What is the thermal balance point temperature formula?

The balance point is where heat pump capacity equals building heat loss. Model each as a line versus outdoor temperature: building load falls to zero at the indoor balance temperature (~65°F), and the heat pump's capacity is interpolated from its two rated points. Solving load(T) = capacity(T) gives the balance-point temperature — a linear interpolation the calculator does automatically once you enter the two capacity points and the design load.