I just wanted to clarify something:
While a I suppose the phrase ITC (Investment Tax Credit), could maybe used for both, generally it is only used in reference for the energy credit you get for business property (including rental property). The rules for that allow a carryback of up to ONE year if it can not be fully used on the current return, and then any excess is carried forward. But it would not apply to any tax due from more than one year ago.
Generally the energy credit for your personal home is called the "Residential Energy Efficient Property Credit", and there is no carryback for that. Any unused excess credit is only carried forward to future tax returns.
