They probably won't be using any 5% spodumene concentrate...

They probably won't be using any 5% spodumene concentrate directly. They will buy 99.9% industrial grade hydroxide or carbonate, almost definitely from China.

But I did have a think about this as you suggested, and I found myself thinking along these lines...

A 21700 cell as used in modern Teslas has a specific energy of ~300Wh/kg

15GWh would return about 50 million cells at 300Wh/kg

The amount of lithium in a 21700 or similar cell is less than 2% by mass.

A typical 21700 cell has a mass of ~65 grams and probably less than 1.3 grams of lithium.

This gives us ~ 4,225 tonnes of 99.9% hydroxide/carbonate required per 15GWh

1 tonne of 5% spodumene concentrate contains ~50kg LCE

The 4,225,000 kg of cathode material therefore, would require about 84,000 tonnes of 5% concentrates.

Looked at another way, 1mt of ore at 1.5% grade would return ~15,000 tonnes of battery grade lithium.

1 tonne of battery grade lithium probably requires about 20 tonnes of 5% concentrate.

So the 4,225 tonnes of battery grade lithium would also require about 84,000 tonnes of 5% concentrates.

Of course these numbers don't take into account losses or inefficiencies of production and I suspect that 300Wh/Kg is being generous.