A farmer plans a solar panel for a small water pump…

Step 1. One cell. $P_{max} = FF \times V_{oc} I_{sc} = 0.78 \times 0.62\ \text{V} \times 5.1\ \text{A} = 2.47$ W.

Step 2. The efficiency. The light on a cell is $G A = 800\ \text{W/m}^2 \times 0.0150\ \text{m}^2 = 12.0$ W. So $\eta = \dfrac{2.47\ \text{W}}{12.0\ \text{W}} = 0.206 = 20.6\ \%$.

Step 3. The cells in series. Each cell gives $0.50$ V at its best point. $\dfrac{17.5\ \text{V}}{0.50\ \text{V}} = 35$ cells.

Step 4. The panel. The current at the best point is the same in every cell, $\dfrac{2.47\ \text{W}}{0.50\ \text{V}} = 4.93$ A. The power is $35 \times 2.47\ \text{W} = 86.3$ W.

Step 5. The energy. $E = Pt = 86.3\ \text{W} \times 6.0\ \text{h} = 518\ \text{Wh}$.