The Grid Will Not Be Disrupted

Why Tesla’s Powerwall Won’t Catalyze a Solar Revolution

The announcement two weeks ago of Tesla Motor’s cheap new lithium-ion storage batteries set the renewable energy world on its ear. Breathless commentators pronounced them a revolutionary advance heralding cheap, ubiquitous electricity storage that would make solar power a 24/7-power source for the masses. Elon Musk, Tesla’s wunderkind CEO, fed these hopes at the glitzy product launch for the 10 kilowatt-hour (KWh) Powerwall home storage battery.

“You could actually go, if you want, completely off the grid,” he told them. “You can take your solar panels, charge the battery packs, and that’s all you use.”

Powerwalls would let developing countries “leapfrog” straight to a solar-plus-storage electricity system, he explained, and the 100-kWh utility-scale Powerpack version would have a world-historical effect. Just 160 million Powerpacks would suffice to “transition” the United States to “sustainable energy,” he said, adding, “With 900 million [Powerpacks] you can…make all the electricity generation in the world renewable — and primarily solar.”

But does all the messianic talk of battery-powered “disruption” and solar triumphalism stack up? Hardly. For all their ballyhooed price reductions, Tesla batteries are still way too feeble and expensive to come even within hyping distance of neither a reliable power supply, nor an off-grid revolution.

On cost, the average residential retail electricity prices in the US are $0.12 per-kWh, while electricity from Tesla’s Powerwall on paired rooftop solar would cost 30 c/kWh or more. Given that 80 percent of pre-orders for Tesla’s batteries are for the utility-scale Powerpack, not the residential Powerwall, battery storage will likely benefit big baseload power plants (the grid) more than solar homeowners. And no matter the staggering cost, battery storage cannot solve the problems of integrating unreliable wind and solar power into the electricity system. In fact, Tesla’s batteries spotlight just how deep and intractable those problems remain.


In the wake of Musk’s announcement, Many in the press and blogosphere hailed Musk for discovering the Holy Grail of renewable energy storage — and for slaying two of the most loathed pariahs in green energy doctrine: nuclear power and the centralized grid. Citing antinuclear activist Arnie Gundersen, Forbes writer Jeff McMahon asked readers, “Did Tesla Just Kill Nuclear Power?” Ecologist editor Oliver Tickell promptly affirmed that, indeed, “Tesla’s Battery Just Killed Nuclear Power” — and McMahon later explained “Why Tesla Batteries are Cheap Enough to Prevent New Power Plants.” Gizmodo purred that “another attractive part for consumers is that this kind of battery will give homeowners complete energy independence, allowing them to sever connections to utility companies.” An article on the investment website Seeking Alpha warned, “Energy storage will likely make the utilities’ grid infrastructures unnecessary in the long run” and decreed that “utilities either work with distributed solar companies or face complete collapse.”

After the first flush of celebration, a closer look at the specs has started undermining the claims of Tesla and its admirers. Contrary to Musk, you would be ill advised to go off the grid with solar panels and batteries. The 10-kWh Powerwall stores enough electricity to supply an average American home, which uses 30 kWh of electricity per day, for all of 8 hours; a day of overcast weather would leave an off-grid solar-plus-Powerwall system without any power at all. And the 10 kWh system can only cycle — charge up with electricity and then discharge — about once per week; it’s designed as a back-up for grid outages, not to store daily household solar generation.

For daily cycling you need the Powerwall’s smaller 7-kWh version. Buy four of those and you would have enough storage to last 24 hours — but a single cloudy day would leave the system as dead as a doorknob. As would a modest strain on the battery: the Powerwall can provide 2 kilowatts (kW) of steady power or 3.3 kW of peak power, which means that if you use it to run a clothes dryer, you’ll have to turn off every other electricity load in the house.

You’ll also pay a lot. Tesla sells the 10-kWh Powerwall for $3,500 wholesale. Inverters and installation roughly doubles that price: the solar installer SolarCity, a Tesla sister company (Musk chairs its board), is selling it for $7,140 retail. A similar outlay will buy you a much more functional gas backup generator that runs steadily regardless of the weather. The workhorse 7-kWh version of the Powerwall sells wholesale for $3,000, perhaps $5,000 installed. Multiply that price by a large number if you want an off-grid storage system that can sustain you through, say, a winter week of cold and cloud.

Even if you don’t go off-grid, electricity stored in a Powerwall is a financial drain. The costs depend on the battery’s lifespan — the number of times it can cycle — and “depth of discharge.” (Discharging all the electricity in a battery reduces the cycles it can produce.) The company has been tight-lipped about these parameters, but Musk has estimated calendar lifespans of about 15 years (they are under warranty for 10 years) and 5,000 cycles for the 7-kWh Powerwall. That works out to a storage cost of perhaps 14 cents-per-kWh (assuming a maximum 100 percent depth-of-discharge, which is doubtful, and ignoring the electricity the battery wastes as it cycles).

Those prices can’t compete against the grid that Tesla is supposed to bankrupt. The average retail price of residential grid electricity in the United States is 12.3 cents-per-kWh. Tesla battery power costs more than that just for the storage. Add in the price of generating rooftop solar power to charge the Powerwall, and the total price of Tesla stored electricity will easily add up to 30 cents-per-kWh, and often more. Storing daytime solar power at 30 cents to replace nighttime grid electricity costing 12 cents is a losing proposition.

It’s also a dud even for homeowners with existing solar systems because of net metering and feed-in tariff schemes that pay them to sell surplus power back to the grid. In California, rooftop PV exported to the grid can earn up to 32 cents-per-kWh, which would put the opportunity cost of storage at close to 50 cents-per-kWh. Price hurdles like these are why SolarCity is not even offering the 7-kWh Powerwall except in Hawaii, where residential electricity costs 31 cents-per-kWh because the grid runs on expensive imported oil.

That calculus may change because of government subsidies, quotas, and rate regulations. California’s Self-Generation Incentive Program rebates up to 60 percent of the capital cost of electricity storage systems; Tesla stands to harvest $65 million in California subsidies by installing batteries for commercial customers like Walmart. A new state requirement that utilities install 1.325 gigawatts of storage (gigawatt-hours not specified) will drive further sales of Tesla’s large Powerpack assemblages there. California may also shift its retail electricity rates to a “time-of-use” (TOU) structure that charges residences much more during late afternoon-evening hours of peak demand than at other times, a rate structure that encourages battery storage.

In some of these TOU schemes the difference in rates between solar noon and 6 p.m. can be as high as 17 cents-per-kWh, roughly break-even for Powerwall storage. The spread between nighttime “super off-peak” rates of 11 cents and peak rates could be even bigger, up to 24 cents. That raises the prospect of non-solar homes buying batteries so they can store grid power at night to subsist on — or sell back to the grid — in the afternoon peak. (Elon Musk’s brother Kimbal suggested just such a use for the Powerwall.) More than the romance of the solar homesteader living in blissful isolation from the grid, this is the idea that entrances battery visionaries: a system of countless household proprietors trading electricity, storing it low and selling it high, savvily arbitraging gluts and shortfalls, smoothing out the gaps and peaks in the grid while driving it towards perfect efficiency.

But there are pitfalls in that model. A successful business case for battery storage is predicated on policies that entrench high electricity prices and wide price spreads. California’s electricity rates are 40 percent higher than the American average and going up, thanks to the state’s aversion to anything that looks like a reliable power plant that might ease supply constraints. To thrive, stored solar electricity will rely on a regulated environment of inadequate infrastructure and energy austerity.

And there’s a more fundamental contradiction in battery populism: if solar homeowners can use batteries to store low and sell high, so can power plants. Indeed, they can do it cheaper. Priced at $250 per kilowatt-hour, 40 percent below the cost of a home Powerwall, Tesla’s utility-scale Powerpack batteries already make the cost of storage much lower for a power plant than for a solar homeowner. (Tom Randall of Bloomberg News calculated that about 80 percent of Tesla’s pre-orders are for the Powerpack, not the household Powerwall.)

In the competition to store low and sell high, baseload power plants, nuclear plants included, would dominate. While a solar producer, whether a rooftop PV rig or a utility-scale farm, could arbitrage the rate spread between solar noon and 6 p.m. peak — provided the sun is out! — a nuclear plant could arbitrage the much wider spread between storing electricity at dirt-cheap nighttime rates and selling at peak rates, whether rain or shine. (No, solar homeowners won’t escape grid transmission costs unless they go completely off-grid. That’s next to impossible, as we’ve seen, and it would also prevent them from arbitraging grid prices.) If storage really catches on in an unbiased electricity market, there will be no huge rate differentials to fund mom-and-pop battery costs, no arbitrage opportunities that have not already been picked clean by the centralized grid.

The grid’s big power plants get that edge from humdrum advantages — economies of scale and expertise; pooled resources; the ability to control generation — that distributed and weather-dependent generators cannot replicate. Batteries won’t bridge that gap. Like every aspect of electricity supply, storage works best with generators that are big, reliable, networked, and controllable.


If the microeconomics of distributed solar-plus-storage look bleak, what about the macro-scale potential of battery storage to usher in the renewable millennium? Can batteries transform intermittent wind and solar power into reliable energy sources for a comprehensive decarbonization of the grid? Will billions of Powerpacks transition the world to an all-renewable energy supply? The answer, unfortunately, is no. The broad strokes of Musk’s battery vision are even less convincing than the details.

Battery storage can certainly play a useful role in the grid. It can, for example, help smooth out moment-to-moment power fluctuations as gusts and lulls ripple across wind farms. Because batteries react in a split second, they can displace much larger amounts of slower-ramping fossil-fired “spinning reserves” — usually gas plants — that normally perform this balancing function, and thus help stabilize the grid against the short-term jolts that are increasingly caused by intermittent renewables. But momentary fluctuations are just the tip of the iceberg of unreliability that batteries would have to bridge in firming up a grid composed mainly of intermittent solar and wind power. Surges and slumps of intermittent power persist for months and across hemispheres, and even a fantastic overbuild of batteries would not be enough to accommodate them.

Consider the 160 million Powerpacks that Musk thinks would transition the US grid to solar and wind, a total of 16 terawatt-hours (trillion watt-hours) of storage. That’s several quantum leaps in storage capacity — but still only a drop in the nation’s gargantuan electricity bucket. America used about 4,100 terawatt-hours (TWh) of electricity last year, so those Powerpacks would be able to backstop a hypothetical all-intermittents American electricity supply for all of 34 hours. (That’s assuming average weather; during a heat wave or cold snap electricity demand would be much higher and the batteries might drain much faster.) The price of the Powerpacks by themselves, installation not included, would be $4 trillion. Want to extend that to two days’ worth of battery power? Throw another $1.6 trillion on the fire. And hope those two days will cover every weather scenario that depresses intermittent generation and hikes demand; otherwise there will be blackouts across the continent.

To see how forlorn a hope that is, we need only look to the voluminous data on Germany’s renewable energy system. Germany’s intermittent power slumps are epic: total output for its wind and solar sectors combined can plummet to less than 5 percent of rated nameplate capacity for an entire week, and even lower for two- and three-day stretches. So let’s look at last year’s stats to see just how much battery storage Germany would have needed to back up its intermittent renewables sector during slumps.

Rather than ask batteries to cope with every supply-and-demand scenario, let’s hold them to the lower standard of simply making up the gap between average wind and solar generation and the reduced generation during slumps. In 2014, Germany’s wind and solar sectors produced a total of 90.9 TWh of electricity, for a daily average of 249 gigawatt-hours (GWh) and a weekly average of 1,748 GWh. According to data from Germany’s Fraunhofer Institute, the lowest daily production of wind and solar power came on January 21, 2014, when their combined generation was 22 gigawatt-hours. To top up that figure to the daily average would have required 227 GWh of Powerpack storage, costing $56 billion. If we were also to ask a battery system to store the largest daily surplus of intermittent generation above the average (on March 16, when wind and solar produced 580 GWh), that would require 331 GWh costing $83 billion.

But wind and solar droughts last much longer than a day. According to Fraunhofer data, the deepest weekly slump occurred in Week 47 (November 17-23) when intermittents together produced 770 GWh. Compensating for that deficit below average production would have required 978 GWh of Powerpack storage costing $244 billion. But then after Week 48’s slight deficit, during which no net charging would have been possible, the batteries would have been completely empty for Week 49’s renewed drought, when a deficit almost as deep, 958 GWh, opened up. The batteries would not have made it through Week 47 anyway, because Week 46, with its deficit of 798 GWh, would have largely drained them before Week 47 even began. If it had had to rely on batteries to back up its wind and solar sectors, Germany would have endured a month of rolling blackouts.

These figures are for Germany’s current wind and solar sectors, which contribute just 15 percent of the country’s electricity production. The costs would rise dramatically for higher penetrations of intermittent power. At a modest penetration of 30 percent, one week of Powerpack storage — still woefully inadequate — would cost about $500 billion dollars. That’s just the (uninstalled) cost of the batteries, which will generate not one electron of low-carbon energy; the wind and solar generators themselves would cost hundreds of billions more. And that entire battery infrastructure would have to be replaced every 15 years or so. To put these expenses in perspective, $500 billion spent on AP1000 reactors at a capital cost of $6,000 per-kilowatt would build 83 gigawatts of nuclear power, enough to decarbonize Germany’s entire electricity supply for 60 years and more.


The numbers show that Musk’s vision of an electricity system of intermittent renewable power backed up by batteries is a pipe dream. Even colossal investments in battery storage will not reduce the system’s dependence on a full-sized “grid battery” of dispatchable power plants whose costs — and ongoing carbon emissions — should be added to the tab of the wind and solar generators that rely on them.

There will be no leapfrogging to distributed solar; if developing countries want to escape perennial energy austerity and insecurity, they will have to deploy grid power plants as fast as they do solar panels and Powerwalls. Batteries don’t make intermittent power cheap and feasible, they make it immensely costly and only marginally more useful. The falling price of wind and solar generators has distracted us from the external costs of trying to shape them into an energy source we can count on. Musk’s coup inadvertently reveals just how steep those costs remain.

The real disappointment in this episode is how advances in energy storage are being shunted down a blind alley by green ideological obsessions. If it comes a long way down in price, battery storage on a significant scale may someday play an appreciable role in meeting peak demand without fossil-fired generation. To accomplish that, a rational energy storage scheme will emphasize not solar systems that store during periods of high daytime demand to service somewhat higher evening demand, but low-carbon baseload plants that reliably store when nighttime demand is very low. That scheme would use storage to augment the potential of nuclear plants, not as a rod to beat them with. It would understand the enduring value of the power grid, that triumph of collective provisioning, rather than trying to tear it down. It’s great that Elon Musk is making better batteries. Let’s think harder about how to use them.

Photo Credit: Kevork Djansezian / Getty Images