There's a magic yet dreaded date that happens each year for energy watchers, when you finally break down and turn on the air conditioner. Every day you don't turn on the air conditioner is like money in the bank — and free money at that. Our average base electricity consumption is 750 kWh per month, month in and month out. When the air conditioner is on, however, we use an additional 2,000 kWh per month. At an average cost of $0.10 per kWh, that works out to $200 per month, or $6.66 per day, to run the air conditioner.
So we check the weather reports and hope for more cool weather as spring advances and it gets hotter and hotter. The magic date arrived this week, we turned on the air conditioning, and we are now looking at an additional $200 per month on our utility bills. In an effort to reduce that pain, we have taken some steps to ensure that our cooling bills are as low as possible.
Thermostat
First and foremost, we have set the thermostat as high as we can reasonably stand it:
This is the downstairs thermostat — the upstairs thermostat is set to 77°F. The theory we are testing is that cold air flows downhill and by setting the upstairs thermostat just a little bit cooler, we will achieve an average temperature of 78°F throughout the house.
Furnace Filters
We learned a painful and expensive lesson this winter — our furnace filters need to be checked every two months at a minimum. Checking the furnace filters shows that they are still pretty clean from when they were changed two months ago, mostly because we turned the furnace off six weeks ago.
I have laid in a stock of furnace filters and will be checking them at the beginning of each month.
Clean Up A/C Compressors
The next thing we did is cut back the bushes around the air conditioning compressors and clean off accumulated pine needles, leaves, and twigs.
A quick spritz with the hose will knock off any loose dirt and get the compressors ready for the summer.
Spot Cooling
In addition to turning, adjusting, and tightening up the ceiling fans, we have also set up pedestal fans in various rooms to ensure a steady, cooling flow of air. I also set up a portable air conditioner in my wife's room to help ensure that she stays cool at night.
My wife suffers from what I will call a mis-set thermostat. She is always a few degrees warmer than everyone else and only really sleeps well in a cool room. She actually enjoys having the thermostat set low in the wintertime because it helps her sleep. Sleeping in the summertime is another problem entirely. What we have done is set up this portable A/C unit to keep her bedroom cool.
We tried this last year and discovered several ways to waste a whole lot of energy. If you have a need like ours, let me give you some advice:
- These units are not very efficient. Ours uses one kilowatt per hour. Always look for a unit with a higher SEER. Spending more money on a more efficient unit will save you in the long run.
- Don't bother trying to keep the "cold" room cool 24 hours a day. Your insulation is probably not that good and you will just waste a lot of energy. Just run the unit when you need it. We have set up a timer to turn the unit on an hour before bed time and turn if off an hour before waking up.
- Keep a fan running in the room in addition to the A/C unit. The more air flow you have, the cooler the room will feel, and higher you can set the thermostat.
- Our upstairs thermostat is in the "cold" room. Since that room will be much colder than the 77°F that the thermostat is set at, our upstairs air conditioner won't be running at night while the portable A/C unit is running. With any luck, these two factors will cancel each other out and we won't see the same cooling bills we did last summer.
Several years ago I ran across an interesting paper that was written by a government agency in Florida about optimizing air conditioning strategy. It greatly improved our air conditioning efficiency and reduced our usage.
I'll summarize the savings strategy below....
Once air conditioning season arrives, seal the house. Do NOT open the windows at night to cool the house. Set the air conditioning system to cool the house below normal at night and allow the house's thermal capacity coast through most of the hot day without air conditioning.
This is contrary to almost every single website about saving energy that I have read. Normally, one sees advice saying to open your windows at night to let night air cool the house. That seems perfectly logical. Night air is cooler and costs nothing to to use. Sadly, that common sense strategy is not always right. For someone in a well insulated home like we have, opening the windows at night ended up INCREASING our total air conditioning usage. Why?
1. Water vapor enters the house along with the night air. The increased humidity decreases comfort and drops air conditioning efficiency the next day because you have to spend energy removing water from the house before you get net cooling from the air conditoner.
If one opens windows and lets cool air in at night, one also allows a great deal of humidity into the house. That water vapor is not only in the air, but it is absorbed into carpeting and upholstery. When the next day's air conditioning begins, a major portion of the air conditioner's energy usage is spent removing all the water vapor that was introduced overnight into the house. Worse yet, increased indoor humidity means things feel hotter at the same temperature than at a lower humidity. That "common sense" night time window opening ends up increasing how long you need to run the air conditioner during the day.
2. An air conditioner compressor is more efficient removing heat when it is cooler outdoors.
Running the air conditioner while the outdoor air is cool, allows the heat pump to be more efficient because it does not have to work against as large a temperature gradient as during the hot mid-day. Shifting air conditioning to the night also moves that load off-peak. In some localities, off peak electricity is also cheaper.
3. Cooler house at night makes sleep a lot easier.
Even with windows open, we could never get truly comfortable. Outdoor air was cooler, but there was always a mugginess and things never got cool enough until past midnight.
Air conditioning at night instead of during the day is worth trying. Your home must well insulated and with shades closed. Our experience has been that cooling the house to about 67 degrees at 10 PM, means we can very comfortably coast through a 90 degree day without hearing the air conditioner come on until 7 PM. It was truly amazing the first week we tried the strategy. A programmable thermostat makes it easy to dive the temp at night and let things coast during the day.
The first day on this regimen will require a longer air conditioner run to initially remove water vapor, but thereafter the house will remain a low humidity, cool refuge all 24 hours. Our home remains very comfortable throughout 24 hours and we only use about 1.5 hours of compressor time total. As a result of adopting night time air conditioning, our electric bill for summer vs winter is at most 5% higher AND I am always comfortable.
Night time air conditioning has both decreased my air conditioning costs and improved comfort level. Sadly, I can no longer find the original paper. I suspect it was pulled because it contained showed energy savings from a strategy contrary to the "obvious" energy saving open your windows at night mantra.
Guy Kuo
I agree with your comment about keeping humidity out of the house during the summer -- I'll have to add a tip about that. And air conditioner compressors do work more efficiently when it's cooler out. I suppose the real question is, just how well-insulated is your house? Could the average home actually "coast" through 12 or more hours of sunshine?
What part of the country do you live in? That could also have an effect.
Yes, a well insulated home and keeping the window shades closed during the day are key. We're in the pacific northwest and have a very well insulated house as well as cellular shades and solar film on all windows. It does indeed get hot in the summer even here, but night time air conditioning has worked very well for us even with 90+ degree days. Having the house at 67 degrees in the morning gives you a fair amount of thermal inertia.
I would never have thought this would work had I not read the Florida article. I'd dive the house to 65 at night but the wife won't tolerate it going that low. 67 was a compromise that gets our house cold enough to coast through a summer day and stay below 80 degrees until the next evening. We hear our neighbor's running their air conditioners all through the day and opening their windows at night. Meanwhile we happily ride through in comfort and only use a little compressor time while it is more efficient. With as much has half the air conditioning energy during a run is simply removing humidity, this can add up to better overall efficiency if your house is insulated well enough.
Guy Kuo
I like what you have to say and would like to subscribe to your newsletter -- no, seriously, would you like to make this into a formal article for Energy Watcher?
Why not use a whole house fan at night to bring temps down and after the house gets down to a low temp, seal the house up and run a dehumidifier to remove the humidity? A dehumidifier cost a lot less to run than an air-conditioner.
Anonymous, very few houses built after the 60s have whole house fans, and the installation costs to make one are not exactly pleasant.
To add to your costs, have you ever looked at how much dehumidifiers are? They're like, 150-200 dollars, and it might take a larger one or more to be effective throughout your house.