The subsidies, which are technically tax credits because they are administered through the tax code, will help low- and middle-income families buy health insurance through the state-based exchanges. Under the new law, nearly every American will be required to have health insurance starting in 2014, or face penalties.
The enrollment season starts Oct. 1.
The subsidies are available to families with incomes up to 400 percent of the poverty level. This year, four times the poverty level is about $62,000 for a two-person family. For a family of four, it's $94,200.
About 18 million people will be eligible for subsidies, according to the Congressional Budget Office.
If families get bigger subsidies than they are entitled to under the law, the amount they have to repay is capped, based on income and family size. If they get less than they qualify for under the law, the government will pay them the difference in the form of a tax refund.
There are also special rules that protect people who marry or divorce from being required to pay back subsidies just because their marital status changes.
There are four thresholds for repaying the subsidies:
* A family of four making less than $47,000 would have to repay a maximum of $600.
* If the same family makes between $47,000 and $70,000, the amount they have to repay is capped at $1,500.
* If the same family makes between $70,000 and $94,200, the amount is capped at $2,500.
* Families making more than four times the poverty level have to repay the entire subsidy.
"It's potentially going to come as a shock to individuals who meet that criteria where their income hits a point where they owe money back," said Rep. Charles Boustany, R-La., chairman of the House Ways and Means oversight subcommittee. "The fact is, with variations in income, people could end up owing money back and that will create consternation and problems for them."
The total amount of money that taxpayers will have to repay is unclear, but congressional estimates offer some clues.
Twice since the health care law was passed Congress has increased the caps for how much people will have to repay. Combined, the two measures are expected to raise more than $40 billion over the next decade, according to Congress' Joint Committee on Taxation.
"I think people will get there," said Livingston, the former IRS official. "They will develop instincts about it the way we all do about any process we go through multiple times. But when it's new, in the early years, this will be a real learning curve."