^ ^ The Big Buddy heater is INDOOR rated . . . AND . . . has integrated Low Oxygen Sensor that automatically shuts off heater in high CO situation. Also has tip-over shut off feature.
I've used Big Buddy in 14 ft hunting camper as only source of heat (no electric and no batteries). Thus, just the passive heat from 18K Btu Big Buddy did great even in extreme temperatures. Even in that confined space (was an older camper so there was air leakage) I felt safe sleeping in camper. Only negative was slight smell of propane.
For 2 car garage, I'd say that Big Buddy is great solution and would highly recommend both the optional hose kit for 20 lb BBQ propane bottle, along with 12 volt DC adapter so that fan be operated with 120 volt plugin. With the electrical adapter, the fan can be used instead of just batteries. Alternatively a small normal fan could push heat around garage, if needed.
Heck, before I got the Big Buddy, I used an oil-filled electric heater to keep my prior 2 car garage above freezing. This took so long and any opening of garage door quickly lost all that heat . . . not good.
Big Buddy is well worth the money due to it's quick and safe heat.