Propane space heaters are very good for quick powerful heat and even really easy to move around units can hear big areas quickly and effectively.
There is a good choice of propane vented space heaters that put out loads of heat - 200,000 British Thermal Units is not uncommon and that is a lot of heat. It is enough to heat 500 square feet comfortable and you can get decent ones for as little as two hundred dollars.
On top of that, propane is a cost effective fuel for space heating. There are also a lot of electric portable space heaters available which do their job very well. But when all is said and done they demand a lot of juice from the grid.
Smaller propane units can be used for outdoor parties to extend the summer into autumn. These can be little squat units or the tall mushroom shaped outdoor patio heaters.
Propane portable space heaters these days come in various styles, offering radiating heat, convection and forced air systems.
At the end of the day though, it is hard to say they are truly energy efficient if they are heating outdoors (competing against mother nature) or heating a big poorly ventilated space like a workshop. It might be a smart move to start improving the insulation properties of the workspace first of all!