Investigations into North America's worst ever power failure are focusing on an early warning system of telephone hotlines.
Authorities say the grid is coping with the back-to-work spike
The hotline system was put in place following the blackout of 1965 and was designed to prevent widespread breakdowns.
Regional power firms have said they are checking their records of conversations with Ohio-based FirstEnergy about problems in the hours ahead of the blackout last Thursday.
A series of failures in four transmission lines - three of them owned by FirstEnergy - have come under suspicion as the power failure's possible starting point.
However, the company is adamant that it should not be blamed for the outages.
$1bn cost of blackout
New York City officials estimated that the economic cost of the blackout would exceed $1bn in their own city alone.
New York City Comptroller William Thompson said that $800m of economic activity did not take place, while $250m worth of perishable goods were lost.
Additional costs, which have not yet been calculated, include lost tax revenue and the cost of overtime for the emergency services, which could run into tens of millions of dollars.
New York, with a population of eight million, represents just one-fifth of the total area blacked out in last Thursday's power failure.
But the biggest cost is likely to be the price of further investment in the overloaded grid system, which some experts say could cost as much as $50bn.
Causes still not known
The head of the US Federal Energy Regulatory Commission (FERC) said it would take "days and weeks" to establish the cause of the power cuts.
FERC chairman Pat Wood added that speculation about the role of FirstEnergy in the blackout was "premature".
FirstEnergy has acknowledged that its transmission lines failed, saying it was "working with the joint US-Canadian task force to gather and look at the data".
But, in a recent statement, it defended itself against accusations that it was to blame for the blackout.
"Contrary to misinterpretations that identified FirstEnergy as the cause of the widespread outage, it is clear that extensive data needs to be gathered and analysed in order to determine with any degree of certainty the circumstances that led to the outage," the
"What happened on Thursday afternoon is a very complex situation, far broader than the power line outages we experienced on our system."
Living with shortages
US Energy Secretary Spencer Abraham is due to meet Canada's energy minister Herb Dhaliwal in Detroit on Wednesday to sketch out how their joint taskforce would investigate the power failure that shut down parts of the cross-border grid within three minutes.
US and Canadian cities continued to restore services on Tuesday, amid fears that a new surge in demand could strain the power grid.
In Canada, the premier of Ontario, Ernie Eves, appealed to people to cut electricity consumption in half until electricity supplies were stable.
Auto firm General Motors halted production at its biggest Canadian plant and cut down work at six others while Ford and DaimlerChrysler also made cutbacks.
US phone operator Verizon was also singled out for criticism for its performance during Thursday's crisis by New York mayor Mike Bloomberg for providing poor emergency services.
Mr Bloomberg is believed to be seeking a meeting with representatives from the company.
"Everybody had a problem with Verizon. We have got to make sure that doesn't happen again," Mr Bloomberg said on Monday.
Verizon has promised to investigate "any glitch we saw on the system" and talk to customers.
In Detroit, Michigan, the big three US car manufacturers told employees to report to work on Monday, after several plants were forced to close on Friday.
But thousands of small firms and restaurants are still counting the cost of wasted produce and struggling to restock their shelves.
Grid still at risk
In Ontario, Mr Eves, who has declared a state of emergency in the province, urged people to avoid using power-guzzling equipment like air conditioners, dishwashers and washing machines.
Officials in the US and Canada have warned of the possibility of rolling blackouts.
"We know that our system will be up and running, but we cannot say with 100% certainty that this can't happen again, until we know what happened and what steps are being taken to prevent it again."