By Paul Keegan, contributor

Now that Bloom Energy has come out of hiding on Fortune.com last Friday and on a recent episode of CBS’s “60 Minutes,” you’d think we’d all be able to start celebrating the invention of K.R. Sridhar’s magic black fuel-cell box. The CEO claims it can provide abundant, cheap, clean electricity that will finally rid us of our dependence on fossil fuels.

But hold the champagne. A few critical questions remain that we hope will be answered at Bloom’s big press event on Wednesday in San Jose, Calif. on the campus of eBay (EBAY), one of Bloom’s first customers.

The event will feature a star-studded lineup that includes California Governor Arnold Schwarzenegger, board member Colin Powell, eBay CEO John Donahoe, venture capital firm and early Bloom believer John Doerr of Kleiner, Perkins, Caulfield & Byers — and, of course, the man on the hot seat, Sridhar himself. Let’s hope one of them can tell us whether the eight secretive years and $400 million Bloom has raised has produced a product that might be commercially viable anytime soon.

Here’s what everyone should be asking:

Can the Bloom Box be cost-competitive without subsidies?

Remarkably, after all this time, Bloom has still not released numbers about how much the Bloom Box costs to operate per kilowatt hour. Here’s what we need to know: How much does a typical Bloom Box — the kind Google (GOOG), eBay, Wal-Mart (WMT), and Staples (SPLS) have been testing — cost. And how many kilowatts of electricity will each box produce (unsubsidized, of course)? Is the cost the $700,000 to $800,000 that “60 Minutes” reported FedEx (FDX) paid for its unit? Would that be for a 400 kilowatt system like the one Google told Fortune.com it bought?

With those two facts, we can factor in the cost of natural gas and determine how much it costs per kilowatt hour to run the system around the clock for 10 years — the length of the Bloom Box warranty. We also need to know the price of that warranty and whether there is another fee for monitoring and maintaining the system, which Bloom handles so customers don’t have to worry about it. On “60 Minutes,” eBay’s Donahoe said the Bloom Box has saved the company $100,000 in energy costs, but that’s meaningless without the details to see how he got there.

Once we get an accurate number for what the Bloom Box costs, we have to compare it to what companies can pay simply by plugging it into the grid — yes, that nasty, coal-burning grid. Jacob Grose, senior analyst with Lux Research, says the average retail price of coal-generated electricity is 11 cents per kilowatt hour, which includes the cost of transmission and distribution. Bloom has to make its boxes cheaper than that — and just as reliable — to prove that it has a winner. Says Grose , who has updated his research since the “60 Minutes” piece on Sunday, “I’m still skeptical that Bloom will be able to compete on price in unsubsidized markets.”

How will people respond when they find out this isn’t a zero-emission generator?

Awestruck news accounts group the Bloom Box with renewable energy sources such as solar and wind. But the most common form of energy fueling Sridhar’s box is likely to be CO2-emitting natural gas.

Sridhar argues that a Bloom Box fueled by natural gas installed in your backyard would produce half as much carbon dioxide as a fossil-fuel power plant. But the comparison isn’t completely apt because states such as California — where all of the units Bloom has sold so far are located — get much of their energy from renewable sources. And he acknowledges that by using natural gas, the Bloom Box does indeed have a carbon footprint. “As long as there is a hydrocarbon fuel,” he admits, “there will be CO2.”

Though some Bloom Box users such as eBay are using bio-gas, not much of that stuff is around and therefore natural gas is likely to be the primary fuel.  Rather than being discussed as a completely new form of energy, shouldn’t the Bloom Box be more accurately considered a more efficient way to burn fossil fuels?

Has the company made a mistake targeting consumers?

How much would it really cost to buy a Bloom Box to power your home — and is that really a plausible market over the next five to 10 years, as Sridhar suggested on “60 Minutes?” Sridhar told Lesley Stahl the same thing he told Fortune.com: that one bread loaf-sized unit capable of producing 1 kilowatt of power could power a single home. We are grateful for the online commentators responding to our article on Friday who pointed out that the average American home consumes far more than one kilowatt. It’s true: If you have ten 100-watt bulbs ablaze in your house, you’ve already consumed 1 kilowatt and haven’t even turned on the air conditioner, dishwasher, or hair dryer yet.

Sridhar says a 1-kilowatt unit would cost $3,000. But if, as many analysts say, the average home might consume five to 10 kilowatts during peak use, the price would be a harder-to-swallow $15,000 to $30,000. It’s unclear why Sridhar is pushing the home market, anyway, since the most fertile ground for now seems to be the corporate and institutional market, which can exploit efficiencies of scale.

The tech world’s been waiting for Bloom to emerge from the shadows; Sridhar will have to prove Wednesday that he’s got something that lives up to the hype.