Did California actually hit 97% renewables in April? Yes and no

With several countries, 15 states and many U.S. utilities pledging to get to a 100% clean electricity system, a lot is riding on the engineers who run the power grid. They have a complex challenge, akin to rebuilding a plane while it’s flying. But real-world examples increasingly point to how it can be done, from South Australia’s rooftop-solar-heavy grid, to Europe’s wind power vanguards, to, most recently, sun-soaked California.

California’s grid hit a major milestone on April 3 when 97% of demand was served by renewable power at 3:39 p.m., according to CAISO, the state’s grid operator. In fact, if you add in hydropower, which CAISO didn’t count, and nuclear, another source of zero-carbon power, there was enough clean energy to cover more than 100% of demand for three full hours that day.

At first glance, this appears to be a major step toward reaching California’s mandated target of 100% clean electricity by 2045. If we can do it for three hours, can’t we do it all year? A closer look at what happened that day reveals a more complicated picture. While California’s milestone is significant, the state still has a long way to go to reach 100% clean electricity.

The meaning of the 97% milestone

Understanding the full picture requires first unpacking how CAISO calculated the 97% figure. California’s in-state renewable energy production was calculated as a percentage of energy demand after accounting for transmission losses. This demand figure omits demand met by rooftop solar, which generates power for more than 1 million California customers. Because large hydropower does not qualify for the state’s renewable portfolio standard, it is also not included in this figure.

Another important caveat: The figure does not account for all demand in California, even leaving aside demand met by rooftop solar. CAISO’s system does not include the areas served by the Los Angeles Department of Water and Power and the Sacramento Municipal Utility District, two publicly owned utilities that together make up roughly 10 percent of electricity sales in the state.

The following chart shows CAISO’s data for in-state renewable generation and electricity demand on April 3. The 97% high point represents an instantaneous maximum of non-hydro renewables.

April 3 was a big wind and solar day for California, coupled with relatively low demand due to mild spring weather. Across the whole day, wind contributed 24% toward meeting demand and solar contributed 22%, followed by large hydro at 8% and geothermal at 4%. Add in minor contributions from biomass, biogas and small hydro projects, and the total renewable percentage for the day was 61%. See the contributions of each renewable type in the following chart. 

Non-renewables still played a big role on a record day in California 

But even on this banner day for renewables, 39% of CAISO’s demand was met by non-renewable sources. And even at the 3:35–3:40 p.m. interval when CAISO hit 97% non-hydro renewables, other power plants in the state were running, including gas, nuclear and hydro facilities.

Adding large hydro and nuclear into the CAISO mix during the renewables peak yields a maximum of 107% carbon-free power that day, as shown in the chart below. During the three hours when clean electricity was being produced in excess of demand, California was exporting its carbon-free energy to neighboring states, almost certainly offsetting fossil power.

However, gas plants never stopped burning during this high-renewables day — see the orange line in the next chart, a CAISO graph of energy supply for April 3. Gas-fired power fell to a low of 2.7 gigawatts around the same time that non-hydro renewables reached their 18 GW peak. That means California still burned enough gas to meet about 15% of demand at the same moment that it had enough non-hydro renewable production to meet 97% of demand; again, the excess was exported to other states. 

On average, California imports much more power from its neighbors than it exports. The red line in the above graph represents imports (and, when it falls below zero, exports). The state imports about 30% of its power annually, and on April 3 it met 21% of energy demand with imports.

On April 3, imports were almost a mirror image of total renewables output: They were higher during the night, while during the sunniest parts of the day, the state exported power instead of importing it. Imports are a mix of carbon-free resources, gas and coal, though it is hard to track exactly what is what on the open Western market. Imported power is an important tool in the reliability and low-cost toolkit, but it has a mixed record as an emissions-reduction strategy.

How can California set more renewable records?

Despite the fact that California’s 97% renewable record isn’t quite what it seems, it is still a significant achievement on the path to 100% clean electricity. Running the state’s grid with 18 GW of utility-scale renewables and 8 to 9 GW of behind-the-meter solar is a huge operational challenge because it implies the vast majority of power was coming from inverter-based weather-dependent resources. Operating a grid under such circumstances is a major research focus for U.S. national labs.

Because a 100% clean grid will have to become even more reliant on these variable resources, CAISO’s success should give policymakers confidence that the grid can remain stable and reliable with most of the power coming from wind, solar and batteries.

But the achievement also reveals that there is still much to be done to reach a truly clean grid. To build on this success, policy will be key. California policymakers should focus on three key areas to support full grid decarbonization: retiring gas plants, starting with those in the most polluted communities; speeding up and diversifying clean energy procurement; and coordinating more effectively with other states in the West.

Gas has stuck around because it’s affordable and offers reliability to the grid. California keeps almost 40 GW of gas in service to prevent energy shortfalls on hot summer days. Gas also provides reliability services in local areas of the grid where it’s difficult (but possible) to replace it with local clean energy resources. A more significant milestone to a 100% clean electricity system will be hours or days with no gas plants running.

Adding to the challenge of retiring gas, California is losing its last nuclear plant — the Diablo Canyon nuclear facility is scheduled to shut down by 2025 — and hydro is becoming less reliable due to drought. 2015 was the state’s lowest hydro year on record, when it only provided 5.9% of in-state power.

State regulators are working to line up replacements for these sources. In February of this year, the California Public Utilities Commission approved a long-range energy plan that would require nearly 24 GW of new renewables and more than 12 GW of new batteries by 2032. If California wants to hasten the transition from gas, it should procure even more capacity from diverse sources like geothermal and offshore wind.

Moving beyond gas is also an environmental justice issue in California. Half of the state’s gas plants are in disadvantaged communities as defined by the CalEnviroScreen mapping tool, so retiring that fossil fuel generation will bring some relief to communities suffering from the nation’s worst air pollution.

California will also need to continue relying on its neighbors as it works to meet its 100% goal. It’s already well interconnected with other Western states, which also have many clean energy resources, and there are significant benefits of resource sharing across state lines.

California’s current imports include fossil fuels, but the West is getting cleaner. 85% of Western U.S. demand is under a 100% clean requirement or order, and Arizona and Idaho are also moving in the same direction because their largest utilities have announced voluntary goals to reach 100% clean power.

Greater regional coordination would allow more effective least-cost planning for a highly interdependent Western system. California’s legislature last discussed forming a regional grid in 2018, but talks fell apart. Now that there’s greater regional alignment on decarbonization, it’s time for California to reopen this discussion and lead the development of a full regional grid.

All in all, April 3 was a big milestone for California, but it doesn’t prove the state can yet reach 100% clean power year-round. Rather, it shows that California is making rapid progress — and will need to continue building its skills in order to integrate lots more renewables over the next decade.