TORONTO - More Canadian municipalities are pledging to power down Saturday for Earth Hour, but an expert says interest in the event may already be fizzling.

Earth Hour has "done a great job of capturing the imagination of an awful lot of people around the world" to raise awareness of climate change issues, said Mark Sarner, who specializes in marketing for non-profits.

But "just more of the same gets old fast," he said Friday, noting most marketing campaigns will fade after a few years without something new to draw the public's attention.

"If I think back to last year's Earth Hour, I don't think it was as big a deal because, you know, novelty normalizes and this is no longer a novelty," he said.

"Unless something really dramatic is done to recreate it and reinvent it, I don't think Earth Hour will be a significant thing in the next five years."

Earth Hour began in Australia in 2007 and has since spread to more than 130 countries.

Canadians organized hundreds of grassroots celebrations -- from candlelit walks to astronomy parties -- for the country's first Earth Hour in March 2008.

Some have become annual traditions, while others have been discontinued.

It's hard to predict how many people will shut off the lights between 8:30 and 9:30 p.m. Saturday, but organizers said they aren't worried about losing steam.

More than 420 municipalities across Canada plan to participate this year, up from roughly 300 last year, said Steven Price of the World Wildlife Fund, the organization behind the event.

About one billion people worldwide are expected to take part, on par with last year, he said.

"I'm confident it will be dark (Saturday) night," he said Friday.

In Toronto, last year's Earth Hour efforts saved 296 megawatts of power, a reduction of about 10 per cent, according to Toronto Hydro.

That's less than in 2009, when the city saved 454 megawatts, or about 15 per cent.

Jennifer Link, a spokeswoman for Toronto Hydro, said she doesn't know whether the downward trend will continue.

Earth Hour shouldn't be judged based on how much energy it saves, said Sarner. Its real goal is to send a message to policy-makers that people care about climate change, he said.

"I would say after five years (Earth Hour) may have done its job," he said.

The challenge now, he said, "is going to be how to sustain, deepen and broaden the momentum of support for climate change-related policies, practices and behaviours."