Is it cheaper to leave AC running all day?

Generally no - it's typically more cost-effective to adjust your AC temperature when you're away rather than running it at full cooling all day, but the optimal strategy depends on your home's insulation and how long you're gone.

The most efficient approach is to raise your thermostat by 7-8 degrees when leaving for work (from 22°C to 29-30°C, for example). This reduces the workload on your AC system during peak daytime hours when electricity rates are highest in Ontario. Your AC won't have to work as hard to maintain an unnecessarily cool temperature in an empty house.

However, avoid dramatic temperature swings. Setting your thermostat too high (like 35°C) when you leave can actually cost more because your AC will work harder to cool down the significantly warmed house when you return. The key is finding the sweet spot where you save energy during the day without creating excessive demand when you get home.

In Ottawa's climate, where summer temperatures can reach 30-35°C, a programmable or smart thermostat is your best investment. These can automatically adjust temperatures based on your schedule, typically saving 10-15% on cooling costs. During Ontario's time-of-use electricity pricing, this strategy becomes even more valuable since you'll use less power during expensive peak hours (11 AM - 5 PM on weekdays).

Your home's insulation quality also matters significantly. Well-insulated homes hold their temperature longer, making temperature adjustments more effective. Poorly insulated homes may benefit from smaller temperature changes to avoid overworking the system.

For optimal efficiency and comfort in your Ottawa home, consider upgrading to a programmable thermostat and ensure your AC system receives annual maintenance to operate at peak efficiency.