This commentary is by Adrienne Karpen, director of engineering at Boulder-based solar measurement company Sinton Instruments, and Ron Sinton, its founder and president.
Electric vehicles are one of the most powerful tools we have to slow down climate change, and Coloradans are embracing them at a remarkable pace. According to the Colorado Energy Office, we surpassed 210,000 EVs in Colorado in late 2025 (about 4% of all vehicles in the state). The City of Boulder indicates that more than 12% of registered cars in Boulder are now EVs.
This is good news for clean air and climate progress, but it also means EV charging is becoming a major new force on the electric grid, both in Colorado and nationwide.
The electric grid isn’t static. Its mix of clean and fossil fuel power changes constantly. That means our climate impact doesn’t just depend on what we drive, but also on when we charge.
The electricity used to charge an EV comes from Xcel’s grid, which is a mix of renewable and fossil-fuel generation. As a result, EV charging today still produces some CO₂ emissions, but less than 30% of the emissions from a gasoline vehicle. As Xcel retires coal plants, those emissions continue to fall. And with just a bit of timing awareness, drivers can charge during the cleanest and lowest-cost hours, moving transportation fuel closer to truly zero-carbon.
The grid isn’t equally clean all day.
Electricity’s carbon intensity changes hour by hour. During the day — especially from morning through afternoon — Colorado’s grid is significantly powered by solar and wind. Daytime solar produces the cleanest, cheapest electricity, while evening hours rely more heavily on fossil-fuel “peaker” plants that ramp up when the sun sets and electricity demand spikes.
Research on grid carbon intensity shows that evening and nighttime hours tend to be the dirtiest, because solar drops off at sunset and calm nights are supplied by fossil-fuel-generated power. Avoiding dirty, low-wind nights is key when considering the best times to charge your EV.
Over the course of a year, a 10 a.m. EV charge has less than half the climate impact of a 9 p.m. charge. At that time of day, emissions fall to roughly one-seventh of those from a gas-powered car.
EVs are already clean, but we can make them even cleaner.
EV growth is now large enough to influence grid planning and peak demand patterns nationwide. Studies from Stanford and MIT show that aligning charging with renewable generation, rather than defaulting to typical evening plug-in habits, reduces grid stress, avoids costly infrastructure upgrades and cuts emissions. This topic has been a major issue in resource planning at the Colorado Public Utilities Commission, where projections for when people might charge EVs can determine how many combustion-turbine peaker plants are required to meet peak demand. These plants are often idle for much of the year and come with high costs.
EV charging can lower the cost of electricity if charged at the right times, but it also has the possibility to increase the price of electricity if it adds to peak congestion hours on the grid. EVs charging on renewable energy can actually determine how much of our future electricity can be renewable.
Informed charging saves money, reduces emissions and strengthens the grid.
For Boulder residents, shifting charging to cleaner hours offers three major benefits:
- Lower electricity costs
Under the new Xcel time-of-use (TOU) rate, midday electricity is not considered peak hours (and is much less expensive) because solar is abundant both locally from rooftop solar and from the grid. Charging during these midday hours can meaningfully reduce household energy bills compared with using electricity during the peak 5 p.m. to 9 p.m. hours. Nights after 9 p.m. also have low-cost hours with TOU rates and are clean on windy nights.
- Lower carbon footprint
Charging during high-renewable hours can cut the carbon impact of each mile driven. Conversely, charging on “dirty nights” — especially low-wind evenings — can multiply emissions compared to daytime charging.
- Better grid stability
Charging when solar and wind are abundant helps utilize more clean energy efficiently and reduce the need to fire up gas peaker plants.
Tools already exist to help us charge clean.
Forecasting tools — such as our publicly available EV charging clean-energy forecast, known as ChargeBug — let drivers see when the grid will be cleanest over the upcoming week. This tool doesn’t require new hardware or special chargers; it simply helps people choose the best time to plug in. This free forecast is based on publicly available information, and the algorithms are transparent — not proprietary.
This isn’t about perfection; it’s about shifting habits. If even a fraction of Boulder’s EV drivers moved their charging from dirty night hours to cleaner daytime windows or windier nights, Boulder’s local progress would mean immediate reductions in pollution from Colorado’s electricity grid.
Boulder has long been a leader in climate action. As EV adoption accelerates, we can embrace clean-energy charging habits that make our transportation system even more sustainable.
EVs are already a climate win (and are fun to drive!). By choosing when we charge, we can turn a good solution into a great one.
The entire topic of EV’s feels completely irrelevant to myself and may of my peers until and unless there are viable, dependable options for charging them for people who rent and don’t have their own way to plug them in.
I completely understand where you’re coming from. For renters and anyone without reliable access to home charging, EVs can feel out of reach. Until charging options expand, that barrier is very real. Improving access to public and workplace charging has to be part of the transition, not something we deal with later.
One thing we also try to highlight in this piece is that as EV adoption grows, when current owners charge has real consequences for the grid. Those patterns shape grid planning, infrastructure investments, and ultimately everyone’s energy bills, whether they drive an EV or not. That’s why giving people better information about cleaner charging times matters. It’s not the whole solution, but it’s one piece of making the system work better for everyone while we keep pushing for broader charging access.
For commuters who have to drive to work on a typical daytime schedule the opportunity to charge during prime solar generation hours is limited. The PUC should require XCEL to incentivize businesses to provide charging stations at work. Also the next step is to help home owners with plug-in EVs utilize vehicle to house capabilities to reduce the evening peak. Some vehicles like the Ioniq5 have this capability. This can be as simple as using extension cords to installing a generator inlet box and transfer switch which generally requires an electrician.
I agree that charging during the day is cleaner, but for a majority of people, it’s not an option on a weekday, when that car is likely parked at work.
Thanks for raising this great point. While daytime charging is often the cleanest option due to abundant solar energy, we recognize that it’s not practical for everyone, especially on weekdays when cars are parked at work. That’s another big reason why we built this forecast: to give nighttime chargers the information they need if they want to charge using renewables. If you see that tonight’s grid is only 10% renewable but tomorrow night will be windy and 50%, if you can wait to charge the next night, you can cut emissions without changing your routine. It’s about giving people the tools to charge cleaner, whenever they plug in.
It’s hard enough to find a charger of any kind in many areas of this state, so thinking about exactly where & how electricity gets generated is the least of my worries. Sure, it’s a nice to have this for the true EVangelists, but for the other 99% of EV owners, just be happy they went EV. Most people already avoid evening charging due to TOU rates and other programs. Sure it would be great if everyone charged at 10am, but for those of us with real jobs, charging at work is typically infeasible or way more expensive than charging at home at night.
Thank you for this article. It makes so much sense to NOT charge when coal is being used but I hadn’t thought about its since charging overnight is recommended.
True. But few people have the luxury of being able to charge cheaply at home during the day. Most commuters charge at home, where energy is cheapest, overnight so their car is ready in the morning. So unless charging rates away from home match the low cost of charging at home, few will switch to charging during the day. Fixed battery storage needs to increase in order to shift renewable generation to time of demand.
Hi Adrienne,
Thanks for creating your terrific app that I use even without an EV! It is super user friendly and makes me feel like I have more choices and don’t need permission to use cleaner electricity. It made me look at my dishwasher and notice that I have a 4-hour delay start button. I had no idea. Now when I can’t run my dishwasher during the day, I hit the delay button before I go to bed so it can run in the middle of the night. I hope my next hot water heater can be “smart,” too.
Great article. I’m fortunate to have an EV and an EV charger at home. Before reading this article, I had my EVs settings to charge anytime except 5-9pm to keep it cheap. But, now (and I work mostly from home), I’ll change the EV charging settings to charge from 10am-4pm. And if I’m going on a long trip and can’t “fill up” my EV battery during the day, I’ll check your Chargebug app to see the best (most windy) nights to charge. Thank you!!