California’s new health insurance marketplace is starting to come into focus as a state agency in charge of implementing President Obama’s federal health reform steadily adds more and more detail to the emerging picture, like a painter filling in a vast canvass.

But exactly how the final image will look to consumers remains a bit murky. And we probably won’t know the answer until after the health benefits exchange, known as Covered California, opens for business Oct. 1.

The big question is whether the coverage the health exchange is offering will be attractive enough – and affordable enough – for millions of uninsured Californians to take the plunge.

Federal law says they must. But consumers can still opt out and pay a tax instead, starting at $95 per adult in 2014 and rising to $695 in 2016, or 2.5 percent of family income, whichever is greater. And since people will be able to buy insurance later if they get a serious illness and the coverage begins to look like a better deal, many will likely do the math before they buy.

Covered California has now released enough information to make those calculations possible for most people.

Earlier this year the agency released details about how the new insurance plans would be structured, and the agency has acknowledged that, for at least some people, the new coverage will be more expensive than what they could buy today. But the new plans will also be broader, and come with wrinkles such as prohibiting insurance companies from capping a customer’s annual or lifetime benefits. And most importantly, the new law requires insurers to offer coverage to everyone, regardless of their health history, which is not the case today.

Once it designed the coverage options, the next step for Covered California was to negotiate with insurance companies interested in serving more than 5 million potential customers who could buy coverage through the agency’s web portal.

And last month, the health exchange announced that it had reached agreement with 13 health insurers. Those 13 include two statewide firms – Blue Shield of California and Anthem Blue Cross – and one, Kaiser Permanente, which will operate statewide except in the central coast counties of Santa Cruz, Monterey, and San Benito.

Several other major insurers, including UnitedHealth, Aetna and Cigna, either decided not to participate or could not reach an agreement with the exchange.

The other ten plans that will be part of the program are regional, and they include some that until now have served only low-income clients through the state Medi-Cal program. These include LA Care, the largest publicly run health plan in the country.

In no part of the state will consumers have fewer than two plans to choose from. In some, as many as six plans will compete for business.

What about the rates?

Premiums will vary based on a consumer’s age, where they live, and which level of coverage they choose. On average, the price for a “silver” plan — the one on which federal subsidies will be based — will be $321 per month for an individual.

But for those who qualify for the subsidies, in the form of a federal tax credit, the rates could be much lower.

In Orange County, for example, a 40 year-old individual with an income of $17,235 year will pay as little as $24 a month for a plan from Health Net, with the subsidies covering the rest of the insurer’s $252 premium. The same person will pay $57 for an Anthem plan, $62 for Blue Shield or $103 for Kaiser.

The subsidies phase out as a person’s income climbs. A single person making $46,000 or more will not qualify for any subsidy. For them, the rates in Orange County for the same plan will range from $252 per month for Health Net to $332 for Kaiser.

That same person will also be responsible for out-of-pocket costs — co-pays and deductibles – up to $6,350 for a year. So, counting premiums and out-of-pocket expenses, the insurance could cost more than $10,000. The maximum federal penalty at that income level for not buying insurance? $1,150.

Of course, the person who opts not to buy insurance risks paying far more than the tax penalty, namely the entire cost of their own care, at least until they can get coverage if they later decide to do so. And while $10,000 is a lot of money, it’s far less than the tens of thousands of dollars that a serious illness or injury could cost a person.

More than 5 million uninsured Californians will be eligible to buy insurance from Covered California and will be making these calculations in the months ahead. About 2.3 million of them will qualify for subsidies. The rest will be on their own.

What they decide is crucial, not just for them, but for the new system that the president’s plan envisions. You can be pretty sure that people who are sick will buy the coverage at just about any price if they can find a way to do so. But if too many healthy people opt out, premiums for those who do buy coverage will have to rise. That could drive more healthy people out of the insurance pool, making coverage for the remaining customers even more expensive. The eventual result: a death spiral.

That’s the worst fear of the people who run Covered California. They say they are confident it won’t happen. But they cannot really know, because no one has tried anything quite like this before.

Daniel Weintraub has covered public policy in California for 25 years. He is editor of the California Health Report at www.calhealthreport.org