FORTUNE — What will General Electric (GE) write on the check it cuts to the IRS this year? I have no idea. And no one else knows, either.

This week, Citizens for Tax Justice, a non-profit group that advocates for higher U.S. corporate taxes, released a report saying GE paid Uncle Sam about $1 billion last year, or about 11.3% of its U.S. profits. GE shot back saying it took an expense of $2.6 billion for U.S. taxes for a rate of 25%. Both are most certainly wrong.

FORTUNE has been down this road before. Last year, my colleague Allan Sloan and ProPublica’s Jeff Gerth spent months trying to figure out whether GE paid taxes in 2010 and how much. They found that GE did pay U.S. federal taxes in 2010 despite headlines to the contrary. Citizens for Tax Justice and GE’s tax rates are based on GE’s accounting statements, not the company’s tax returns, which neither GE nor the IRS are required to release publicly.

Each year, GE, and indeed all public companies, tell investors how much they think taxes will reduce their profits. That’s their “tax expense.” But it’s just a provision – a set aside. Not all of that money gets sent to the IRS immediately. Some of it never will. The reason has to do with the fact that there are two sets of accounting rules – one for the IRS and another for investors and the SEC. And there are plenty moves that are perfectly legal and will lower a company’s taxes, but are nonetheless not allowed by financial accounting rules. As a result, companies routinely overstate how much they pay in taxes. GE, for example, has $5.2 billion in what it calls “unrecognized tax benefits,” which is money it told investors it spent on taxes but never did. “It’s very dangerous to use a company’s stated tax expense to make statements about what it actually pays in taxes,” says Douglas Shackelford, who studies taxes and accounting at the University of North Carolina.

And yet, that’s exactly what both Citizens for Tax Justice and GE are doing. The difference between CTJ’s 11.3% estimated tax rate and GE’s own claim of 25% has to do with the difference between current and deferred expenses. The current tax expense are taxes that the company says it owes now. Deferred taxes is money that the company may owe in the future. The latter is like putting aside money to pay taxes on the investment gains you have in your 401(k), even though you won’t have to actually pay until you retire and don’t really know exactly how much.

Citizens for Tax Justice says only GE’s current tax expense should be included in its rate, since that’s the money the company is mostly likely to actually pay the IRS. GE says you should include both its current and deferred taxes because that includes all the money it’s putting aside to pay 2011 taxes, which is the company’s best guess at what it eventually will owe. The CTJ number probably under counts some money that will eventually be paid. The GE number almost certainly double counts because some deferred tax expenses eventually become current tax expenses. Tax expert Robert Willens says he thinks using the current tax expense only is probably closer to reality. But the larger point is that both numbers are most definitely wrong.

The best figure we do have for what GE pays in taxes comes from its financial statements and it’s something called “cash taxes paid.” In 2011, that figure was $2.9 billion. Compare it to the $20 billion GE made in 2011 before taxes and that works out to a rate of 14.4%. But that’s for all of GE’s taxes everywhere. GE won’t say how much of that money was for local or federal U.S. taxes or to foreign governments, to which GE appears to pay the majority of its taxes these days. What’s more, some of that money could have been for back taxes. And some could eventually be refunded.

Says GE spokesman Andrew Williams, “When we file our returns later this year, we expect a U.S. tax liability. We do not disclose the amount, nor does any other company.”

And until the government or the Financial Accounting Standards Board forces GE to say more, frustratingly, that’s the best we are going to get.