Is it better to turn off the AC during the day, using it only at night, or to keep it on day and night?

Asked by Pete Gonzales
Chico, CA

I am debating whether it is better to cool the house from a higher daytime time only at night, or if a more constant temperature both day and night is more efficient.


Cynthia Phakos

Answered by Cynthia Phakos

Los Angeles, CA

Koffka Phakos Design

September 2, 2010

Though it seems like your question would have a straightforward answer, you have hit on a complex topic with many variables.

Maintaining a lower temperature during the day may be costing you more than just the additional energy used.

  • Southern California Edison (SCE), your local provider, has established a baseline for energy usage by area and consumer profile, which, if exceeded, triggers the higher rates of their tiered system.
  • A start in answering this question would be to call SCE to find out if you have exceeded your baseline energy usage.

A more energy efficient home

As we are becoming more sophisticated in evaluating our homes for energy efficiency, you may be interested in further understanding how you might make yours a more energy efficient home.

  • In regards to your air conditioner, a home’s HVAC system accounts for 30% of its energy usage. If your air conditioner is over ten years old a new one could save 20 to 40% of your energy bill, whereas if it is over 20 years old a new one may save you up to 50% of your energy bill (according to the U.S. Dept. of Energy).
  • A programmable thermostat will give you more control and, for example, will allow you to turn on the air conditioning just before returning home, rather than running it all day.
  • You could further reduce your energy costs with proper insulation, roofing, and ventilation. If this is more than you yourself want to handle, contact an energy auditor to assist you (try

As the summer sun in California is very high in the sky, solar gain occurs primarily through the roof.

Ways to upgrade your roof

I live in Los Angeles, and by simply adding insulation to my roof I rarely use my air conditioner in the evening when I return from work.

  • Insulation: Insulating materials are measured by R-values, which quantify their resistance to heat flow. Title 24, California’s Energy Code, requires R30 insulation at the roof. If your roof is not insulated, you will find that the addition of insulation will reduce heat gain significantly. Add the best insulation space allows. As one of the biggest problems with insulation is its proper installation, visually inspect it for gaps, as this will impact its performance.
  • Cool Roofs: During the summer, a typical dark roof can be up 150 to 190 degrees Fahrenheit, while cool roofs peak at 100 to 120 degrees Fahrenheit. A cool roof minimizes the solar heat gain of a building by reflecting incoming sun rays and re-emitting the remaining absorbed portion. This can additionally reduce your air-conditioning requirements by 15%. Attic spaces and ducts will stay cooler. GAF carries a line of asphalt shingle, Timberline Cool Series, made with specially-designed roofing granules that have greater reflectance than those of traditional shingles, at a competitive price. If you have an existing flat roof, a cool roof coating such as Kanopy Reflective Roof Maintenance Solutions by Karnak can be applied with rollers, sprays, or brushes.
  • Radiant Barriers: This is a reflective coating on the underside of roof sheathing that inhibits heat transfer by thermal radiation and can cut monthly air conditioning bills up to 17%. If you are reroofing and redoing the sheathing, install LP’s OSB (oriented strand board) TechShield. It is made of a thin, durable sheet of aluminum laminated to the board. There are other materials on the market that can be added to the underside of your existing roof that work in the same way. ESP’s Low E Reflective Insulation is just 3/16".  Made of a nontoxic polyethylene foam, laminated between two surfaces of 99% pure aluminum facings, it resists all three types of heat transfer: convection, conduction, and radiation.

Check GreenHomeGuide's best attic insulation Q&A to see what other homeowners and contractors are saying about foam insulation, radiant barriers, etc."

Other energy-saving upgrades

  • Whole House Fans: A Whole House Fan installed at the highpoint of your home will exhaust the hot air as well as drawing cooler night air in through open windows. This can be done when you return from work in the evening. The fan pulls the hot daytime air out of a building into the attic space, which causes positive pressure in the attic forcing the air out through the vents, while at the same time producing a negative pressure inside the building drawings cool air in through the open windows.
  • Ceiling fans: The breeze from a ceiling fan speeds the evaporation of perspiration on your skin, making your natural cooling mechanism operate more efficiently. You will find a ceiling fan directly over your bed will make you feel very comfortable. Look for the Energy Star rating when purchasing your ceiling fan and you will further save on your electric bill. Since the fan works directly on the body, and doesn’t affect the temperature of the air, it should only be on when someone is in a room.

Many of the methods and products discussed qualify for SCE’s rebates and federal tax incentives.

  • Visit the Flex Your Power website.
  • SCE’s lists residential rebates on its website here.


For more information:

Read Steve Saunders's Q&A "Does a programmable thermostat really save 20% on my utility bills?"

Tagged In: heating cooling, summer energy

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