Ottawa residents, landmarks and businesses, including Parliament Hill and the Fairmont Château Laurier, flicked off their lights, causing a four-per-cent drop in electricity use.
About 892 megawatts of electricity were consumed from 8 p.m. to 9 p.m. on Saturday, compared with 929 megawatts during the same time last week, Hydro Ottawa said.
"We can say there was a noticeable drop," said Susan Barrett, a spokeswoman for the city-owned utility company.
Temperatures for both Saturdays at the time, as measured at the Ottawa airport, were around -4C, according to Environment Canada.
Marie-Lynn Boudreau was among those in Ottawa who turned off their lights and appliances for Earth Hour. Under the flicker of candles, Ms. Boudreau hosted a party with 20 guests.
After watching a green movie, Garbage Revolution, earlier in the night, the group played charades during Earth Hour.
"We ended up keeping up the lights off for longer than the hour just because the atmosphere was so fun."
Ms. Boudreau said there would have been more awareness if the city had signed on earlier, although she appreciated the efforts of those who participated.
Ottawa was among the 116 municipalities in Canada that took part in the event, with Toronto boasting an 8.7-per-cent drop in power use. About 50 million people participated worldwide, according to the World Wildlife Fund.
Hydro Ottawa said power consumption continued to be lower than the previous week, even after Earth Hour. About 860 megawatts were consumed between 9 p.m. and 10 p.m. compared with 891 megawatts last Saturday.
An average Ottawa home uses about 0.75 megawatts a month.
Mayor Larry O'Brien said he was proud of Ottawa's participation and hoped the city could do better next year.
"Based on this year's effort, we will be able to set an ambitious goal for next year and no doubt Ottawa will once again come together and rise to the challenge."
Provincewide, the demand for electricity dropped five per cent, or 900 megawatts, between 8 p.m. 9 p.m. on Saturday.
"These results are beyond our wildest expectations," said Mike Russill, president and CEO of WWF-Canada. "Earth Hour was an astronomical success because it tapped into, and gave a voice to an existing public concern. The fact that millions of people turned out their lights for the occasion shows that Canadians share a profound desire for action on climate change."
Ontario saved about 700 megawatts for the whole day Saturday. Typically, the province would have consumed a little more than 17,000 megawatts, but instead used about 16,300.
Natalie McGowan, owner of an Ottawa business that sells green products and mother of 19-month-old triplets, turned off all her lights and caught up on much-needed sleep during Earth Hour.
Her triplets had conveniently fallen asleep beforehand.
"That was my perfect chance," said Ms. McGowan. "I don't sleep, that is why I slept."
As for those who say Earth Hour is a gimmick, Ms. McGowan is quick to respond.
"Any event that raises awareness can't be negative," she said. "Gimmick or not, if it raises awareness, it achieved its goal."
But while thousands of people and businesses across the country were trying to cut their power use, residents in Calgary actually used more.
According to Calgary's energy supplier, consumption in the city increased from 1,050 megawatts at 7:45 p.m. to a peak of 1,060 megawatts 30 minutes later. That compares with a range of 976 megawatts to 1,023 megawatts at the same times last Saturday.