Why Spain's blackout left people's phones without internet or signal

Telecom operators often have measures in place to weather such incidents, industry experts say, but there are limits to how much they can do to keep services available.

Backup power

Many base stations -- the cell towers that connect phones wirelessly to the wider network -- are fitted with backup batteries that let them keep operating for a few hours without grid power.

This is especially common in countries with less reliable supply, or that are subject to natural disasters like earthquakes, said Ian Fogg of consultancy CCS Insight.

Even in stable Germany, though, "our antenna locations are usually equipped with batteries," Deutsche Telekom spokesman Christian Fischer told AFP.

"Short-lived, local power outages can be compensated for by the internal reserves on some installations," allowing mobile data service and voice calls to continue, he added.

Hardware for the cabled "backbone" of the network and infrastructure such as data centres may be equipped with heavier-duty batteries or their own generators that can keep the lights on longer.

Like other major providers, Deutsche Telekom also has temporary mobile cell towers, generators and other network equipment ready for deployment to disaster areas, Fischer said.

But end users' devices like phones and routers will still need power of their own to access the network.

Who you gonna call?

Even if your own mobile network has power and your phone is powered and connected, a widespread blackout could leave people you try to call on other networks without connection, or take internet services and websites offline, Fogg said.

Phone networks can be set up so that users can receive emergency messages over the network of a rival operator if the one they subscribe to is unavailable.

But "that depends on how everything is configured before the outage," and "only helps you if some of the operators are affected but not all of them," Fogg added.

One alternative becoming available on the latest smartphones are so-called non-terrestrial networks (NTNs) which can allow for limited data and texts over a satellite connection.

'Virtual power plant'

Different hardware providers are building technology that could see mobile networks themselves contributing to the stability of power grids.

Nokia last year announced software it dubbed "Virtual Power Plant" that would help cell towers and other hardware feed power into the grid from their batteries or solar panels, generating revenue for operators and helping stabilise supply.

Rival Ericsson has a similar offering named "Site Energy Orchestration", while Finnish operator Elisa already has a "Distributed Energy Storage" system of this kind in service.