The pump efficiency is the volume pumped per unit of energy used. For pumps, it’s the number of liters or gallons per watt of electricity. 90% of the cost of a pump over its life time is electricity. If a pump costs twice as much to operate as before, even if it pumps the same flow as before it would be important to know this. Other than increased electric bill, a major variation of the pumps’ efficiency indicates that something other than flow is highly abnormal.