I recently had an addition put on over the garage (520 sq ft). I did a heat load calc on the space and oversized the unit by 20%. Based on this I chose a Mitsu 15K heat mini split (I did not want to go larger to prevent short cycling in the summer). My only concern was that I live in MA and the temps in the winter often get below freezing, teens. The contractors that I had over said it would be no problem. After this winter when the temps dropped below zero, the heat pump seemed to do fine. My question is I know below 32 the heat pumps loose output. I am not sure if the unit ran all the time to keep up but is this the most efficient source of heat for a small space. I did not want to go electric as KW/H in MA is 13 cents. Should I look into some supplemental heat for the really cold days? If so any recomendations?