A pump lifts 800 kg of water to a tank on a roof 18 m above the pump.

Step 1. (a) $PE = 800\ \text{kg} \times 9.8\ \text{m/s}^2 \times 18\ \text{m} = 141\,120\ \text{J}$.

Step 2. (b) The pump must do at least this much work, $141\,120\ \text{J}$; a real pump does more, because some energy becomes heat.