See https://www.irs.gov/publications/p502#en_US_2021_publink1000178902
See also https://ttlc.intuit.com/community/tax-credits-deductions/discussion/are-whole-home-generators-tax-de...
See also https://ttlc.intuit.com/community/tax-credits-deductions/discussion/i-had-a-generator-installed-for-...
