People in Ottawa might not have saved as much electricity as they were told during last month's Earth Hour.

Hydro Ottawa issued a news release late on March 26, saying it registered a drop in demand during Earth Hour of four and a half per cent.

The city and the mayor referenced that number in their own release, praising people's contribution to the worldwide event.

Hydro Ottawa calculated the four and a half per cent drop by recording demand at 8:30 p.m. and then again an hour later.

It had gone down by four and a half per cent, which they say show Earth Hour was a success. Toronto Hydro used the same method to calculate its Earth Hour savings and reported a five per cent drop.

But demand almost always decreases between 8:30 p.m. and 9:30 p.m.

Stephan Schott, a professor of public administration and energy issues at Carleton University, said he was surprised, because to come up with an accurate figure one would need a larger set of data adjusted for temperature.

"I don't think it's a very meaningful figure at all, because we have to compare that to other Saturdays in March," said Schott.

In other words, there's no baseline.

Measured differently, Earth Hour energy actually up

"We did change [the calculation] last year because there is that demand to have results the night of, and we also are aligning ourselves more with how it's reported across the province," said Susan Barrett, Hydro Ottawa's acting manager of communications and public affairs.

However, the province's Independent Electricity System Operator this year encouraged utilities to compare demand with March 22, 2008, a Saturday night with similar weather.

Using the IESO method, Hydro Ottawa customers used only about one per cent less electricity during the 2011 Earth Hour than in an hour on that weekend three years ago.

Kingston, Ont., a city that used the IESO method in its calculation, posted a 3.1 per cent drop in electrical consumption.

Barrett said the utility is standing by its calculation.

Earth Hour is an annual global event hosted by the World Wildlife Federation. People are asked to turn off unnecessary light and electronics to conserve power and demonstrate support for the fight against climate change.

The IESO, using its own standards, reported a lower electricity demand of 2.1 per cent, or 360 megawatts, for all of Ontario during the 2011 Earth Hour. The drop was equivalent to the peak energy needs of a city the size of Windsor, the IESO said.