When do I claim the credit when the enabling property and enabled property were installed in different tax years?

If the enabling property and the enabled property were installed in two separate tax years, then you can treat both as having been installed in the later of the two years. For example, if you installed a new panelboard in 2024, and in 2025, you install a new electric heat pump water heater you'd treat both as having been installed in 2025.