In partnership with PLMA, this group is for practitioners from energy utilities, solution providers, and trade allies to share load management expertise and explore innovative approaches to program delivery, pricing constructs, and technology adoption.


It takes big energy to back up wind and solar

image credit: © Malpetr |
David Wojick, Ph.D.'s picture

Independent expert on the interface between science, technology and Federal policy. Projects include research, complex issue analysis and writing. Former faculty Carnegie Mellon, also Senior...

  • Member since 1998
  • 12 items added with 14,506 views
  • Mar 26, 2021

Power system design can be extremely complex but there is one simple number that is painfully obvious. At least it is painful to the advocates of wind and solar power, which may be why we never hear about it. It is a big, bad number.

To my knowledge this big number has no name, but it should. Let’s call it the “minimum backup requirement” for wind and solar, or MBR. The minimum backup requirement is how much generating capacity a system must have to reliably produce power when wind and solar don’t.

For most places the magnitude of MBR is very simple. It is all of the juice needed on the hottest or coldest low wind night. It is night so there is no solar. Sustained wind is less than eight miles per hour, so there is no wind power. It is very hot or cold so the need for power is very high.

In many places MBR will be close to the maximum power the system ever needs, because heat waves and cold spells are often low wind events. In heat waves it may be a bit hotter during the day but not that much. In cold spells it is often coldest at night.

Thus what is called “peak demand” is a good approximation for the maximum backup requirement. In other words, there has to be enough reliable generating capacity to provide all of the maximum power the system will ever need. For any public power system that is a very big number, as big as it gets in fact.

Actually it gets a bit bigger, because there also has to be margin of safety or what is called “reserve capacity”. This is to allow for something not working as it should. Fifteen percent is a typical reserve in American systems. This makes MBR something like 115% of peak demand.

We often read about wind and solar being cheaper than coal, gas and nuclear power, but that does not include the MBR for wind and solar. What is relatively cheap for wind and solar is the cost to produce a unit of electricity. This is often called LCOE or the “levelized cost of energy”. But adding the reliable backup required to give people the power they need makes wind and solar very expensive.

In short the true cost of wind and solar is LCOE + MBR. This is the big cost you never hear about. But if every state goes to wind and solar then each one will have to have MBR for roughly its entire peak demand. That is an enormous amount of generating capacity.

Of course the cost of MBR depends on the generating technology. Storage is out because the cost is astronomical. Gas fired generation might be best but it is fossil fueled, as is coal. If one insists on zero fossil fuel then nuclear is probably the only option. Operating nuclear plants as intermittent backup is stupid and expensive, but so is no fossil fuel generation.

What is clearly ruled out is 100% renewables, because there would frequently be no electricity at all. That is unless geothermal could be made to work on an enormous scale, which would take many decades to develop.

It is clear that the Biden Administration’s goal of zero fossil fueled electricity by 2035 (without nuclear) is economically impossible because of the minimum backup requirements for wind and solar. You can’t get there from here.

One wonders why we have never heard of this obvious huge cost with wind and solar. The utilities I have looked at avoid it with a trick.

Dominion Energy, which supplies most of Virginia’s juice, is a good example. The Virginia Legislature passed a law saying that Dominion’s power generation had to be zero fossil fueled by 2045. Dominion developed a Plan saying how they would do this. Tucked away in passing on page 119 they say they will expand their capacity for importing power purchased from other utilities. This increase happens to be to an amount equal to their peak demand.

The plan is to buy all the MBR juice from the neighbors! But if everyone is going wind and solar then no one will have juice to sell. In fact they will all be buying, which does not work. Note that the high pressure systems which cause low wind can be huge, covering a dozen or more states. For that matter, no one has that kind of excess generating capacity today.

To summarize, for every utility there will be times when there is zero wind and solar power combined with near peak demand. Meeting this huge need is the minimum backup requirement. The huge cost of meeting this requirement is part of the cost of wind and solar power. MBR makes wind and solar extremely expensive.

The simple question to ask the Biden Administration, the States and their power utilities is this: How will you provide power on hot or cold low wind nights?

Mark Silverstone's picture
Mark Silverstone on Mar 30, 2021

Storage is out because the cost is astronomical.

Not necessarily true in the next 10 years or so.  Especially if green hydrogen is an option. 

"Utility-scale battery storage costs decreased nearly 70% between 2015 and 2018"

It is still high, but coming down fast.

David Magill's picture
David Magill on Mar 30, 2021

"Storage is out because the cost is astronomical" "Not necessarily true."

I have never seen the quantity of storage required quantified. If we build out solar + wind sufficient on average GWh to provide for the annual demand +15% and have backup sufficient for annual demand (yes, that's for 365.25 *24/4 times the 4 hours usually quoted for storage, that should be an upper bound for how much storage is needed. Going below that boundary means we need sufficient additional solar and wind to be able to charge the storage perhaps in the middle of a long snowy and cloudy windless string of days in the winter (in Alaska? c'mon man). 

I'd be interested to know if anyone has a link to where this is thought through -- without the handwaving of "we'll import it from our neighbors."

Bob Meinetz's picture
Bob Meinetz on Apr 20, 2021

David, because a grid powered by "renewables + storage" exists nowhere in the world, quantifying what it might cost involves a lot of assumptions. Here's a sketch for what storing one day of electricity to power California might cost:

CA annual electricity consumption: 259,500,000 MWh
Daily consumption: 259,500,000 / 365 = 710,000 MWh
Current cost of installed Li-Ion grid battery storage (EIA): $625,000 / MWh
Cost for one day of battery storage capacity:
710,000 x $625,000 = $443.8 B

  • Enough storage to power California for one day of cloudy, windless weather would cost almost 1/2 trillion dollars, about twice California's total annual budget.
  • It would need to be completely replaced every 7-10 years.

Powering an electrical grid with batteries is indeed too expensive - by several orders of magnitude.

David Wojick, Ph.D.'s picture
Thank David for the Post!
Energy Central contributors share their experience and insights for the benefit of other Members (like you). Please show them your appreciation by leaving a comment, 'liking' this post, or following this Member.
More posts from this member

Get Published - Build a Following

The Energy Central Power Industry Network is based on one core idea - power industry professionals helping each other and advancing the industry by sharing and learning from each other.

If you have an experience or insight to share or have learned something from a conference or seminar, your peers and colleagues on Energy Central want to hear about it. It's also easy to share a link to an article you've liked or an industry resource that you think would be helpful.

                 Learn more about posting on Energy Central »