What are the guidelines on the proper sizing of emergency/supplemental strip heaters on a new HP/Air Handler system?
Old HP is 4T and does a decent job down to mid 30's. Old Air Handler has 20KW worth of strip heating which actually seems like overkill when they kick in.
Manual Load J confirms that 4T is the right capacity system for my home and climate.
If I assume that the strip heating (ie in emergency mode) needs to completely heat the house in the event of a HP malfunction and that 4T (48,000 BTU) is the correct capacity then that suggests that strip heating needs to be about 15KW. (ie 48,000 BTU X KW/3414 BTU = 14KW).
It looks like you have a good handle on the concept. I would go with the 15kw. This should be all that you need unless your area is subject to extreme cold for long periods. Then you would want the 20kw.