Tesla and independent energy firm NatPower have reached a deal to build 25 gigawatt-hours of battery storage in Italy and Britain, the first phase of a program worth up to $5 billion.

Under the multiyear agreement, NatPower will deploy Tesla’s Megapack systems and use Tesla’s trading software to decide when to buy and sell electricity — extending Tesla’s reach deeper into Europe’s fast-growing grid storage market.

A $5 billion program aiming past 100 GWh

The companies announced the deal Tuesday through Reuters. The first phase covers five initial projects totaling 25 GWh across the two countries, with a construction cost of $4 billion to $5 billion.

That’s only the start. NatPower says the broader program ultimately aims to exceed 100 GWh of storage capacity, with revenue that could top $15 billion over 20 years.