Flex Alerts and Your AC: How to Stay Cool While Easing the Grid (and Your Bill)
A Flex Alert doesn't mean shutting off your AC. Pre-cool before 4 PM, raise the thermostat to 78°F from 4-9 PM, close blinds, and run fans. Here's a simple Inland Empire game plan that eases the grid and lowers your bill.
Owner & Lead HVAC Technician, Alex Air & Heating · EPA 608 Universal Certified · Ontario, CA
TL;DR
The best flex alert AC tips: don't shut the AC off — pre-cool your home before 4 PM, then from 4 to 9 PM raise the thermostat to 78°F or higher, close sun-facing blinds, run ceiling fans, and hold off on the oven, laundry, dishwasher, and EV charging. A Flex Alert is a voluntary CAISO request to conserve when the evening grid is strained. Because 4-9 PM is also the priciest time-of-use window in California, these same habits trim your bill every hot day, not just during alerts.
- A Flex Alert asks for voluntary conservation from about 4 to 9 PM, the 5-hour window when demand peaks and solar drops off.
- Pre-cooling before 4 PM lets a well-sealed Inland Empire home coast through the peak with the thermostat set to 78°F or higher.
- Setting the thermostat lower than 78°F does not cool faster — it just runs the compressor harder during the most expensive hours.
- Ceiling and portable fans let most people stay comfortable up to about 4°F warmer, and fans use a fraction of an AC's power.
- The 4-9 PM window is the top-priced tier on most California TOU rate plans, so these habits save money every day, not only during alerts.
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How should you use your AC during a Flex Alert in California?
During a Flex Alert, you don't have to shut off your AC. Pre-cool your home before 4 PM, then from 4 to 9 PM raise the thermostat to 78°F or higher, close sun-facing blinds, and run fans. Hold off on big appliances. That eases the strained evening grid and lowers your peak-hour bill.
What exactly is a Flex Alert?
A Flex Alert is a voluntary request from the California ISO (CAISO), the nonprofit operator of the state's electric grid, asking residents to conserve electricity during periods of grid stress. They're usually issued for 4 to 9 PM on hot days, when air-conditioning demand stays high but solar generation drops off after sunset. It is not a rotating outage or a blackout notice — it's the opposite. When enough households voluntarily ease their use in that window, the grid stays balanced and forced outages are avoided.
Here in the Inland Empire — Ontario, Fontana, Riverside, Rancho Cucamonga and nearby — evenings stay hot well past sunset, so the 4-9 PM crunch lands right when your home still wants cooling. The good news: you can keep your family comfortable and still cut your load during those hours with a little planning. And to be clear, a Flex Alert asks for participation, not sacrifice — no one is checking your meter or penalizing you. It works precisely because thousands of households each trim a little at the same time, which is enough to keep the whole system from tipping into an emergency.
Your hour-by-hour Flex Alert timeline
The whole strategy comes down to timing. Do the cooling work before the peak, coast through 4-9 PM, then return to normal. Here's the plan for a typical hot Inland Empire day:
| Time block | What to do with AC & appliances | Why it works |
|---|---|---|
| Before 4 PM | Pre-cool the house: set a comfortable-to-slightly-low temperature. Run the dishwasher, laundry, and oven now. Charge the EV. | Cheaper off-peak power and abundant daytime solar. The home stores cool air for the evening. |
| 4-9 PM (peak) | Raise the thermostat to 78°F or higher. Close sun-facing blinds. Run ceiling and portable fans. Skip the oven, laundry, dishwasher, and EV charging. | This is the strained, most expensive window. A pre-cooled, shaded home coasts while the compressor barely runs. |
| After 9 PM | Return the thermostat to your normal setting. Resume laundry, dishwasher, and EV charging as needed. | Demand falls, prices drop, and outdoor air cools, so the AC recovers efficiently. |
Close the blinds and let fans do the work
Two low-tech moves make the 78°F setpoint feel fine. First, close blinds and drapes on windows facing the sun — west- and south-facing glass pours heat into the house through the afternoon, and blocking it keeps your walls and floors from soaking up warmth. Second, turn on ceiling and portable fans in the rooms you're using. Moving air lets most people feel comfortable a few degrees warmer, so the fan does the job the compressor would otherwise strain to do. The two work together: a shaded, pre-cooled room with a fan running at 78°F often feels the same as a sun-baked room at 74°F with no air moving — except the first one costs a fraction as much to hold through the peak.
- Fans cool people, not empty rooms — turn them off when you leave, since they don't lower air temperature.
- A ceiling fan draws a tiny fraction of the power an AC compressor uses, so it's nearly free comfort.
- Close interior doors to unused rooms so your cooled air stays where the family is.
- Setting the thermostat far below 78°F does not cool faster — it just runs the compressor longer at the worst time.
Why these habits save money every day, not just on Flex Alert days
Most California utilities bill on time-of-use (TOU) rates, where electricity from 4 to 9 PM is the most expensive of the day — often by a wide margin. That's the same window a Flex Alert targets, for the same reason: it's when the grid is tightest. So pre-cooling and setting back during those hours isn't just a Flex Alert trick — it trims your bill on every hot day of the summer. Think of it this way: a Flex Alert is just the grid asking loudly for something your rate plan already rewards you for doing quietly. Shift the oven, dishwasher, laundry, and EV charging out of that five-hour block and pair it with a pre-cooled house, and the priciest hours of your bill barely register any load at all. If you want to go deeper on the billing side, see our guide to lowering your summer AC electric bill and our take on the best summer thermostat temperature.
Let a smart thermostat run the plan for you
Remembering to pre-cool at 3 PM and set back at 4 PM every day is a lot to juggle. A smart or programmable thermostat automates the entire timeline — it pre-cools before the peak and eases off to 78°F from 4 to 9 PM on its own, and many models respond directly to utility peak events and TOU rate plans. If you're weighing the upgrade, our comparisons of the best smart thermostats for 2026 and whether a smart thermostat is worth it walk through the options. Heading out of town? Don't leave cooling running full-blast — set the right vacation thermostat settings first.
The easiest way to win every Flex Alert — and every hot afternoon — is to let the thermostat do the timing. Our team sets up pre-cooling and 4-9 PM setback schedules across the Inland Empire. See smart thermostat installation.
For homeowners in Ontario, Pomona, Rancho Cucamonga, Fontana, Riverside, and Chino, the routine is the same all summer: pre-cool a well-sealed, shaded house before 4 PM, hold 78°F or higher through the evening peak with blinds closed and fans running, and never disable cooling entirely for elderly family members, infants, or anyone with a health condition. Do that and you'll stay comfortable, help the grid avoid outages, and keep your peak-hour costs down at the same time. If your system struggles to hold a pre-cooled house through the evening — cycling constantly at 78°F, or leaving upstairs rooms hot — that's often a sign of leaky ducts, low refrigerant, or an aging unit, and it's worth having a local technician check before the next heat wave hits.
Frequently asked questions
A Flex Alert is a voluntary call from the California ISO (CAISO), the operator of the state's power grid, asking residents to reduce electricity use during times of grid stress. They're typically issued for 4 to 9 PM on hot days, when demand is high and solar generation fades in the evening. It is not a rotating outage — conserving voluntarily is exactly what helps the grid avoid forced outages.
No. You never have to shut cooling off entirely, and you shouldn't for households with elderly people, infants, or health conditions. The goal is to shift and reduce use, not eliminate it. Pre-cool your home before 4 PM, then raise the thermostat to 78°F or higher from 4 to 9 PM. The house coasts on stored cool air while the compressor runs far less during the strained window.
Pre-cooling means running your AC harder earlier in the day — setting a comfortable-to-slightly-low temperature before 4 PM — so the home and its walls, floors, and furniture hold that coolness into the evening. When the 4-9 PM peak hits, you raise the thermostat and let the house coast. A well-sealed, shaded Inland Empire home can stay comfortable for hours this way with the AC barely cycling.
In the late afternoon and evening, electricity demand is still high from air conditioning and people returning home, but rooftop and utility solar generation drops sharply as the sun sets. That gap strains the grid. It's also why most California utilities price 4 to 9 PM as their most expensive time-of-use hours, so cutting back then both helps the grid and lowers your bill.
No. An AC cools at the same rate whether you set it to 78°F or 68°F — the lower number just makes it run longer to reach a colder target. Setting it very low during a Flex Alert wastes electricity in the worst hours and drives up your bill. Pick your target temperature, then use fans, closed blinds, and pre-cooling to stay comfortable instead of pushing the thermostat down.
Yes. A smart or programmable thermostat can schedule pre-cooling before 4 PM and an automatic setback to 78°F or higher from 4 to 9 PM, so you don't have to remember. Many models also respond to utility peak events and time-of-use rate plans on their own. That automation captures the savings every hot day, not just when CAISO issues an alert.