Complex sentence: Properly installed and maintained pumps for water can be around 70-90% efficient, while less-cared for pumps can be less efficient due to wear on the impeller and general back leakage through over large clearances.
Split into simpler sentences: Properly installed and maintained pumps for water can be around 70-90% efficient. Many pumps are much less efficient than they could be due to wear on the impeller and general back leakage through over large clearances.