Allotments 4 All

I've been given a 1.9kw Propane heater for my 10x8 greenhouse but I need to get a bottle.
I was wondering what other users experiences were, with regards to size and rate of consumption.
I appreciate conditions vary but for comparison my greenhouse will be bubblewrapped for insulation and needs only to be kept just over frost-free. I'm also in relatively mild buckinghamshire.
Any help/guidance would be greatly appreciated.