Solar Mandates: An Idea Whose Time Has Come?

Solar panels are perhaps the first image that springs to mind for most people when they think about the green energy revolution that is underway. Moreso than large-scale solar farms, most people think of rooftop solar, which is growing rapidly in New York and is already near-ubiquitous in some of the sunnier parts of the country. Given the benefits of rooftop solar and its increasing popularity, it seems like great news that California recently mandated that all new houses either have rooftop solar or be connected to community solar starting in 2020.

As with all things clean energy and climate-related though, the devil is in the details. Up to a certain point, rooftop solar benefits everyone – homeowners who have it save money, utilities don’t have to invest as much in new power generation and transmission (which saves all ratepayers money), and everyone benefits from fewer greenhouse gas emissions and cleaner air. But two problems emerge when rooftop solar clears a certain threshold.

First, the electric grid needs to be maintained no matter what and large-scale power generation will always be necessary because not all homes are suitable for on-site renewable energy. This means that the same effect that saves ratepayers money as rooftop solar grows eventually flips and begins to cost them money – as fewer and fewer customers pay the utility, the customers who can’t go off-grid need to pay more and more for the same amount of grid maintenance. These customers are disproportionately lower-income and likely to live in cities. Germany, which at one point had the most aggressive incentives for rooftop solar in the world, has been struggling with this effect for several years.

Second, solar power creates what Americans call the “duck curve” and Europeans call “devil horns.” While we encourage you to click the links to see a better graphical representation of where these names come from than your humble NYLCV bloggers are capable of producing, the basic story goes like this: for the traditional day that utilities plan for, electricity use increases steadily early in the day, hits a small peak in late morning, falls a bit in early afternoon, and then ramps up again starting in late afternoon, eventually hitting the largest peak of the day roughly during primetime TV hours. In the summer this big peak occurs earlier in the day because air conditioning consumes so much energy and peaks during the hottest part of the day.

Since the sun shines brightest in the afternoon, the peak of solar power production coincides with the small early afternoon dip in demand and the beginning of the late afternoon ramp-up. As more and more solar comes online, the amount of power that utilities need to purchase in the afternoon falls more and more – so far so good. But this also means that the dinner-time ramp-up becomes steeper and steeper as solar becomes less productive at the same time that energy demand is increasing. Right now, the only power plants that are capable of rapidly injecting thousands of megawatts into the grid on short notice burn natural gas and, to a lesser extent, coal. This effect has led Germany to actually build new coal-fired power plants in the last decade, effectively wiping out the emissions reductions they’ve achieved over the same time frame.

To get to our long-winded point, does this mean California’s mandate is a bad idea? Not necessarily. Battery storage, which is becoming less expensive and more effective, could be used to store some of the solar energy produced in the early afternoon and release it back into the grid during the late afternoon ramp-up. Renewable energy could also be priced to deter the installation of systems that exacerbate this effect and incentivize systems that reduce it, something that New York is actively exploring. While this problem is very solvable, in the near term it might make more sense for California to focus on other ways that housing policy can fight climate change, such as increased density and complete streets. For a full consideration of the pros and cons of California’s policy, we highly recommend David Roberts’ explainer at Vox.