1. The IRS uses Energystar for their official answers, so will I, below.
2. Residential clean energy credit can be taken multiple times and it carries over when not used.
From Federal Tax Credits for Energy Efficiency we see:
Energy Efficiency Home Improvements
Claim 30% up to $1,200 for these qualifying energy property costs and certain energy efficient home improvements. The credits are nonrefundable, so you cannot get back more on the credit than you owe in taxes. You may not apply any excess credit to future tax years.
Residential Clean Energy Equipment Upgrades - save 30%
If you invest in renewable energy for your home (solar, wind, geothermal, fuel cells or battery storage technology), you may qualify for an annual residential clean energy tax credit of 30% of the costs for qualified, newly installed property from 2022 through 2032. The credit percentage drops to 26% for property installed in 2033 and 22% for property installed in 2034.
For these upgrades, you can carry forward any excess credit and apply it to reduce the tax you owe in future years.