“June 22, 2018 — Today, the Internal Revenue Service (IRS) released guidance establishing when construction starts for solar projects under tax law…. As explained by SEIA, the guidance, Notice 2018-59, provides two methods to establish when construction of a solar facility starts in order to qualify for the solar investment tax credit (ITC):
(1) Starting physical work of a significant nature; or
(2) Meeting the “five percent safe harbor test” – i.e., paying or incurring 5% or more of the total cost of the facility in the year that construction begins.”

https://solarindustrymag.com/irs-new-solar-itc-guidance-a-boon-for-developers/