I had a heat pump installed a few months ago. The installer set the auxillery heat to take over at 25 degrees. The heat pump has no problem heating my house at this temperature, but I am worried that it is less efficient at these low temperatures than my gas furnace. My worry is not about the money. It is the amount of carbon released. I think my furnace is 78% efficient and the heat pump is 13 seer and hspf 8.5.

Is my heat pump more or less efficient at 25 degrees than my furnace?

The installer set the auxillery heat to take over at 25 degrees.

I think I should have said the gas furnace not auxillery heat takes over at 25 degrees.

My worry is not about the money. It is the amount of carbon released

If you are that worried about the carbon release then I would invest in a 95 to 98% efficient furnace and shut off the heat pump at a higher temperature. The smoke stack power plants that generate electric are the main problem. Now if you live in a area with hydro electric or a nuke generating the power you are in good shape.