Do I have to pay back ACA subsidies?

When you enrol, the Marketplace pays an advance premium tax credit to your insurer based on the income you expect. At tax time the real figure is compared with the estimate. Earn less than expected and you receive the difference as a refund; earn more, and some or all of the excess is repaid, subject to caps that depend on your income level.

The way to avoid a surprise is boring and effective: report income changes when they happen rather than at the end of the year. A mid-year update either raises your monthly help immediately or reduces what you owe the following April.

An estimate for planning, not a quote. Medicare and the Marketplace set your real figures.