The Treehouse Blog

I’m trying to catch up on some reading, and am working through a number of articles concerning energy sources, the power grid, and so forth.  One almost constant source of annoyance I find in these articles is that the difference between power and energy are either ignored, misused, or left hopelessly ambiguous.  The writers are not totally to blame, since our vocabulary for discussing these concepts is weak and it takes a lot of effort to clearly convey the correct meaning.  Consider the following example:

The U.S. electric system has 2.5 gigawatts of pumped hydro storage capacity.

“Capacity” when used to discuss an energy storage apparatus can have two meanings.  It could mean the power capacity of the system, which is the amount of energy that can be released from storage and converted to electricity instantaneously.  For pumped hydro storage, this would be determined to a large degree by the size of the turbines, generators, and penstocks.  This quantity represents, for instance, the number of lights a plant could handle without dimming – it says nothing about how long it could keep them lit.  A common unit of power capacity is gigawatts.

The other type of “capacity” is the energy storage capacity of the system.  For pumped hydro storage, this is the value related to the size of the water reservoir.  This is the quantity that represents how long a plant could keep a given number of lights lit, for instance.  A common unit of energy capacity is gigawatt-hours.

Assuming the author chose the units correctly, this quote tells us that the power capacity of US pumped hydro is 2.5 GW.  Unfortunately, something isn’t right, because the Energy Information Administration says the number is actually 20 GW.

Consider another example:

The Norton project could store 2.7 gigawatts of power in an abandoned limestone mine.

I’m afraid this one is beyond redemption.  You don’t store power, you store energy.  But the units given are for power, and from further research in other articles, the power capacity of the plant is expected to be 2.7 gigawatts.  I was not able to find a source that estimated the plant’s energy storage potential.

Any time you read the words “power,” “energy,” or “gigawatt” in an article, read it very critically.  The commonly cited statistics for power plants are always their power capacity, and even for energy storage plants, storage capacity is seldom stated.  One likely reason is that power capacity is relatively easy to determine – it is likely written on a nameplate on the generator somewhere.  Calculating the energy capacity of a storage facility would be more challenging, and might not be a number required by regulators – and hence one that is seldom available.

Both quotes are from this wired.com article, but many articles discussing energy storage have the same problem.