timmmay: Why are heat pumps more efficient when cooling? In both cases they're moving heat from one side of the heat pump to another, rather than creating or destroying heat.
When heating, the heat pump has to extract heat out of the air outside which is usually very cold. The fins often freeze up and it has to warm them with a standard heater coil (defrost light) and so if its 10 degrees outside, and you want to warm it up by 10 degrees inside, alot more energy is required to extract this heat.
Its still more efficient than a heater though.
When its hot inside, and you want to cool it, it is easier to extract the heat because there is much more of it there and so it requires less energy to make a difference.