A system comprising: a reservoir (14) for holding a liquid; a pump (20) arranged to pump the liquid out of the reservoir (14) or into the reservoir (14); a level sensor (26) configured to detect the level of the liquid in the reservoir (14) and to generate level data; a current monitor (28) configured to monitor the current drawn by the pump (20) and to generate current data; and a controller (30) configured to: receive level data and determine the static head from the level data, and generate head data from the static head; receive first current and head data corresponding to the current drawn by the pump (20) as a function of the static head for a first period of operation of the pump (20); receive second current and head data corresponding to the current drawn by the pump (20) as a function of the static head for a second period of operation of the pump (20), wherein the second current and head data is recorded when the liquid in the reservoir (14) is between a first liquid level and a second liquid level, and wherein the first current and head data is recorded when the liquid in the reservoir (14) is between at least the first liquid level and the second liquid level; and process the first current and head data and the second current and head data to determine first difference data (PPI) representing the difference between the current drawn between the first liquid level and the second liquid level in the first period of operation and the second period of operation, the first difference data providing a pump performance indicator giving an indication of the condition of the pump.