Are solar-only water heating systems eligible for OG-300 certification?

Yes. With the release of the 2020 version of the ICC 900/SRCC 300 standard, OG-300 certification is now available for solar water heating systems that do not use a backup (auxiliary) water heater. Note that local codes in North America may limit or prohibit the installation of solar-only systems. However, the use of such systems is fairly common in the Caribbean region (including Puerto Rico and the US Virgin Islands) and in Mexico.

Note that ENERGY STAR certification is NOT currently available for residential solar water heating systems without a backup heating source of some kind.