Elon Musk has suggested that lithium-ion grid batteries can power the entire island, but that might not be quite as easy at he makes it sound.

When Hurricane Maria hit Puerto Rico, it caused huge damage to its power grid. So bad, in fact, that restoring power to everyone on the island is expected to take months. Tesla has already sent hundreds of its Powerwall battery packs to help the cause; they can be paired up with solar panels to provide microgrids to some parts of the island.

But could Tesla do more? When asked on Twitter if Tesla could help rebuild Puerto Rico’s grid with solar and battery systems, Musk replied:

The Tesla team has done this for many smaller islands around the world, but there is no scalability limit, so it can be done for Puerto Rico too. Such a decision would be in the hands of the Puerto Rican government, Public Utilities Commission, any commercial stakeholders and, most importantly, the people of Puerto Rico.

Puerto Rico’s governor, Ricardo Rossello, seems interested, telling the Tesla CEO: “Let's talk.”

But let’s hold on a second. It’s true that Tesla has helped supply some islands with grid-scale batteries, but the installations are small: a six-megawatt facility on Ta’u, Samoa, allows the island to run entirely off wind power, while a 52-megawatt-hour battery in Kauai, Hawaii, stores solar energy to reduce the need for diesel generators. Tesla’s biggest grid battery, currently in production in South Australia, will have a total capacity of 129 megawatt-hours.

But Puerto Rico isn’t a small island. It has a population of 3.4 million people, and its power generation was 20 billion kilowatt-hours in 2014.

Calculating how big an installation Puerto Rico would require to cover its energy needs isn’t just a matter of back-of-the-envelope math. But however you slice it, it would have to be much, much larger than the Australian system Tesla is currently working on.

Its size also means it would be expensive. Elon Musk has publicly stated that Tesla grid battery systems currently cost $250 per kilowatt-hour for systems of more than 100 megawatt-hours. That figure pegs the South Australia scheme at over $32 million.

There’s also a big question mark over longevity, because lithium-ion batteries aren’t usually used to provide baseload power, which would require frequent charging and discharging. Tesla doesn’t say how many cycles the batteries in its grid battery systems can tolerate before they degrade and reach the end of their useful life. But as with other lithium-ion batteries, it’s likely in the thousands—probably around 5,000, the same as Tesla’s consumer-focused Powerwall units. That’s not bad in a domestic setting, but it could be quickly devoured in Puerto Rico.

Finally, it could take an incredibly long time to build the thing. Tesla promised to build the South Australia facility in 100 days or else waive its fee, and that build-out so far appears to be going well. But building a full grid system could take considerably longer. And given estimates that restoring the grid could take up to six months (not including Tesla’s involvement), one is left wondering if the cost, complexity, and longevity issues don’t make the suggestions rather more bluster than substance.