I believe placed in service. From IRS:

 the residential energy efficient property credit allows for a credit equal to the applicable percent of the cost of qualified property. Qualifying properties are solar electric property, solar water heaters, geothermal heat pumps, small wind turbines and fuel cell property. Only fuel cell property is subject to a limitation, which is $500 with respect to each half kilowatt of capacity of the qualified fuel cell property. Generally, this credit for alternative energy equipment terminates for property placed in service after December 31, 2021. The applicable percentages are:

1. In the case of property placed in service after December 31, 2016, and before January 1, 2020, 30 percent.
2. In the case of property placed in service after December 31, 2019, and before January 1, 2021, 26 percent.
3. In the case of property placed in service after December 31, 2020, and before January 1, 2022, 22 percent.