Do I need a back up source of heat?
Yes, you do need a back up source of heat. There are two reasons:
Should your heat pump ever fail, your back up will ensure your house stays warm even in cooler temperatures.
Heat pumps extract heat from the outdoor air. As the temperate drops, your heat pump may require more support to heat your home.
There are heat pumps on the market that operate up to -30 degrees, however as temperature drops, the amount of heat that is dispersed also drops. Daikin cold climate series of heat pumps will perform at 100% capacity at -15 degrees.