:bananen_smilies019: Hard question to answer !! It depends on the size of the heater itself, the area you are trying to heat, how long you run it, plus your starting tempature. My lower shop is 36 X 24 with a 14 X 12 finishing room open to it. I have a 45,000 BTU torpedo heater rated for 1000 square feet in there I use when it gets near 0 outside. The tempature stays about 55 to 60 degrees in the shop at all times. I turn on the heater and let it run for about 15 minutes, this brings the temp. up to 65 or so. If you can't stay warm at that temp., you aren't working hard enough. I change the 20# tank once during the winter. My upstairs garage is a 36 X 24 X 10 spancreet area and also stays around 50 degrees on the coldest days. I have another torpedo heater I use if I need to work on one of my cars or tractors. This one is a duel setting unit offering 30,000 or 60,000 BTUs of heat. I can turn this one on to the high setting and work in a t-shirt in about 10 minutes before having to turn it off. A 20# tank will last several winters. These propane heaters have very little odor to them, and would have none at all if they didn't have an agent added to them so you can smell a leak. If your power was out for an extended amount of time, in cold weather, you could rent a 100# tank to keep things from freezing. Many contractors use this method to keep their projects warm so they can work in the winter cold. A fuel oil heater is very smelly and stays in your cloths and the area you heat for days. Hope this helps. Jim S