Nearly every basic calculator has a set of solar panels built into it. Even so, these calculators usually come with a battery inside that actually powers the device. These panels help extend the life of the calculator by slowly recharging the original battery. The manufacturer's intention is to make the calculator last long enough that it gets replaced for a reason other than the battery, often being dropped and broken.
Use most solar powered calculators like you would any other calculator; direct exposure to light typically doesn't matter.
Recharge a battery powered calculator by leaving it in a bright location but out of direct sunlight.
Recognize when you have a true solar calculator. The numbers will fade when the light is blocked and there will often be a noticeable lag between pushing a button and its appearance on a calculator. When using a true solar calculator operate it in a bright location but still out of direct sunlight.
Clean off the panels occasionally to keep them working well. Dust, dirt and oils from your fingers can cause a significant amount of refraction, and the solar energy is lost. Wipe them off with a paper towel or even the end of a shirt, the plastic covering over the panels will protect them.
If you expose the calculator to direct sunlight for prolonged periods of time it may harm the delicate receivers inside the panels.