California, where generators are expected to get half of their electricity from renewables by 2030, offers a pretty good illustration of the problem. It’s called the “duck curve.” It shows what adding renewables to the electric grid does to the demand for other sources of power, and it does look like a duck.

As more and more solar capacity is fed onto the grid, it will displace alternatives. An extra watt from the sun costs nothing. But the sun doesn’t shine equally at all times. Around noon, when it is blazing, there will be little need for energy from nuclear reactors, or even from gas or coal. At 7 p.m., when people get home from work and turn on their appliances, the sun will no longer be so hot. Ramping up alternative sources then will be indispensable.

The problem is that nuclear reactors, and even gas- and coal-fired generators, can’t switch themselves on and off on a dime. So what happens is that around the middle of the day those generators have to pay the grid to take their power. Unsurprisingly, this erodes nukes’ profitability. It might even nudge them out of the system altogether.

How does a renewables strategy play out in the future? Getting more power from renewables at 7 p.m. will mean building excess capacity at noon. Indeed, getting all power from renewables will require building capacity equal to several times the demand during the middle of the day and keeping it turned off much of the time.

Daily fluctuations are not the end of it. Wind power and sunlight change with the seasons, too. What’s more, climate change will probably change their power and seasonality in unforeseen ways. Considering how expensive wind and sun farms can be, it might make sense to reconsider a strategy that dashes a zero-carbon energy source that could stay on all the time.

A report published last month by the White House’s Council of Economic Advisers suggests there is space for more renewable energy on the grid. New technologies — to store power when the sun is hot or to share it across wider areas — might allow for a bigger renewable footprint.

But there are limits. “There is a very real integration cost from renewables,” said Kenneth Gillingham, an economist at Yale who wrote the report. “So far that cost is small.”