> Gigawatts and gigawatt hours are just different magnitude units for the same quantities, like meters and millimeters.

I liken it more to distance vs speed. Power, like speed, is a rate, in particular the instantaneous rate at which energy is transferred.

Expressed using the same scheme we use for speed (distance per time : power per time), power is watt-hours per hour (or 0.0036 megajoules per hour).

I'm guessing we express power differently because James Watt codified the standard unit for power and the standard for electrical energy came from the time-integral of his definition of power.

This is what confuses many people, especially with solar and battery storage. "I've got 400 watts of solar feeding into my 400AH of batteries with a 3000w inverter" Most people will have no idea how much usable power they get and how much energy storage they need to meet their needs. (this example is mostly from an RV perspective)

In general, people are not used to formally separating power and energy as concepts, much less as units of measure.

This makes perfect sense because most of the measurements of things we own or consume are counted in units like area, volume, and energy, which are inherently more tangible than an instantaneous rate like "power". Speed is a bit of an exception to this, but I think that has to do with the ubiquity of speedometers in cars.

To the extent that people broadly understand power as distinct from energy they tend to do so in a mechanical, not an electrical context, and in less precise terms ("That's a powerful truck"). However, some exceptions to this are light bulbs and electric space heaters, where people generally understand power as being proportional to brightness and heat output.

In a way, it's an incredible testament to the engineering of electrical systems that people have been able to largely ignore the difference between power and energy, or just need to flip a circuit breaker if something exceeds a limit.

Amp hours is particularly annoying IMO because you can't convert it to energy without knowing the voltage.

The reason that battery capacity is measured in amp hours is that you can't know the exact voltage of the battery as it drops with lower levels of charge.

Newer large-capacity battery systems seem more likely to specify battery capacity in (k)Wh. I think this is because they come with sophisticated battery management systems that keep the battery within a narrow charge (20%-80%) range, so therefore they can be assured of the bounds of the voltage.

Also, Li-ion and LiFePO4 batteries have a much different discharge curve than earlier chemistries [1], with much more constant voltage up to 80% discharge.

1. https://www.powertechsystems.eu/wp-content/uploads/sites/6/2...

If voltage drops with lower levels of charge, then so does the current (assuming a load with a constant resistance). Isn't that just another reason to use Watt hours instead?

> "Ive got 400 watts of solar feeding into my 400AH of batteries with a 3000w inverter"

Assuming 12V, that's about 400Ah*12V=4800Wh of storage. 4800Wh/400W = 12 hours to charge those, which seems reasonable for an RV. The thing that seems a bit oversized is the 3000W inverter.

I meant the "giga" units relative to their "mega" counterparts. Same physical quantities but multiplied by 1000.

Hmm, in that case your comparison should be from MW to GW, not GW to GWh, right?

I referred to megawatts and megawatt hours in my first two sentences. "Gigawatts and gigawatt hours" was indicating counterparts to MW and MWh. Though apparently that grammatical construction was not as clear as I initially believed.

Ah, OK. Thanks for clarifying!

Everything about these storage projects is available as public records from the CPUC, down to the site plan drawings.

Thanks! I've been noticing these projects mostly from press releases that surface through Google News, and those don't always provide both numbers. It's good to know that there is a detailed source of open data.