Question: Not knowing what my income is from year to year, I could estimate my income to be lower than the actual amount. If it turns out the IRS says I owe them money because they paid me too much subsidy, will I also be charged interest, penalty, or fine?
Answer: No. There is no penalty or interest, because the IRS will recover the subsidy overpayment as part of your federal tax payment for that tax year. it would not be overdue.