A heat pump is a heat engine that absorbs energy from a thermal source TH, converts the energy in work, and rejects the remaining heat to a lower temperature thermal sink TL.  A heat engine follow the first law of thermodynamics such that QH = QL + W.  The efficiency of a heat pump is defined as:

A heat pump is a heat engine typically used for air conditioning.  A heat pump pulls energy from the lower thermal reservoir and rejects it to the higher reservoir.  Because this is counter to a natural flow between two temperature gradients, additional work from an independent source is required to run the cycle.  The desired output for a heat pump is the heat rejection (typically to a building).  Work can be derived from a heat pump, but that the desired quantity.  A “coefficient of performance” can be calculated as

K=QL/W

the added heat per required work.

A refrigerator works in the same fashion of a heat pump, except the desired quantity is the heat absorption from the colder reservoir.  The coefficient of performance for a refrigerator is:

K=QH/W