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Comment by mathgradthrow | Hacker News Reader

Gigawatts are not a unit of energy.... something something zalgo.

Installed capacity is sometimes measured by power output rather than energy storage capacity. Both measures are relevant.

And yet only one is ever mentioned when batteries come up, despite the context being heavily implied to be "energy" not "power".

I always find talk of power weird. It does make sense when you need to replace some plant going down... But in general storage capacity is much more important. That is how many hours and at what price can they provide demand for. With hours here being some reasonable number above say four...

What I've seen in several articles is that peak power production seems to be as much a limiting factor as stored energy. I'm still haven't found a good explanation why that is though. So if anyone could educate me, I'm all ears.

Let's take an absurd extreme case to illustrate - if you had a battery that could store 1GWh but could only discharge up to 1mW in an hour, how would you power an aluminum furnace with this battery?

Assuming you are using batteries for continuous solar power, you basically need the batteries to be able to store enough power to last through the night, and to be able to match the solar power plant's peak power - otherwise, the grid can't power the same things during the night that it can during the day, which was the whole reason we wanted the batteries in the first place.

Because grid batteries are currently used as peak supply - if a production of power plants suddenly falls of by 10 GW or there is 10 GW additional demand, you have to quickly provide that 10 GW of power before other (cheaper) additional production can be ramped up.

My suspicion is battery life estimating is complicated and proprietary. So your depreciation due to wear depends on both the rate of charge and the rate of discharge.

My best guess is they shoot for a discharge rate of roughly C3. Meaning they can deliver the rated power for 3 hours. Suspect they keep the charge between 10 and 90%. So I think you can multiply watts by 4 to get watt-hours.

That said I've looked for actual numbers in kwh and not found much.

Depends on who you ask. For the grid operators since they balance instantaneous supply and demand they only care about power available. Expecting the producers to maximize their profit and hence save the batteries until they are most needed.

When explicitly designing an electrical system, say an off-grid cabin, both the energy stored and instantaneous matching of supply needs to pencil out.

In the large scale grid example we expect the market to handle the energy balance. Adding transmission links, secondary markets and so on when the market does not adequately match demand.

Generally speaking today batteries used for grid scale storage store 4 GWh per 1 GW. This number comes from maximizing the utilization of the grid connection and matching the hours per day with highest electricity rates.

Exactly, especially with some types of batteries this is relevant. Redox flow batteries that are being experimented with for grid storage now use a reservoir for storing the anolyte and catolyte fluids. The bit that allows those to interface where the power is generated is much smaller though. So you can take the same battery and trivially double or triple the storage capacity by simply using bigger reservoirs and more liquid but the power delivery would stay the same. These types of battery are potentially great for longer term storage but they aren't necessarily able to deliver the stored energy quickly. It could take days/weeks to deplete them and a similar amount of time to charge them back up.

Lithium ion grid batteries are sort of the opposite. They can deliver a lot of power but usually only for a few hours at maximum capacity until they are depleted. So, you see companies install a few hundred gwh of capacity being able to deliver most of that power in a few hours.

They’re a unit of power which is what is being described here. People want to know how much they can draw per unit time from each source.

Wind Turbines a GW

Nuclear Plant b GW

Battery Storage c GW

That’s a power measure so you use power units. It would make no sense to use energy units.

It's presented in Gigawatts here, but I expect the actual metric is probably something like "Gigawatts available for 99th centile of duration of demand peaks", as this would allow you to talk about capacity in the same units as grid supply and demand. The whole point of grid batteries is for near-instant availability, so total energy available is moot if you can't access it right away - therefore a metric of energy would be less useful. Anyway, that's my speculation.

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