Key Points You Need to Know When Talking About Energy: Power vs. Energy

I'm going to start this series by going over some very basic fundamentals of energy production, distribution, and consumption.  The goal here is to highlight those facts which are most important in order to be an intelligent consumer of energy news and an intelligent participant in energy debates.

Power vs. Energy

I don't think I should waste time by offering yet another explanation of electrical terms such as current, voltage and resistance, since these explanations are available in many forms across the internet.  If you need a refresher on what those terms mean, I recommend searching on the terms "electricity" and "water analogy".  The water analogy is not perfect, but it's a standard and it's good way to keep these things straight in your head.  Here is one such explanation I found among many: 

Understanding The Basics Of Electricity By Thinking Of It As Water

What I would like to particularly point out, however, is the distinction between Power and Energy.  To quote the above site:

POWER is like the volume of water that is flowing from the hose, given a specific pressure and diameter. Electric power is measured in watts (W). And larger systems are measured in kilowatts (1 KW = 1000 watts) or megawatts (1 MW = 1,000,000 watts).

ENERGY is like measuring the volume of water that has flowed through the hose over a period of time, like filling a 5 gallon bucket in a minute. Electric energy is often confused with electric power but they are two different things – power measures capacity and energy measures delivery. Electric energy is measured in watt hours (wh) but most people are more familiar with the measurement on their electric bills, kilowatt hours (1 kWh = 1,000 watt hours). Electric utilities work at a larger scale and will commonly use megawatt hours (1 MWh = 1,000 kWh).

Because I think this distinction is important, I'll add a supplementary example to illustrate the difference between power and energy.  Consider: how would a Lamborghini perform as a way to haul a heavy trailer across country, say for a move?  In terms of raw horsepower, I'm sure the sports car has the capability of moving even heavy loads (if you could figure out a way to attach the trailer to the car in a way which the forces involved don't rip the car apart).  But, for how long could the sports car keep this up?  Because the sports car is designed in such a way as to maximize power output for relatively brief periods of acceleration, it does not produce this power efficiently--it guzzles gas for maximum acceleration output.  Further, in order to minimize weight, these sports cars all have tiny gas tanks.  The amount of energy that these types of car produce on any given full tank of gas, is therefore pretty small.  In order to haul a trailer across country, you'd have to be constantly stopping at gas stations along the way to fill up. There are some stretches of road in the States in which it is doubtful that a sports car could actually make it between gas stations.

Therefore, high power does not equate to high energy.

Why this matters

Now, there is a specific reason why this distinction is critical to keep in mind when reading news reports or arguments on energy, specifically regarding renewable energy sources.  It is a constant bad habit of people reporting on energy stories or talking about energy problems, that they frequently report on the *power* component of a power station and not the *energy capacity* component.  The specific thing to watch out for is: does a news story report electrical generation in megawatts (MW), or in megawatt-hours (MWh)?  Far too many people use the former when they should be using the latter.

For example, I have seen articles claiming that such-and-such percentage of electricity production in a certain location comes from renewables.  Diving into the data myself, I have found that these numbers are frequently produced by adding up the "nameplate" capacity of all the powerplants of a particular type and comparing them.  These values are reported in a number of easily available sites; for example, here's a list of powerplants in Texas which is available on Wikipedia: List of Power Stations in Texas

The problem with this is that the nameplate capacity for a powerplant is its *maximum power output* at any given time.  It does not represent the energy per hour that the plant should be expected to produce.  For more traditional power plants, this wasn't such a critical distinction, as most of these plants operate pretty continuously, and so you can compare energy output of these plants mostly based off of their maximum rated continuous capacity.  That is, a 2000MW coal plant will probably produce about as much energy as a 2000MW oil plant (though even here some major caveats will apply).

This is not at all the case, however, with wind and solar plants, which are only rarely able to operate at nameplate capacity; even the *average* output can vary pretty severely, but something like 1/4 or 1/3 of maximum output is often about right to get to real energy produced.  Many analyses which look at relative amounts of energy produced by renewable vs. non-renewable sources therefore drastically overstate the amount of actual energy being produced by renewables because of the confusion between power and energy capacity.

How this applies to energy storage

This same confusion also causes dramatic overestimates of the sufficiency of current utility-scale battery backup.

For example, the current largest utility-scale battery power plant is the Moss Landing Battery Storage project in California.  This power plant has a nameplate capacity of 400MW.  If you look at the list of wind power plants in Texas from the above Wikipedia link, you'll see that the average power capacity of these plants is about 280MW.  Does this mean that the Moss Landing plant could replace about 1.5 wind power plants?  No.  The *energy* capacity of Mass Landing is only 1600MWh.  This means that *if* the plant starts at full charge, it is only able to produce its 400MW to the grid for a period of 4 hours before it is completely drained of energy.

This is, in fact very much like the Lamborghini example above.  Technically, the power plant does have the power to replace a wind plant for a while, but planning on using it to replace wind power on a regular basis is highly optimistic given its total capacity.

Conclusion

The bottom line here is, if you see a news story or argument about energy production that purports to show percentage of production or consumption, be sure that you double-check the units.  If the argument is based on megawatts instead of megawatt hours (as many are), chances are that the author is fundamentally misunderstanding the problem at hand and is looking at the wrong values.