In the climate where I live it is seldom below 20 degrees Fahrenheit. However, we do get a cold spell everynow again, sometimes lasting 3-4 days. I understand a heatpump is less efficient at lower outside temperatures. Below 20 degrees our heatpump runs about 90% of the time. I can raise indoor temp a few degrees and run the heat strips for about 10 minutes, then the heatpump will shout off for about 2 hours until the house reaches the original cooler temp. I also have 2 LP gas fireplaces one on each floor that put out about 30K BTU each. Each fireplace has a temp setting, so they need not be on 100% of the time. Both will maintain house comfortably, or if real cold (below 10 degrees) assist the heat pump and the heat pump will stay off for long periods (4-6 hours) while assisted by the fireplaces.

My question is this, below 20 degrees which is most economical, let the heatpump run 90% of the time, run the heat strips 5-6 times a day to relieve the heatpump, run both the heatpump and the gas fireplaces, or run the fireplaces only, or just leave the thermostat on emergency heat running the strips but not the pump?

The cost of electricity is relatively reasonable here: $0.086 per KWH, LP Gas delivered is right now about $2.89 per gallon. I keep the house a constant 66 degrees. During the winter my electric bill is about $175.00... the house all electric, with a well for water. Summer electric bills with no AC are about $75.00. So I figure my heating cost during the winter months is about $100.00

Many of the HVAC businesses here are recommending using a heatpump system above 40 degrees, and using an LP gas furnace below 40 degrees.

The key to the answer is your last paragraph. Heat pumps do lose efficiency when the heat source (the outside) is cold. Running a heat pump 90% of the time consumes a lot of electricity.

The heat strips (I assume they are electric) are not going to be as cost-effective (BTU per dollar) as the LP gas or the heat pump. I'd suggest maximizing your LP GAS usage when the outside temperature is below 20 degrees. I'd run both the gas fireplaces and the heat pump.

My gut tells me the HP with aux heat will give you the lowest BTU cost because I assume the efficiency of those LP fireplaces are low. But to give you a better idea what is the SEER/HSPF of your HP and what is the efficiency (AFUE) of those fireplaces if known, and I can repost some calculations.

The most efficient setup in your case would be your HP and high efficiency furnace (90%+ AFUE) with an economic (not comfort) balance point of somewhere in the teens to 30F depending on your SEER.

What temp. should you turn on the heat strips in cold weather and the heat pump off?