Since Tesla launched its “Powerwall” system in 2015, home battery storage for solar panels has risen from an interesting hypothetical to one of the most talked about energy products out there, driving interest from tens of thousands of potential buyers and Vermont’s largest utility.

The flurry of interest surrounding home energy storage prompted me and my colleagues at the University of Texas at Austin to attempt to cut past the hype and understand exactly how storing solar energy to reduce reliance on the utility would really affect both consumers’ utility bills and the wider electricity system. Our efforts culminated in a paper published today in the journal Nature Energy titled “The impacts of storing solar energy in the home to reduce reliance on the utility.”

Our study found that storage provides a benefit in the form of reduced power flows in the distribution grid, which can lead to utility infrastructure cost savings. However, because some energy is lost every time a battery system charges and discharges, storing solar energy for later use in the home actually increases energy consumption versus just sending it directly to the grid. And with today’s fossil-fuel powered electric grid, the increase in energy consumption also leads to an increase in overall emissions.

These findings challenge the conventional notion that energy storage is inherently clean, so let’s unpack them.

An important challenge we had to overcome was understanding exactly how home energy storage might operate, considering that its operation depends on exactly when households use electricity, and how their use lines up with when their solar panels produce energy. To overcome this challenge, we used real electricity consumption and solar production data collected from 99 Austin households over the entirety of 2014. Then, we modeled two plausible ways that storage might operate to isolate households from the utility, as illustrated below.

We developed two distinct ways that home energy storage might operate to reduce reliance on the utility. The “target zero” method naively zeros-out net power demand whenever possible until the storage runs out of energy. The “minimize power” method more strategically doles out stored energy so that net power demand is low over the entire 24 hours. Credit: Fares and Webber 2017

Under both of the storage operation methods we developed, the homeowner gets to use more of their solar energy without sending it to the grid. But the total amount of solar energy produced actually goes down, because about 15 percent of the energy that goes into a state-of-the-art battery system is lost to inefficiencies. Thus, the net amount of energy used actually increases for every household in our study, because less solar power is produced overall. The average increase was 324–591 kilowatt-hours (kWh) annually, or about an 8–14 percent increase versus a household with solar panels but no energy storage.

We modeled the increase in energy consumption from adding energy storage for each of the 99 households in our study. The average increase ranged from about 324 kWh under "target zero" operation to about 591 kWh under "minimize power" operation, as shown by the shaded grey areas. This increase is equivalent to an 8–14 percent increase versus a household with solar panels but no energy storage. Credit: Fares and Webber 2017

To make up for the solar energy lost in the battery some extra energy must come from somewhere. That somewhere is the electric grid — and today’s grid runs on fossil fuels. We calculated how storage would affect the emissions footprint for each household based on the current electricity fuel mix in Texas, where each of the 99 households are located. We found that adding storage would lead to an increase in carbon dioxide, sulfur dioxide, and nitrogen oxide emissions for an average household.

In other words, if a household with solar panels wants to reduce its emissions footprint, adding energy storage is a bad idea.  

But it’s worth noting that storage combined with solar panels is still a heck of a lot cleaner than having no solar panels at all. So if there is no way to add solar panels without adding storage too, storage would actually provide an indirect energy and emissions benefit.

The increase in energy consumption from adding energy storage is much smaller than the decrease caused by adding solar panels in the first place, so storage would provide an energy and emissions benefit if it directly enabled a new solar installation. Credit: Fares and Webber 2017

But we are very far off from the point where storage will be strictly required to install more rooftop solar panels. A study cited in our paper found that the California utility Pacific Gas & Electric could achieve 100 percent rooftop solar penetration (measured as the ratio of solar capacity to peak system demand) by simply adjusting its distribution transformers more often to maintain an adequate voltage. The total cost would be $442,000 annually, or just 0.007% of the utility’s $6 billion operating budget.

In short, it’s important to not put the cart before the horse when it comes to home energy storage. Storage can provide a lot of benefits when it comes to integrating renewable energy into an already heavy-renewable grid, but fortunately we don’t need storage to integrate rooftop solar panels today. Thus, we shouldn’t add storage for environmental purposes unless it has clearly demonstrable benefits. If we’re not careful, we could actually increase energy consumption and emissions.


The full technical paper referenced in this post can be accessed for free for a limited time at this link.


Associated image credit: Claus Ableiter and Daniel Boyd via CC-BY 2.0.