If I installed a PV system in 2019 using a loan w/ 0% interest for 12 months and then moved the remaining loan balance to a HELOC, could I deduct the HELOC interest?

[Critter-3]

No ... this was not to buy, build or improve the home.  You bought a PV system you can move to a different property and it is not an improvement to the home ... it is like a car ... when you sell the home it can go with you.

[Opus 17]

Even if we assume that the solar system is a substantial improvement to the home, it does not qualify as “acquisition debt” for purposes of the mortgage interest deduction because it would have to have been added to the mortgage within 90 days and you waited too long.