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Comment by michaelt | Hacker News Reader

I think it's less a question of batteries being economical, and more a question of the relative economics of batteries vs solar panels.

After all, if the highest demand is between 16:30 and 19:00 you could use batteries to store power at 12:00 and sell it at 18:00 - or in famously sunny Australia you could build enough solar panels that solar output at 18:00 matches power demand.

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.

(Also, from a politican's perspective, making batteries highly economical is how you get batteries built. And an awful lot of pro-environment policies involve raising taxes, banning things and creating new chores; it's nice to have some green policy announcements that actually benefit voters in the short term.)

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> you could build enough solar panels that solar output at 18:00 matches power demand

No you could not. For half the year the sun has set by 18:00.

I mean in the dead of winter, yes. For six months of the year? Definitely not.

Definitely so. Unless you are on the equator, the sun is up for less than 12 hours a day from the autumnal equinox to the spring equinox. The sun will set before 18:00 local solar time. So apart from funkiness with time zones and summer time (which extends a couple of weeks past the autumnal equinox in Aus), yes, roughly half the year.

You're not crazy in the broad sweep of your idea, but actually because the sun isn't a point of light you're also not strictly correct, for example in Singapore the day is always more than 12 hours long.

Singapore is close to the equator (1°17′N) — the days are roughly 12 hours long all year. No, it's not exact. They vary from +3 to +12 minutes. It's close enough (<2% error).

It's not entirely due to the apparent size of the sun — refraction due to the atmosphere has a slightly greater effect.

(Singapore is also in the 'wrong' timezone. The sun sets around 7 pm every day, giving it effectively permanent daylight saving time.)

But regardless, Australia is not near the equator. The timezones are mostly ok. In most of the country (for most of the population anyway) the sun sets before 18:00 for roughly half the year. No amount of solar is going to power the evening peak demand without storage.

You won't get 12% return if your panels generate electricity which is only paid between 18 and 19, because there is already overcapacity between 16:30 and 18.

If you build enough solar that the output at 18:00 matches the demand, it will not have the same ROI as if you are using batteries.

    > 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.

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