It's hardly news that America is awash in cheap shale gas. Since 2007, drillers have been extracting natural gas (and oil) from shale-rock formations around the country, using new fracking and horizontal drilling techniques. The boom has boosted incomes in states like Texas, upended the electricity sector and helped cut U.S. carbon emissions.

That cheap gas is also saving Americans money on their energy bills. But how much? Ask the experts, and you get a fairly wide range of answers.

Case in point: New research from Boston Consulting Group (BCG) suggests that shale-gas benefits might be much, much bigger than we think. The study estimates that the natural gas boom is now saving the average U.S. household $425 to $725 a year. That's enough to blunt most of the impact from, say, the expiration of the payroll tax cut back this past January.

And the study suggests that savings will rise over the next decade:

But this is also significantly higher than previous studies have found: In October, 50 experts and 14 modeling teams convened at Stanford's Energy Modeling Forum to discuss the shale boom. Their average estimate of the economic upside? About $70 billion per year — and that included all benefits, with household energy savings only a small portion of the total.

Why the variation? Here's one key difference: The BCG study estimates that the average American household spent $9,000 on energy in 2012. One-third of that was direct expenditures — gasoline, electricity, natural gas for heating. But there are also the indirect energy costs "embedded" in various goods and services. And that adds up.

The glut of cheap gas is having a clear effect on direct spending — lowering heating bills by an average of $186 per household, BCG found. But, the study notes, cheap gas is also having an effect on those indirect costs, by, say, lowering the cost of feedstock for petrochemicals or the cost to operate factories. (The study compared current natural gas prices with those forecast before the fracking boom became widespread.)

"I think a lot of people don’t understand how much the consumer is paying for energy embedded in the supply chain," says Hal Sirkin, a a senior partner at BCG and co-author of the study. "That gives you a different view on the benefits of cheaper energy."

This is essentially the flip side of the old debate over how much a carbon tax would cost Americans. If you think those supply-chain effects are large, then swings in energy prices can matter enormously to households — in both directions. Conversely, studies and models that find a small impact from carbon pricing are likely to find a more muted impact from the shale boom, too.

Meanwhile, the BCG study also estimates that 30 percent to 50 percent of the savings from lower natural gas prices are being passed along to consumers so far. In theory, says Sirkin, that should rise over time as competition increases. That suggests that the shale boom has yet to fully embed itself within the economy.

In any case, add this study to the pile of research debating the effects of the shale-gas boom. Here are some of the (often contradictory) findings we've seen so far:

-- Manufacturing: PricewaterhouseCoopers put out a report back in 2011 suggesting that cheap natural gas could lead to a manufacturing renaissance in the United States and create 1 million new jobs. Others have been skeptical of this claim: Goldman Sachs has argued that there's little sign that energy-intensive industries are rebounding.

-- Local economies: The fracking boom — for both oil and gas — is creating good-paying jobs and boosting incomes in states such as Texas, North Dakota, Pennsylvania and Ohio. Yet some research has suggested that Western towns that went through the last resource boom in the 1980s were actually worse off in the long run after the crash.

-- Climate change: The glut of natural gas is displacing coal in the electricity sector and reducing U.S. carbon emissions. But some modelers think the climate-change impact of the shale boom will be relatively minor in the long run, in part because all that gas could also displace carbon-free energy like nuclear, wind or solar. What's more, methane emissions from natural-gas operations could undercut the climate benefits of shale gas.

-- Environment: There are the various (and often conflicting) studies on the pollution impact of fracking. The Environmental Protection Agency, for its part, is treading cautiously in this area.