If a short circuit is placed on a solar panel, it produces the highest output current. However, the output voltage into a short circuit is zero. Since power is amps times volts, the output power is zero.

If the panel's output feeds an open circuit, the output voltage is a maximum. Again though, since no current is being drawn, the output power is, once again, zero.

There is a point (the maximum power point) where the output voltage times output current yields the greatest output power. This typically is around 80% of the open circuit voltage.

A panel's open-circuit voltage, short circuit current, and maximum power point all vary with light intensity as well as temperature. More light yields more power, higher temperature yields lower power.

A Maximum Power Point Tracking charger adjusts the loading on the panel to allow the panel to operate at its maximum power output. As light intensity varies, the charger re-adjusts the loading to keep the panel at its maximum power point.

Note that there are times when the battery is fully charged or the application load is low. This results in the loading on the panel to be less than its maximum capability. Under this condition, the panel's output voltage tends to rise towards its open-circuit value. The panel is operated at its MPP only when it's fully loaded.