Can solar-only (no backup) systems receive ENERGY STAR certification?

No. The ENERGY STAR Residential Water Heater Certification is only available for water heaters that use at least some amount of fuel – electricity, gas, oil or propane. Therefore, solar-only water heating systems are not eligible for ENERGY STAR Certification from ICC-SRCC.

But – solar-only systems are eligible for OG-300 certification.