COP expresses the efficiency of a refrigeration system in a single number: heat removed (kW) divided by electricity consumed (kW). A COP of 2.5 means that for every 1 kW of electricity the compressor draws, 2.5 kW of heat is removed from the room. The higher the value, the lower the electricity bill. In the air-conditioning world the same ratio is given as EER (BTU/h divided by W) or the seasonal SEER.
COP is not constant; it falls as the difference between evaporating and condensing temperatures grows. The same compressor that gives a COP of 3 in a 0 °C room drops to 1.2 in a −25 °C blast freezer, and it cannot deliver at 42 °C the efficiency it delivers in 30 °C outside air. That is why a clean and adequately sized condenser, not holding the room colder than necessary and correctly set defrost directly affect the COP, in other words the bill.
When comparing two units by COP, look at values given at the same condition. The catalog lists compressor power input next to capacity; you can take the ratio yourself. Solutions such as electronic expansion valves, floating head pressure control and variable-speed compressors raise the COP noticeably at part load.