Solar panel installations in the US are mainly regulated at the state, county and utility level. Some states have adopted specific licensing requirements for solar power installers and for solar panels and related equipment. However, many states have not yet done so. If you click on your state you will see the licenses or registrations a solar power installer must have in your state and also the accreditation requirement for solar panels and inverters in your state.
As a general rule if a state does not have specific licensing requirements for installers then it would fall back to general licensing requirements that would apply to all electrical work. You also need to be careful because even if there is no state wide licensing requirements local utilities and counties may have their own requirements. A short cut to a definitive answer may be to either call your utility or to request quotes through this site from local installers that know the rules of your local utility. In relation to the licensing of solar panels and inverters many states rely on the Californian Energy Commission lists for approved solar panels and approved solar inverters. This list in turn relies compliance with standards UL 1741, UL 1702 and IEEE1547. This list is searchable through this site or through the GoSolar California website.