SACRAMENTO (CBS13) — Covered California enrollees who underestimated their income when signing up for California’s implementation of the Affordable Care Act may be hit with sticker shock when tax season rolls around.

The program began in 2014, making this the first tax year under the new law. For some, it could mean less of a refund coming back from the tax man.

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Anthony Lopez is one of more than 1 million Californians who are enrolled in Covered California.

“I’m happy with what I got so far,” Lopez said.

After guessing his income for this year, the college student and part-time caregiver shells out only $54 a month for a Silver Pro plan. The federal government picks up the rest of his premium in the form of tax credits.

Tax attorney Betty Williams says that credit could bring a crushing surprise when it comes to enrollees’ tax refunds.

“They won’t be happy because suddenly they won’t have that money coming to them,” she said.

Refunds for some may be affected, depending on how accurately they estimated their income.

“If it turns out their income was higher than they anticipated and too much was paid, they essentially have to pay it back,” she said. “And the way it’s paid back is through whatever type of refund they might have.”

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That means if you told Covered California you were going to make one annual income, but really made more money, you may have to give some of your tax credits back.

“Likely to be most affected with this are people who changed jobs and perhaps got a higher-paying job and didn’t make an adjustment,” she said.

Or people who simply made more than they thought, like Lopez. He ended up working more hours than he planned, but didn’t think to report the change to Covered California.

“I do caregiving for two patients, and it just goes by the hour and our budget is based on California and their state budget,” he said.

Covered California spokesman Dana Howard says participants should be updating their income throughout the tax year if that income changes.

“Customers and the consumers are advised when there is an income change during the course of the year to get in touch with the service center to notify us about that change,” he said.

Lopez relies on his tax refund, and he’s concerned.

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“I’m a student, so the tuition and my job—I just always, I like to have the tax rebate,” he said.