This tax season, for the first time since the health law passed five years ago, consumers are facing its financial consequences. Whether they owe a penalty for not having health insurance or have to reconcile how much they got in premium tax credits against their incomes, many people have to contend with new tax forms and calculations. Experts say the worst may be yet to come.

When Christa Avampato, 39, bought a silver plan on the New York health insurance exchange last year, she was surprised and pleased to learn that she qualified for a $177 premium tax credit that is available to people with incomes between 100 and 400 percent of the federal poverty level. The tax credit, which was sent directly to her insurer every month, reduced the monthly payment for her $400 plan to $223.

A big check from a client at the end of last year pushed the self-employed consultant and content creator’s income higher than she had estimated. When she filed her income taxes earlier this month she got the bad news: She must repay $750 of the tax credit she’d received.

Avampato paid the bill out of her savings. Since her higher income meant she also owed more money on her federal and state income taxes, repaying the tax credit was “just rubbing salt in the wound,” Avampato says. But she’s not complaining. The tax credit made her coverage much more affordable. Going forward, she says she’ll just keep in mind that repayment is a possibility.

It’s hard to hit the income estimate on the nose, and changes in family status can also throw off the annual household income estimate on which the premium tax credit amount is based.

Like Avampato, half of people who received premium tax credits would have to repay some portion of the amount, according to estimates by The Kaiser Family Foundation. Forty-five percent would get a refund, according to the KFF analysis. The average repayment and the average refund would both be a little under $800. (KHN is an editorially independent program of the foundation.)

Tax preparer H&R Block has also looked at the issue. It reported that 52 percent of people who enrolled in coverage on the exchanges had to repay an average of $530 in premium tax credits, according to an analysis of the first six weeks of returns filed through tax preparer. About a third of marketplace enrollees got a tax credit refund of $365 on average, according to H&R Block.

The amount that people have to repay is capped based on their income. Still, someone earning 200 percent of the poverty level ($22,980) could owe several hundred dollars, says Karen Pollitz, a senior fellow at the Kaiser Family Foundation. People whose income tops 400 percent of poverty ($45,960 for an individual) have to pay the entire premium tax credit back.

Experts say the message for taxpayers is clear: if your income or family status changes, go back to the marketplace now and as necessary throughout the year to adjust them so you can minimize repayment issues when your 2015 taxes are due.

Many people are learning about what the health law requires and how it affects them for the first time when they come in to file their taxes, says Tara Straw, a health policy analyst at the Center on Budget and Policy Priorities. For the past 10 years, Straw has managed a Volunteer Income Tax Assistance site in the District of Columbia as part of an Internal Revenue Service program that provides free tax preparation services for lower income people.

Some of the recently initiated owe a penalty for not having health insurance. For 2014, the penalty is the greater of $95 or 1 percent of income. The H&R Block analysis found that the average penalty people paid for not having insurance was $172. Consumers who learn they owe a penalty when they file their 2014 taxes can qualify for a special enrollment period to buy 2015 coverage if they haven’t already done so.  That would protect them against a penalty on their next return.

People may be able to avoid the penalty by qualifying for an exemption. Tax preparers rely on software to help them complete people’s returns, including the forms used to reconcile premium tax credits and pay the penalty for not having insurance or apply for an exemption from the requirement. For the most part, the software is up to the task, Straw says, but it comes up short with some of the more complicated calculations.

Case in point: applying for the exemption from the health insurance requirement because coverage is unaffordable. Under the health law, if the minimum amount people would have to pay for employer coverage or a bronze level health plan is more than 8 percent of household income they don’t have to buy insurance. That situation is likely to be one of the most common reasons for claiming an exemption.

But to figure out whether someone qualifies, the software would have to incorporate details such as the cost of the second lowest cost silver plan (to calculate how much someone could receive in premium tax credits) and the lowest cost bronze plan in someone’s area. The software can’t do that, so tax preparers must complete the information by hand.

“That one in particular has been vexing,” says Straw.

The gnarliest filing challenges may yet come from people with complicated situations, such as those who had errors in the IRS form 1095A that reported how much they received in premium tax credits, experts say.

Take the example of a couple with a 20-year-old son living at home who bought a family policy on the exchange. If midway through the year the son gets a job and is no longer his parents’ dependent, the family’s premium tax credit calculation will be off. The family needs to work together to figure out the optimal way to divide the credit already received between the two tax returns. The goal is to maximize the benefit to the family and minimize any tax credit repayment they may face.

“A lot of tax software is just not designed for that kind of trial and error,” says Straw.

Please contact Kaiser Health News to send comments or ideas for future topics for the Insuring Your Health column.