Lets say you have a 3 ton heat pump which requires a 30 amp circuit plus a 10 kilowatt electric heater which is probrably at another 40 amps, electric dryer, range, lighting - a 100 amp panel probrably would not dut it on a cold Christmas day with the heat pump, electric heaters and stove cooking the Christmas turkey. No problem with just the heat pump and the gas furnace and not the electric strip heaters.
Natural gas backup.
Aire Serv of SW Connecticut- Gas heat, dual fuel and central a/c systems installed and serviced
I was told by my HVAC guy who installed my funace asd I asked him about a HP along with my funace and what the pay back is and he said the pay back with a hp & Furnace NG is less than 5 years of course that is all based on the age and type of system it is replacing but the pay back is faster than you may think. I have to add I have a 100 amp panel with 50 amps going out to my shop running 220 machinery and have a electric oven and frig, micro, 2 pc's, ET center, AC direct vent water heater coming off my 100 amp panel and hever had a problem.