In the mornings, the pumps at HDB blocks can be heard working furiously. A lot of electricity is used during this peak period.
Why not pump the tanks on the rooftop full during the off-peak hours, when electricity costs less?
The savings from off-peak energy usage will, in the long run, more than make up for the cost of the extra tanks that will need to be acquired to do this.
It also smoothens the electrical generation, as power is consumed more consistently. This means there is no need for extra capital-intensive power generation and cabling costs to address peak hour demand.
It is also good for the environment.
If solar heaters are attached to the water tanks; these tanks could be heated so households need not even turn on their water heaters.
Again, this would result in less energy being used.