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Comment by bradfa | Hacker News Reader

I'm glad they're reporting it as the energy storage capacity and not the power output capacity. Too many news sites like to report the power output capacity in stories like these, which is horribly misleading, and then often they also frame it as though this is equivalent to adding actual power generation to the grid, which is even more misleading.

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One reason they report in MW instead of MWh, is that grid operators primarily spec systems by their continuous MW, and only secondarily mention their cumulative MWh. So that's the way they talk about them, and reporters just mimic that. Both are important, but the top-line number is MW, for sure.

One reason for this is that for a lot of grid-support functions, instantaneous or short-term power is vastly more important than long-term energy capacity. The other reason is that traditional generation is only sized by MW; you just keep pouring fuel into it forever, so the notion of MWh doesn't even enter the picture. They're not even thinking in those terms.

You and I, as battery natives, think about it the other way around. But we're not grid operators.

It's probably also a view that is shifting as batteries move from primarily providing short-term grid stability (where power available is probably the more important metric) to longer term storage.

Yup! Some breaker trips, and a 500MW plant goes offline. How much battery storage do you need to compensate? 500MW. The fact that it stores 500MWh and therefore only lasts an hour comes afterwards, that is plenty of time to ask all the other fossil power plants to scale up a bit.

Having worked in the industry for a bit, the reason people talk about "MW of batteries" is because they're usually designed to deliver the capacity for a fixed number of hours. Earlier batteries would do 1 hour (so a battery would typically be 1MW/1MWh) and newer sites coming online are usually 2 hours (so 1MW/2MWh). As battery deployment grows the money is increasingly just in energy arbitrage, so there's a lot of pressure for longer duration storage. If you see a headline MW figure it's a safe bet that it's between 1-2x that in MWh.

Isn’t the MWh more like 2-4x in the US for grid sized batteries?

I can see why one might intuitively think this, but actually this is just a bad take. For most practical purposes, the main value added by these batteries is not really the size of their storage capacity (afterall, their capacity is tiny relative to the grid's needs), but their ability to quickly sink or supply massive amounts of power.

If these batteries had 10x the storage capacity, but 1/10th the power output, they'd miss out on their biggest advantage, which is capturing mid-day solar oversupply, and providing virtual grid inertia.

Long duration, lower-power batteries will have an important role to play once grids get further along in renewable adoption, but for now, short duration power-supply is the main value they add, and it makes perfect sense to quote battery numbers in terms of power. (not that I'd complain about providing capacity numbers too)

I mean, both is important. Pumped hydro can have quite large amounts of storage, but supply comparatively little W of power.

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