As explained by both the IRS—which wrote the regulation governing the Obamacare subsidy–and the Congressional Research Service, which published a July 31 report on the matter (Health Insurance Premium Credits in the Patient Protection and Affordable Care Act), the Obamacare insurance-premium subsidy essentially works as a cap on the percentage of annual income an eligible person is required to pay in health-insurance premiums.
This percentage-of-income cap gradually increases as a household’s income increases from 100 percent of FPL to 400 percent.
For households with income between 100 percent and 133 percent of the poverty level, for example, insurance premiums are capped at 2 percent of household income. From there, the cap gradually rises until it tops out at 9.5 percent of income for households making between 300 percent and 400 percent of the poverty level.