Garage Heating

Now for reference BTU rating (for example 45,000 BTU) is heater input per hour.
Heat output is btu input x efficiency 45,000 x 80% = 36,000 btu,for a non-condensing furnace like these. Condensing units can be upwards of 98% efficient.

One gallon of propane contains 91,000 BTU.

20# cylinder of propane holds 4.7 gallons or 427,700 btu or 9.5 hours @ 45,000 btu input.

Vaporization Rate: as you see from this link http://www.generatorjoe.net/html/PropaneNGUse.html (scroll down to vaporization rate of cylinders) a 20# cylinder will not run a 45,000 btu heater. It takes two 20# cylinders at room temp in parallel to supply a 45k heater, at 10 deg it takes five.