The wiki says it's 484 megajoules, delivered in 3 seconds. That is equivalent to 134 mega-watthours,

For a potentially more useful arbitrary metric, that's a bit less than two Model S batteries. Except it delivers all the energy in seconds, and charges in 45 seconds, and isn't limited to a few hundred charge cycles. It's a kinetic flywheel storage system, but I wonder if it would be built today using a giant lithium-ion pack (many Tesla PowerWalls, basically) or a supercap array.

134 kilowatt-hours.

1 megawatt-hour is 3600 megajoules.

Thank you for the correction.

It seems like this sort of energy could be very feasible with supercaps, lithium-ion, or lead acid, at that level.

Like you said in your other comment, the energy is a couple Tesla batteries. It's the power and cycling requirements that are hard.

Doubt it. A large purpose of this kind of system is to make it non explosive,

Flywheels fail by basically exploding.

I don't think this is that arbitrary - presumably the charge is delivered over 3 seconds, and the question is, how much power did that represent - 12,000 homes is a pretty good way of mentally visualizing how much power is being delivered.

If only we could have arbitrary units that everybody would understand without ambiguity? Like kWh/MWh or Joules... no?

I guess it depends on what you mean by "everybody would understand". Most people I know would get glazed eyes if I used terms like those. How many people really have context for understanding what those terms mean? Most people can understand what meter is since there are so many items laying around that are approximately 1 meter long.

You conveniently ignored part "without ambiguity".

I don't think it is much to expect that one would be able to connect a electricity bill where one pays for monthly energy usage stated in kwh to article where energy is also stated in kwh.

I think it is too much. I pay my hydrobill and I still have no idea how to visualize a hwh. However saying 12000 homes for 3 seconds makes sense for me. It is roughly the same power that a large neighbourhood would use.

Also, what does the ambiguity matter in this situation? This isn't a technical paper.

I'd wager few people look at the kilowatt hours on their electric bill. They look at the dollars. I know that my monthly electric bill is about $100. I would have no idea how many kilowatt hours without looking.

Hah! I've never looked at the amount of kwh I've used on my bill. All that matters is the amount of money I owe.

I work in the energy metering industry, but 12,000 homes over 3 seconds is still a more meaningful measurement of power for me than 72 kWh (or whatever the value they were trying to capture was). It really is a lot of power to send in 3 seconds.

I am European and I don't have a clue how much energy US home uses. Is it one in Alaska, Hawaii or maybe New Jersey?

Let alone: Is it a sprawling mansion or a one person apartment?

My go-to number is 1,234 J it's equivalent to a major league baseball player hitting a ball as hard as he can.

In other words, enough to power half a home for 20 hours. :/

The time used isn't completely arbitrary, the system launch time is ~3 seconds.

Also lovely how carefully they avoided the pedestrian 'one home for ten hours', or the overwrought '36,000 homes for one second'.

Goldilocks and Aristotle on board.

It's because the charge is delivered over 3 seconds, that they used 3 seconds.