Several years ago, I came across a recently built office building with a condenser farm outside (had around 6-8 units). The installing contractor had never installed back up heat, and it was now 15* outside. Heat pumps couldn't keep up and it was going to be over a day till I could get and install enough electric heat packs. So I went to grainger, bought 2 large torpedo heaters, built a cover around the units to keep the heat in, and fired them into the covered area. Brought the building right up to temperature.