Heat Wave Energy Tips for Ontario: Stay Cool, Safe and Keep Costs Down

The most useful heat wave energy tips for Ontario homes are to pre-cool the house before peak electricity hours, set the air conditioner to about 25–26 °C rather than lower, block the sun before it hits the windows, and use fans to make a warmer setting feel comfortable. Together, these keep a home livable through several days above 30 °C without letting the hydro bill climb as fast as the temperature.

Ontario heat waves often bring humidex readings in the high 30s or 40s, and overnight lows that stay above 20 °C, which means homes never get a chance to cool down on their own. Air conditioners run for long stretches, and many of those hours fall in the most expensive part of the day. Here is how to manage both comfort and cost, and when heat stops being an energy question and becomes a safety one.

What does air conditioning cost during a heat wave?

A typical central air conditioner for an Ontario house draws roughly 2.5–3.5 kW while running. Using 3 kW as an example, at the Regulated Price Plan prices in effect from 1 November 2026 (commodity only):

Price period (summer)	Price	Cost per hour of AC running
TOU off-peak (weekdays 7 p.m.–7 a.m., weekends)	9.1¢	about 27¢
TOU mid-peak (weekdays 7–11 a.m. and 5–7 p.m.)	14.6¢	about 44¢
TOU on-peak (weekdays 11 a.m.–5 p.m.)	18.9¢	about 57¢
ULO ultra-low overnight (11 p.m.–7 a.m.)	3.7¢	about 11¢
ULO on-peak (weekdays 4–9 p.m.)	37.7¢	about $1.13

During a heat wave, an AC may run the equivalent of 12–16 full-power hours a day. At 3 kW and a rough all-in cost of about 16.5¢/kWh after HST and the Ontario Electricity Rebate, that is roughly 36–48 kWh, or about $6–$8 a day. A three-day heat wave on top of normal summer use can add $20 or more to a monthly bill. On the Tiered plan, the summer threshold is only 600 kWh a month, so heat wave cooling often pushes usage into Tier 2 at 13.2¢.

A window air conditioner typically draws 0.5–1.2 kW, and a box or pedestal fan 50–100 W, which costs about a cent an hour.

Pre-cool, then coast

Pre-cooling uses the house itself as a thermal battery. You cool it a little more when electricity is cheaper and the outdoor temperature is lower, then let it drift up slowly during the expensive hours.

On Time-of-Use (summer weekdays):

From 7 to 11 a.m. (mid-peak), set the thermostat 1–2 °C below your normal setting.
At 11 a.m. (on-peak starts), raise it to about 25–26 °C, or a degree or two higher if you are out.
From 7 p.m. (off-peak), return to your normal evening setting.

On Ultra-Low Overnight:

Run cooling at your normal setting through the morning and early afternoon (mid-peak).
Pre-cool by 1–2 °C between about 2 and 4 p.m.
From 4 to 9 p.m. (37.7¢), raise the setting by 2–3 °C, close every blind, and avoid cooking with the oven.
Use the overnight window from 11 p.m. to cool bedrooms.

Pre-cooling works best in well-insulated homes and is less effective in homes with large west-facing windows or a poorly insulated attic. If you are on ULO and your home heats up fast, a summer of heat waves can wipe out the plan's overnight savings. See is Ultra-Low Overnight worth it.

Tip: Setting the thermostat to 18 °C does not cool the house faster. Most air conditioners have a single speed or a few fixed stages; a lower setting just makes the system run longer and overshoot.

Heat wave energy tips for Ontario homes: keep the heat out

The cheapest cooling is the heat that never gets in.

Action	Effect	Cost
Close blinds and curtains on east windows in the morning and south and west windows in the afternoon	Blocks a large share of solar heat through glass; exterior shading works best	Free
Keep windows closed during the day, open them at night if it is cooler outside	Flushes heat without running the AC	Free
Avoid the oven and dryer during the day	Each adds heat the AC must remove	Free
Use bathroom and kitchen fans briefly	Removes heat and humidity at the source	Pennies
Install exterior awnings, solar screens or reflective window film	Reduces heat gain on sunny windows	$50–$500+ per window (estimate)
Turn off unneeded lights and electronics	All electricity used indoors ends up as heat	Free

Our guide to window coverings and film compares options in more detail.

Use fans and the AC well

Ceiling fans should spin counter-clockwise in summer (looking up) to push air down. Moving air can make a room feel several degrees cooler, but fans cool people, not rooms. Turn them off in empty rooms.
Keep the indoor fan on "auto," not "on," so the AC can remove humidity between cycles.
Change the furnace filter. The same blower and filter serve the AC. A clogged filter during a heat wave can cause the indoor coil to freeze.
Check the outdoor unit. Keep at least 60 cm (2 ft) of clearance, and gently rinse dirt from the coil with the power off. Do not cover it or block airflow with plants.
Close doors to unused rooms with window units, but leave central-system registers mostly open.
Portable air conditioners are generally less efficient than window units of similar capacity. A ductless heat pump is far more efficient for a hot room that never cools, such as a top floor.

Caution: If your AC is running but the air coming out is warm, or you see ice on the copper lines, turn the cooling off and switch the fan to "on" for an hour to thaw the coil. Check the filter, then call a contractor if it happens again. Running a frozen system can damage the compressor.

For long-term fixes, attic insulation and air sealing reduce heat gain from the top of the house, and a correctly sized, efficient system makes heat waves easier. Our air conditioning buying guide covers sizing and SEER2 ratings, and our summer cooling savings guide covers the rest of the season.

Health and safety during extreme heat

Extreme heat is a serious health risk, especially for older adults, infants and young children, people with chronic illnesses, and people who live alone. Environment and Climate Change Canada and local public health units issue heat warnings in Ontario. When one is in effect:

Drink water regularly, even before you feel thirsty.
Know the signs of heat exhaustion (heavy sweating, dizziness, headache, nausea) and heat stroke (confusion, fainting, hot skin, no sweating). Heat stroke is a medical emergency: call 911.
Check on neighbours, family members and friends who are vulnerable, at least once a day.
If your home is above about 26 °C and you cannot cool it, spend part of the day somewhere air-conditioned. Many Ontario municipalities open cooling centres; check your city's website or call 211.
Never leave children or pets in a parked vehicle.

Heat waves can also strain local electrical equipment and coincide with summer storms. Have a plan for an outage, especially if someone in your home relies on cooling or powered medical equipment. See our power outage preparedness guide.

Frequently asked questions

What temperature should I set my air conditioner to during a heat wave?

About 25–26 °C when home is a reasonable target for most households, with fans for extra comfort. Raise it a few degrees when away, but not so high that the system cannot catch up. People who are vulnerable to heat may need a cooler setting.

Is it cheaper to leave the AC on all day or turn it off?

For a few hours away, raising the setting by 2–4 °C usually saves energy compared with leaving it at your normal setting. Turning it off completely during a multi-day heat wave can let the house, furniture and walls absorb so much heat that the AC struggles to recover and runs through the expensive evening hours.

Do fans use a lot of electricity?

No. A typical box, pedestal or ceiling fan uses about 50–100 W, roughly a cent an hour. Fans are among the cheapest ways to feel cooler, as long as they are turned off in empty rooms.

Will the IESO cut my power during a heat wave?

The IESO operates Ontario's electricity grid and plans for summer peaks; widespread rotating outages are rare. Local outages are more likely to come from storms or overloaded neighbourhood equipment. Follow your utility's updates during extreme heat.