What temp to use emergency heat on heat pump?

Emergency heat on a heat pump should typically be used when outdoor temperatures drop below 30°F (-1°C) or when the heat pump is struggling to maintain the desired indoor temperature due to extreme cold. It's also advisable to activate emergency heat if there is a malfunction in the heat pump system, especially if indoor temperatures are dropping rapidly. Using emergency heat can help ensure comfort but may lead to higher energy costs, so it's best used only when necessary.