Under the new military retirement system justapproved by Congress, service members will have to make a choicefew of them are prepared for: Whether to take traditional pensionchecks or opt for a lump sum that’s worth up to half of what thepension benefit would bring.

Military members are known for devotion to duty. But servicemembers in many areas depend not just on their military pay but onfood stamps and other outside sources of income to make endsmeet.

Now they have to decide, at retirement, whether to take some ofthe money now or a much larger amount later—a choice that wasopposed by both the Defense Department and many veterans’advocates.

However, Congress included the option—for a benefit whose valuehasn’t even been determined, but is likely to be less thanadvantageous to troops—when it gave its okay to the plan.

The Military Times reported that the Pentagon is goingto have to come up with a number for those lump-sum checks,something that’s bound to be troublesome.

Read: Senator Sanders pushes repeal ofMultiemployer Pension Reform

For one thing, while private-sector corporations calculate howmuch they have to offer employees in a pension buyout, the discountrate—which assumes that money available today is worth more thanmoney that’s not available till some future date—has to be tied tofinancial market indicators, such as bond rates.

That means the discount rate can come in between 2 and 4percent, according to Bill Hallmark, who tracks pensions for theAmerican Academy of Actuaries.

Read: Another banner quarter for pension buyoutdeals

Hallmark, who was quoted in the article saying that how thediscount rates for military lump sums would be determined “isuncertain,” added that the Pentagon could likely use asignificantly larger discount rate—perhaps even more than 10percent.

That would mean military retirees opting for a lump sum wouldget a lot less than their comrades who take pension checks.

Of course, they’ll have to be told about how it works, whichmeans the DoD, while it would save a considerable amount of moneyby giving out lump sums instead of pensions, might instead finditself sticking to the old system if service members turn down thenew option.

The new system goes into effect in 2018.

All current troops are grandfathered into the existing system,and have a choice whether or not to go to the new system—which doesprovide 401(k)-style benefits for almost everyone who serves morethan two years, or a pension check that will be 20 percent smallerthan the current one.

At present, service members with less than 20 years of servicereceive no benefits when they separate. Anyone joining in 2018 orlater will come in under the new system.