This is just a little something I dashed off in a Fb thread in a skoolie group, that I thought it might be nice to refer back to sometime. Someone said they thought solar panel output voltage maxed out at 19V and I said...
Some solar panels are really two panels in one. Many of them are arranged top/bottom and have bypass diodes so that one end of the panel can be shaded and you can still get output. Panels are made up of cells which put out 0.5-0.6 volts each, and are connected in series. It takes sustained voltages at specific levels to charge batteries, and the voltage drops due to load where charging occurs.
Traditional "12 volt" solar panels have a rated maximum open circuit (disconnected) voltage ("OCV") of around 19 volts. Under load this drops to around 13-14 at the point of charging, which matches old school flooded lead acid batteries. This particular design voltage allowed simple on/off charge controllers to function without much intelligence, simple/slow switching, and slow sensing of voltage. Modern "MPPT" charge controllers sample voltage so frequently and make changes in output so rapidly that they can turn very high voltages into correct battery charging voltages.
Most full size modern panels seem to have OCV over 40 volts. Keeping the series OCV under the maximum voltage of your solar controller is how you avoid frying it if you have a problem with batteries or connections. You add together the OCV of all the panels in series.
(I added some links and made some minor edits to this web version.)