You don’t know what you’ve got until it’s gone, and the total eclipse is a stark reminder of that adage when it comes to the key role solar energy currently plays in the US.

More than 31 million people — nearly 10 percent of the population in the US — live in an area that will experience the total solar eclipse today. Millions more live near dirty power plants that could be tapped to make up for a loss of solar power.

Grid managers have had to find backup sources of energy to cope with the eclipse. It shows us how far the nation has come in cleaning up its power grid — and what we’re still in dire need of to complete that task.

All 50 states will experience some degree of disruption to solar power generation during the eclipse, according to the National Renewable Energy Laboratory (NREL). It forecasts a whopping 93 percent peak power reduction from solar panels within the Texas grid, where the solar eclipse will first cross into the US before slicing a diagonal path across the nation toward Maine. Peak power reduction is expected to reach 71 percent within the eastern power grid and 45 percent in the western grid.

The eclipse only reaches “totality,” when the Sun is completely blocked by the Moon, for several minutes in each location. But a partial eclipse can persist for several hours. While solar generation falls, electricity demand is expected to rise. Households and businesses with photovoltaic panels won’t be able to depend on their own solar systems as much — they’ll need to rely more on the grid.

That kind of mismatch in supply and demand is what can lead to outages. Grid managers have had a lot of time to prepare for this eclipse, so experts aren’t expecting any blackouts. Hydropower and gas are supposed to make up for most of the the shortfall in solar energy. NREL expects gas to cover about 30 percent of the loss in utility-scale solar generation.

Put simply: more gas, more pollution. On a national level, that’s not good for US climate goals, which aim to slash greenhouse gas emissions roughly in half by 2030 compared to 2005 levels. When it comes to soot and smog-forming pollutants, the effects are more concentrated in communities that border fossil fuel power plants.

Around 32 million people in the US live within three miles of a peaker plant, a facility that typically runs on gas and fires up during energy demand “peaks” like the one the solar eclipse is expected to trigger. Peakers are some of the dirtiest power plants in the nation, and a majority of them are located in communities of color and low-income neighborhoods.

The last time a total eclipse took place in the US in 2017, gas replaced the majority of the solar energy lost. But a lot has changed since then. To start, the path of totality is significantly wider this time around — meaning a much larger area is affected. Moreover, solar energy has become the cheapest source of electricity in history. The US has way more of it now, around 2.5 times as much solar generation capacity as it did in 2017.

While solar panels don’t pump out greenhouse gases or worsen air quality for nearby residents like fossil fuel power plants do, solar comes with its own challenges. Namely, it goes away when the Sun’s not shining. That’s not just a problem during a solar eclipse, of course.

Thankfully, the US has also made some progress on that problem. Battery storage in the US has grown from .6 GW during the last solar eclipse to 15.4 GW today. Even so, a lot more energy storage is needed. The eclipse is forecast to either fully or partially block sunlight to utility-scale solar farms with a combined capacity of 91.3 GW, according to the Energy Information Administration. For a sense of scale, that’s nearly all of the nation’s utility-scale solar capacity (although the US has around 139 GW of capacity when including small-scale solar).

As an alternative to gas peaker plants, pumped hydropower storage is set to make up for 42 percent of the shortfall in solar energy during the eclipse. That involves pumping water from a lower elevation to a higher elevation and then letting it flow through a turbine to generate electricity. The system essentially works like a giant battery, and without it, today’s loss of solar power likely would have led to even more consequences to air quality and climate.

Ramping up capacity to generate and store renewable energy so that there’s always a reliable supply is one of the biggest challenges facing power grids today. The solar eclipse is just one test of how prepared the US is to meet this challenge. It also shows how the only other alternative — continuing to rely on dirty sources of energy — comes at an unfair cost to many Americans.