> If batteries have a solid 9% return on investment, but solar panels have an even better 12% return on investment, panels will outpace batteries even though the batteries are a decent investment.

Sorry, normally I hate this follow-up on HN, but can you share a source? I tried to Google for sources, but there is a pretty wild range of ROI in different countries/regions. My point: Ideally, can you provide personal/anecdotal experience, or something that is specific to a country or region?

EDIT

I forgot to say: I like your idea of intraday arbitrage using batteries! It is a very cool idea. Surely, this could be well modeled to know your expected ROI before investment/build-out.