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The Problem with California's Energy Storage Mandate

California Storage Mandate

A transition to a low carbon energy system will require a large increase in energy storage, and policies to promote storage technology are necessary. This applies more obviously to renewable energy – we are all aware that the sun goes down – than nuclear energy. Nuclear power plants however run must economically at full tilt all the time, therefore there is a preference to be able to store that electricity when people don’t need it for when people do not need it. Therefore recent moves by the state of California to promote energy storage should be welcomed. However a closer look at the details raises some serious questions.

California’s policy is as follows. By 2024 California’s three investor owned utilies must invest in 1.325 billion watts (1.325 GW) of energy storage capacity. The problems with this mandate are multiple. Let’s begin with the most obvious and fundamental. Energy is not measured in watts. By convention we measure it in joules, or the more mediaeval unit called the British Thermal Unit. A watt is instead a unit of the rate of energy use. If I was to use an anology then I would say this mandate is rather like a fitness instructor telling me to get fit by running at 15 kilometres per hour each day. To see this consider the most commonly used form of energy storage: pumped storage.

Pumped storage is a rather straightforward, if expensive, way to store electricity. You use electricity to pump water up a hill, this is then stored in a reservoir, and when you want electricity you let the water flow back down the hill and electricity is generated by that water turning turbines. Typically this stores electricity with an efficiency of around 75%.

However meeting this 1.325 GW energy storage mandate entirely with pumped storage would give very uncertain results. Consider the pumped storage stations in the United Kingdom. Not blessed with easily pronounced names their capacities and storage capabilities are: Ffestiniog (capacity: 0.36 GW; storage: 1.3 GWh), Cruachan (capacity: 0.4 GW; storage: 10 GWh), Foyers (capacity: 0.3 GW; storage 6.3: GWh), Dinorwig (capacity: 1.8 GW; storage: 9.1 GWh). These numbers make the problem clear, there is a very unclear relationship between capacity, which is what California is mandating, and actual energy the station can store, which is what the mandate is supposed to deliver. Cruachan can store slightly more energy than Dinorwig, however Dinorwig can supply energy five times faster than Cruachan. In other words if California was to meet its mandated energy storage capacity with systems similar to Dinorwig it would actually be able to store five times less energy than if it used systems similar to Cruachan.  Quite clearly a quantiative target that can result in energy storage levels that vary by a factor of five has some serious problems.

The mandate also veers into near meaninglessness when some other forms of energy storage are concerned. Power to gas is an energy storage system with some benefits over existing energy storage methods, and is currently being tested in Germany and elsewhere. In pumped storage you convert electrical energy into potential energy, then into kinetic energy and then back into electrical energy. In power to gas you convert electrical energy into chemical energy, in the form of methane. This process is about 60% efficient, but burning it for heating or electricity in power plants will result in further losses. The fact that you can do whatever you want with this methane raises an obvious problem. How would you decide how this contributes towards the energy storage mandate? There appears to be no sensible answer, and I can only conclude that for this energy storage technique the mandate literally makes no sense.

Energy storage in most cases – though not for power to gas – is rather like a battery. As everyone knows some batteries last a lot longer than others, but can still power the same devices (until they run out of energy of course). Few people would buy batteries in a shop without thinking about how long those batteries would last. The same basic considerations should also apply to energy storage mandates, but apparently they do not.

Robert Wilson's picture

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Josh Nilsen's picture
Josh Nilsen on Oct 22, 2013 4:58 pm GMT

This is a significant point.  You are 100% correct about the ambiguity that exists in this mandate.  In reality it is a pure gift to energy storage providers, a gift that is usually reserved for oil and gas lobbying.

Imagine that, renewables getting unfair advantages at the expense of fossil fuels.  I thought it would take longer than this.

@MWh – Lithium ion batteries can barely store any time right now.  The 36MW facility in Texas to store wind electricity can only run for 15 minutes.  But on the other side of the coin, you can charge it up at night when wind electricity is close to free or even at a negative value (too much for the transmission lines to hold vs. capacity needed so they have to turn the farm off).

Really the economics for storage could be wildly different across the country.  We need significantly more variables to be open to the public than are currently available.

Mike V's picture
Mike V on Oct 22, 2013 7:41 pm GMT

FYI – California already has some significant pump hydro facilities.

  • Castaic Pumped Storage Plant         1200 MW
  • Helms Pumped Storage Plant           1200 MW 
  • Edward C Hyatt  Hydro Plant              780 MW
  • William R. Gianelli pumped Storage    424 MW
  • John S. Eastwood Pumped Storage    200 MW


Clayton Handleman's picture
Clayton Handleman on Oct 22, 2013 10:37 pm GMT

Pretty surprising that they missed this one. You are right on and this is just another case of the challenges a lack of energy literacy causes in a technological society that is trying to keep its democracy strong.  Can’t do it if the voters don’t know the language of energy.


Regarding the quoted portion of your post, I have added a bit of clarification to it in terms of the language of energy in the electric utility world:

“Let’s begin with the most obvious and fundamental. Energy is not measured in watts. By convention we measure it in joules, or the more mediaeval unit called the British Thermal Unit. A watt is instead a unit of the rate of energy use”

When talking utility power and energy we use the following expressions of power and energy:


kilowatts or kW = roughly the consumption of a hair dryer (thousands of watts)

megawatts or MW = nameplate rating of a utility scale wind turbine us typically 1MW to 5MW these days (millions of watts).

gigawatts or GW = roughly the scale of a nuclear power plant or largish coal power plant – also about the consumption of a small city  (billions of watts)

terawatts or TW = the US average electrical power consumption is about 1/2 terawatt (trillion watts)

It is rare for utility and electric power engineers to talk about electricity in terms of the SI energy units of Joules.  Typically they describe the energy in terms of the time it takes to deliver that energy at a constant power level.  The basic and most common unit that is used is the kilowatthour or kWh.  This is the amount of energy consumed by a 1kW device operating for an hour.  A hair dryer has a power draw of about 1.5kW so it uses 1.5kwh of energy in an hour.

MWh = megaWatt hour = 1.5 MWh is required to run 1000 hair dryers for an hour or 1 hair dryer for 1000 hours.

GWh = GigaWatt hour

TWh = TeraWatt hour


So as you mention, to put the storage requirements in terms of power or energy only is problematic.  Here are a few examples of the dramatic differences in power and energy for a variety of systems:

A bolt of lightning = approximately 1GigaWatt second or roughly 277 MWh of energy delivered at a rate of about 1 – 5 TerraWatts (remember the lightning bolt lasts for far less than 1 second).  The energy is comparable to that delivered by a nuclear power plant for roughly 1 second.

Space Shuttle during launch phase – Solid Rocket Boosters along with the shuttle main engines: ~14GW for about 120 seconds.  This is about .46 GWh which is roughly the energy output of a nuclear power plant running for about 1/2 hour.

A Tesla battery holds about 85kWh of charge with peak power of 310 kW or .310MW.  That is at peak acceleration.  On the highway, figure about 240 miles per charge or roughly 21 kW to cruise at around 60 mph.  This is roughly .0021% of the output of a nuclear power plant – the energy required to charge the Tesla is 85kwhr or about the energy delivered by a nuclear power plant running for about 1/3rd of a second.





Nathan Wilson's picture
Nathan Wilson on Oct 23, 2013 2:53 am GMT

In defense of the California utility regulators, perhaps they decided to leave the details to the utilities.  They have about 62 GWatt of total summer generation capacity (2005 data that was handy), so I would think that this 1.3 GWatts of storage is just one of many storage building initiatives (assuming they eventually reach 50%  or more variable renewbles on the grid).

They already have 5.5 GWatts of wind, and something like 2 GWatts of solar, so surely their engineers can think of good ways to use storage to improve grid operation.

Even if they decide that a “degenerate solution” like buying an average of 30 minutes of storage capacity for this 1.3 GWatt traunch (for frequency regulation and ramp-rate control) is all that they can cost-justify at the moment,  that storage will still be beneficial when at some point in the future, there are more renewables on the grid, and the need for more storage (perhaps with 4-8 hour capacity) is realized.  (So we should not be discouraged, even if today the perferred solution is big steam drums for their fossil fuel plants; eventually a good option will be solar-salt thermal energy storage connected to a Gen IV nuclear plant.)

Also, as to the best metric to describe a “power-to-gas” (or more generally “dispatchable electric fuel synthesis”), I think stating the Wattage of the system is meaningful even if the fuel is used for non-electical applications.  Fuel synthesis provides a beneficial means to utilize power that would otherwise be curtailed, so power is certainly an appropriate metric.

Clifford Goudey's picture
Clifford Goudey on Oct 23, 2013 2:07 pm GMT

The article’s title was appealing and I expected a meaningful critique of the mandate, not a trite argument on semantics.  Maybe the GW vs. GWh disparity was a typo, much like those found in the article’s first paragraph.  More likely this was exactly what the California PUC intended, as power capacity is as relevant in determining the value of storage as is energy storage itself.  Indeed, under the proposed framework the ESPs and CCAs are required to report their storage procurements in both MW and MWh.  To suggest that a PUC does not know the difference between the two metrics is a stretch.


Robert Wilson's picture
Robert Wilson on Oct 23, 2013 4:39 pm GMT


Wattage for power to gas is still problematic, assuming you mean the wattage of the input to the plants. Power 2 gas can just keep producing gas, there is no upper limit. The same is not true for battery style storage, which would include pumped storage. So the wattage does not provide an apples to apples comparison. 



Thomas Garven's picture
Thomas Garven on Oct 23, 2013 4:40 pm GMT

Interesting discussion. 

I worked in the public utility sector in California for about 25 years.  It was very rare to hear anyone in the utility sector including engineers use the term kWh.  The time component was usually just understood to exist in the power generation business.  

Just like in my above sentence, “public utility sector in …”.   I never really stated the experience was in the electricity sector; but many people might have assumed it was from just reading the sentence and the general topic nature of this article.  I believe we short form many of the expressions we use without really realizing it.  

Have a great day everyone.  

p.s. yes it was in the electric utility business, LOL. 

Robert Bernal's picture
Robert Bernal on Oct 27, 2013 5:19 am GMT

Nuclear can be designed to be load following (and) to encourage the greater variability coming with increased renewables, which could substantially reduce the need for expensive storage only installations.

Clifford Goudey's picture
Clifford Goudey on Oct 27, 2013 12:45 pm GMT

Robert, thank you, that is a most interesting article.  So it appears there are no technical reasons to prevent NPPs from operating as load followers and indeed in countries like France that’s the way it’s done.  Not so in the US and I wonder if the regulatory prohibition suggests they are incapable of doing so or if it is simply a way to allow them to operate at maximum profitability. 

My understanding it that there are legitimate technical reasons why coal and combined-cycle plants can not be operated in a load-following mode, though maybe that too is simply based on an economic preferance of the owners.

Either way, this little-talked-about capability of NPPs has implications on the role of storage in managing the grid.

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