What exactly does the IRS letter change? The non solar energy credit only?
There is a $500 lifetime limit on the credit ... look at the form 5695 from prior years ... did you already use it up?
Did you forget to enter the prior year credit on line 18 (from the worksheet) https://www.irs.gov/pub/irs-prior/i5695--2018.pdf ... save a PDF of the return now if you don't have one and look at the form & worksheet.
Line 18
If you took a nonbusiness energy property credit in 2006, 2007,
2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, or 2017,
complete the following worksheet to figure the amount to enter
on line 18. If the total of the credits on line 9 of the worksheet is
$500 or more, you generally can't take this credit in 2018.
