So I could buy a battery for ~$1,850 today that won't even break even in a decade (and hope everything goes well with this plan, not guaranteed in the slightest), or I can buy a 10 year US treasury note that'll give me another $1,000+ in profit at the end of the decade.

Remind me why I should buy the battery? It clearly can't be because it makes financial sense to just have the battery.