Large employers are going into this year's health insurance enrollment season with more uncertainty than they have in years. New Obamacare rules are making it hard to get a handle on how much their benefit costs could increase.
"We have employers extending coverage to a lot of people who may not have had coverage before, and needing to plan for who might take coverage," said Tracy Watts, a leader in health-care reform issues at Mercer, the benefits consulting unit of Marsh & McLennan. "That is a real wildcard, to be able to figure that out."
Employers are expecting to pay an average of 4.6 percent more for health-plan benefits in 2015, according to Mercer's latest National Survey of Employer-Sponsored Health Plans. The big unknown is how many workers they'll have to pay for.
Read MoreBusinesses not 'clamoring' for Obamacare
This is the first year large firms are required to offer coverage to most full-time workers who put in 30 hours or more per week, under the Affordable Care Act, after the Obama administration delayed the so-called employer mandate for one year.
Nearly 1 in 4 large employers waited until this year to comply with new Obamacare health-plan requirements, and the majority of those playing catch up are in retail and hospitality, industries with large numbers of hourly workers.
"They might be doubling or tripling the number of employees to whom they are extending coverage. Not that all of them will enroll," said Beth Umland, Mercer's director of health and benefits research. "But when you cover more employees, you spend more money."
Read MoreObamacare's fewer fans, more customers
In August, Wal-Mart Stores warned that its health-care costs would increase $500 million this year, after more workers signed up for benefit plans than expected. The retailer did not say how big an increase that represents in percentage terms, beyond saying that the figure was a material.
For 2015, Wal-Mart announced that it would no longer offer health insurance to employees working fewer than 30 hours per week. Many large employers surveyed by Mercer are trying to limit eligibility for benefits by limiting hours for part-time employees and new hires.
Typically 20 percent of workers opt out of employer health benefits, but the Obamacare individual mandate may push more employees to sign up for coverage at work. The penalty for not having insurance increases to a minimum of $325 in 2015, up from just $95 this year.
"I think 2015 will be the big impact year," said Watts. "Then things will probably settle after that."
Read MoreYou're gonna pay for that—Obamacare's Cadillac Tax
The Mercer survey points to signs that employers are getting used to the idea of Obamacare, even as a new excise tax on high-priced plans will likely complicate coverage for 2016.
Only 4 percent of large employers believe they will stop offering employee health plans in the next five years, according to the survey; that's down from 6 percent in 2013.