Renewable energy battery backup

-is it feasible?

I put this page together because there seems to be a need for a straightforward explanation of why the economics of this idea simply do not add up. Also, why it is unlikely that they could ever add up, not even if costs were reduced by a factor of ten or more.

The various renewables promoters are suggesting that the UK should aim for '100% renewable energy by 2050' on the strength of claims that battery storage technologies will overcome the intermittency of wind and solar energy.


  • Wind and solar outages can last several weeks
  • To be a 100% renewable Grid, batteries would have to fully cover these outages
  • A week's Grid backup with Tesla Powerwalls would cost around a trillion pounds
  • To fully cover the longer wind outages would cost upwards of four trillion pounds
  • Even if battery tech advances bring costs down to one tenth, that is still uneconomic
  • Other non fossil fuel technologies are orders of ten or a hundred cheaper, and more likely to succeed

Why renewable energy storage would be needed

Solar PV provides a burst of energy centered around mid-day. This may extend for as long as twelve hours in summer, but the maximum output is only achieved for a few hours. Cloud cover can reduce the output by a large factor, hence energy yields differ from day to day. In midwinter, output is typically one tenth of that in summer.

Hence we see here that firstly, backup must cover the period between sunrise and sunset every day. That is the minimum backup requirement. The worst case requirement is that of winter, when two or more months may suffer near-zero solar power.

Wind also suffers intermittency, although it follows a rather different pattern from solar. Greatest outputs tend to be in the winter and spring, with less in summer and autumn. At any time of year, a high pressure region settling over the UK can result in several days with little or no wind. In some instances this has extended to three weeks of calm.

The Grid has little inherent storage capacity. For the most part, generating capacity has to be adjusted in real time such that supply equals demand.

Renewables can usefully serve as part of Grid capacity, up to a point. Provided that the percentage of renewables is significantly less than that of other power sources in the mix, the other sources can quite simply be adjusted to make-up for the changes in renewables output. This arrangement is what we presently have, and it works well.

However, when the requirement becomes one of entirely replacing other energy sources with renewables, that option no longer exists. In that case, there HAS to be some way of storing the renewable energy for use when the wind doesn't blow. 


German electricity  production

UK electricity production

UK historical electricity production

How much power?

UK electrical requirements are for a base night-time demand of about 25GW*, rising to 35GW on a summer day, and up to 55GW on a cold winter day. Of that, about 8GW is available from nuclear, plus a couple of GW from hydro and international feeds.

Thus, if we are to eliminate fossil fuels that leaves an amount ranging from 15GW to 45GW to be supplied from renewables. Or, from batteries when there is no wind or sun.

* To give an idea of scale, a gigawatt is essentially a million one-bar electric fires, or ten million traditional light bulbs.

For how long?

The backup requirements for both wind and solar energy types require that for complete security of supply, other sources, or stored renewable energy, must be able to stand-in for several weeks at a continuous stretch.

Logistically, if we wanted to be genuinely 100% renewable, we'd need battery backup to cover about a month of cloudy skies and nil wind. However, such extended outages only occur once in a while, so to be more realistic let's specify a week's battery backup, and agree to use fossil fuels to cover longer outages. After all, that shouldn't happen too often.

So as a ballpark, a week's backup is what we'll settle for. The UK baseload, minus nuclear and hydro, we could take as 25GW on average. So, we need a battery with a capacity of 25 x 24 x 7 gigawatt hours. 4,200GWh.

How large would such a battery be, and how much would it cost?

Traditional lead acid

A car battery stores about one millionth of a gigawatt-hour,and costs maybe £50 wholesale. It is probably the cheapest option. Lead acid technology is however not ideal for this kind of duty owing to its problems with deep discharge and limited cycle life. Lithium would be a better option, although more costly.


Turning towards batteries actually intended for this kind of purpose, the lithium-based Tesla Powerwall stores 14 kWh, or 14 millionths of a gigawatt-hour, and costs $5,500 USD at one-off prices.

The individual cells typically used to build such packs are known as the 18650 style, and cost from £1 up in bulk quantities, although the quality brands will typically be £2 to £3 each. They each hold around 2.5Ah at 3.7 volts, which equates to around 10 watt-hours. Thus a Powerwall-equivalent pack would contain 1,400 such cells, at a cost of £2800 if the unit cost is £2. This fits in quite well with Tesla's retail price for the Powerwall, so let's just assume that we couldn't undercut Tesla's prices by all that much even if we built our own.

Flow batteries

Another possibility being widely touted by the various Green Tech websites is the Flow Battery. Using liquid electrolyte stored in external tanks, this does lend itself to larger deployments. Figures for actual cost of these units are quite hard to come by, but the lowest projected cost I've found for a unit in-development is $250 per kWh, or about $3000 for the equivalent to a Powerwall in storage capacity. That might come down with development of this relatively new technology. For the moment though, all known available flow batteries are more costly per unit storage than lithium.

OK, Tesla it is.  

So, let us assume that we could talk Elon Musk down to $4000 each for a bulk purchase of Powerwalls. How many would we need for our week's Grid backup?

14kWh is 14 millionths of a GWh, so to supply 4,200GWh for our week of no wind or sun, we would need 4200/14 million of them. That's 300 million Powerwalls for a week's Grid backup.

Thus, every person alive in the UK has to buy five Powerwalls and we're done. No problem. Of course that includes OAPs, the unemployed, and children on pocket money.

In national budget terms, the cost would be 300 million times $4000, or $1.26 trillion. For the sake of argument, call it a round trillion pounds Sterling.

Bear in mind that's for a week's backup. If we wanted to go the 'really, really green' route and ensure that we never (well, hardly ever) had to resort to burning stuff dug out of the ground, the cost would be more like four trillion.

How much is this amount of money worth, in relative terms?

In other words, what could we do with an equal amount of money, but unfettered in our choice by EU renewables mandates? 

Renew our entire electrical generating capacity

Conventional gasfired generating capacity costs somewhat less than a billion per gigawatt. So, we could replace the UK's entire 60GW-or-so baseload machinery with new, cleaner, fossil fuel tech. In fact, doing this would hardly dent the 'renewables battery account' bank balance. 

Carry out a fullscale fusion test

Even after updating our fossil fuel capacity with cleaner, greener equipment, we would still have plenty left over to build our own ITER, Polywell or Stellarator  and finally find out if fusion works.

Develop thorium technology

Even after the fusion test there would be plenty money left over for a thorium reactor. Ballpark cost, one two-hundredth of the week's worth of renewables backup. In fact it might be better to try this first, since if it works, then cancel the new fossil fuel stations anyway and concentrate on thorium generating capacity. Chances are it would work too, because it's already been done. Whereas, Grid-scale battery backup has never been done and probably never will be. 

Something else entirely

Instead we might decide to go explore the Moon, with our own homegrown Apollo project. Shall we say 17 launches of a 300ft, 2,500 tonne rocket, including six lunar landings? Cost, adjusted for inflation since the 1970's? About a fifth of the single week of renewables backup.

No, I am NOT joking. we are talking five times the cost of putting astronauts on the Moon for the basic model of the renewables battery backup.

Is it wise to set a target of 100% renewables, plus backup?

I think these figures should be enough to convince even the most ardent renewable energy supporter that it is not.

The battery backup scheme is UNSUSTAINABLE.

Even if the cost could be reduced by a factor of a hundred -which would be equivalent to buying 18650's at 2p each- it would still be very questionable economics. Thus, unless some really, really radical battery technology is developed, it will never be sustainable. Then again, why take a huge gamble on a technology that may prove impossible to deploy, when we know that thorium is possible? That, to me, sounds like folly. Folly based on an ideological obsession with wind and solar energy, rather than a rational decision based on facts. The rational person takes the option with a realistic chance of succeeding.

Why, then, is it being promoted?

The battery backup idea is a gambit being played by the renewables manufacturers. It's reasonable to assume they know the idea is impractical. Their concerns though, are to sell generating equipment, and this is one way of convincing the public and the decisionmakers that their products can be made to provide continuous power.

What will the vendors say IF, after the contracts have been signed, no battery backup materialises? Might it be, 'Not our problem mate. We just install the turbines.'

I think it might.

They would be correct on that one. It's ours. Our problem. We the people, who have to foot the bill.