If you're asking, is it safe to use, the answer is yes. Most solar generators are safe to operate even when it's being charged. Of course you should always check the manual first. If it says you can use when charging, then there's no problem. If the manual says do not use while charging, then don't do it.
If you charge the generator through solar panels, there will be no problem. In the worst case, clouds pass overhead, slowing or stopping charging. If there is not enough energy in the generator, your device will no longer operate. But there is no damage to the generator or your device.
You have to be more careful if charging on AC / DC power. As long as the power outlet is stable there won't be issues. Do not plug the generator on a faulty power outlet as it could lead to damage. The outlet could overheat and that's dangerous. As long as the power socket is good, there should be no problem with charging.
No surprise, one of the ways to charge a 100 watt solar generator is with solar panels. Some generators have built-in solar panels. If so, just point the solar panel to the sun (make sure it's the right angle) and let the sun do the work.
A 12V lighter is another charging option, although it is not as common as AC / DC and solar panel sockets. If it is available at your power station, you can charge it in your car or any vehicle with a 12V lighter. Usually however, car chargers are found on portable solar chargers, not generators.
Solar generators, whether it's portable or for home backup power, will only get better. Their power continues to grow even as prices drop. Charging with a solar generator will not damage it, but be prepared to wait. Unless it is an emergency, we recommend that the generator rest during charging.