the most convenient as you can just buy a simple fan heater and plug it in and it will only click on when the thermostat tells it to - so once set up you never have to think about it. With electric fan heaters, the more you spend the more reliable and the more accurate they are. The Cheapest heaters may have thermostats that are only accurate to +/- 3 degrees which may result in fluctuations of 6 or even 8 degrees in the temperature of your greenhouse. This is perfectly fine for most growers, but if you really need reliability and accuracy then you may have to spend around £200+ for an electric heater that is more accurate and reliable.
If you don¹t have electricity in your greenhouse then you can choose form either paraffin or gas to fuel your heater. Paraffin heaters are the cheapest of all, but you have to fill them up every day and re-light them.
Gas heaters can be good with a pilot light lit, the burner only lights when the thermostat deems it cold enough so you can set the greenhouse to your perfect temperature. The CO2 emitted by a gas heater is also beneficial to plants. For bottled gas you need to use Propane as butane can freeze in the cold weather. You can also choose a natural gas model for plumbing into mains gas if you are lucky enough to have that in your greenhouse.