Nearly half of India has been plunged into darkness after a second massive power cut left hundreds of millions of people without electricity.

Travel networks descended into chaos while companies, schools and homes were forced to conduct what business, lessons and housework they could by candlelight as government technicians battled to restore power before nightfall.

It is the second such meltdown in as many days.

Following Monday's failure, power officials managed to restore the northern grid by evening, but at 1:05pm local time on Tuesday, the grid collapsed again.

Tuesday's failure hit a wider swathe of the country than Monday's, this time including eastern as well as northern states, the state-run power transmission company said in a statement.

The cause is unclear, although there have been unconfirmed reports that the blackouts may be down to some states using more power than authorised.

The meltdown has affected Indian capital Delhi as well as major cities such as Kolkata.