Where will you be when the hour arrives? Wherever it is, you might want to take a flashlight–LED, of course. In it’s third year, the organizers of Earth Hour are shooting for one billion people to turn out the lights in this global demonstration in support of decarbonization.

It’s being promoted as a kind of switchplate referendum. Begun in Sydney, Australia, in 2007, it’s a simple concept, which may be part of its appeal: Wherever you are, at 8:30 local time tomorrow (Saturday) evening, turn off the lights for one hour.

Last year organizers estimated that 50 million people complied. California icons like the Golden Gate Bridge went dark. This year, about two dozen California cities and counties have signed up to participate, as well as the State of California.

They’re not telling you to turn off your computer or iPhone, however. Electrons we save at the light switch might be made up for on the Internet, which will likely be abuzz with a worldwide conversation documenting the event on blogs and social networks like Twitter (tag your updates with #earthour and #location). Photo sites will be bombarded with picture uploads.

Personally, I’ll be in the high desert of New Mexico, beyond the sight of any town or even neighbors, so the event won’t make for much of a snaphot.

It might also be a bit anticlimactic for the folks who run California’s power grid. I asked Gregg Fishman at the California ISO (Independent System Operator) if they’d be able to see a dip in the load at 8:30 p.m. tomorrow. He’s not counting on it: “It will probably have some impact but it’s really hard to measure,” he told me.

This time of year, the state is usually pulling about 27-28,000 megawatts at that hour on a Saturday evening. In fact, if you look at the ISO’s grid status graph, you can see a little spike around 8 p.m., as people normally start turning lights on. But Fishman says that even though “most of the load is lighting” at that hour, it may be hard, even for grid technicians, to measure the actual effect of Earth Hour.

But of course, that’s kind of beside the point. The event isn’t designed to achieve palpable energy savings for one hour. It’s supposed to be a visual show of support for policies designed to reduce energy consumption and the global carbon footprint. Recent surveys have shown that economic woes have pushed concerns about global warming and the environment to their predictable recessionary lows, at least in this country. Tomorrow night we’ll find out how the rest of the world feels about it.