The Air Conditioning Battle

Summer is approaching (though from the unseasonably cool temperatures here, you could have fooled me) and with the approach of summer means summer cooling bills. Many, many households across the United States will be trying to maximize the cooling they can get from their central air units, window air conditioning, and fans in order to minimize their energy bills.

There are a number of steps that are often tossed out there as “standard” advice, such as installing a programmable thermostat, finding the upper limit of what you can tolerate inside, opening windows in the night time to let in cooler air, closing the curtains and/or blinds to block direct sunlight, and so on.

In fact, I listed several of these tips in a recent article, Seven Simple Tactics We Use to Keep Summer Energy Bills Low.

After posting that article, I received some feedback from astute frugal readers out there. Most of that feedback centered on whether or not it made sense to leave the air conditioning on all day when you were at work or just turning it on as soon as you get home. Some people leaned strongly one way, while others disagreed.

First of all, a programmable thermostat helps to solve the problem somewhat no matter where you live. A good programmable thermostat allows you to set your home’s climate control to stay off completely during the day unless the temperature gets above a certain very high threshold. That way, it only runs during the day on the very hottest of days. On other days, it just kicks on as normal before you get home, lowering the temperature down to what you’d like. If you don’t mind getting home to a somewhat warm house, set the program later. Most programmable thermostats are pretty easy to use once you play around with them a bit.

However, there are a few additional ingredients that are worth adding to the mix.

The biggest factor is the policy of your utility company. When are the energy rates highest? Some energy companies have relatively low rates in the morning, but jack them up in the afternoon when people come home from work, while others have high rates all day.

If your company raises rates in the afternoon, you’re likely better off running it on an all-day cooldown so that you’re not running the air conditioning for a long solid block in the afternoon. If it’s high all day, a single chunk of air conditioning in the afternoon might be more effective.

Next, how big is your home? A larger home is going to hold the temperature steadier than a smaller home, but if you allow it to rise too much, it will take a long time to cool down.

The smaller your residence, the more sense it makes to cool it down at the end of the day instead of keeping it steady throughout, whereas in a larger residence, keeping the temperature lower might make sense. This lines up particularly well with the changing energy rates described above. I would strongly discourage you from cooling down a large home in the afternoon in Nevada!

The next thing to consider is what times of the day shade covers your home. Shade will cause the temperature to rise more slowly than it would under direct sunlight. Does your home receive shade in the middle of the day? If so, that mid-day temperature probably won’t spike as high as it would without that shade.

If your home is shaded, an afternoon cooldown will probably work best as your house won’t get as hot in the mid-day sun. If your home is under direct sunlight all day, you’re much more likely to have a high temperature spike during the middle of the day, meaning that all day cooling (and closed curtains!) might be your best bet.

All of these factors – and more – play a role in which method actually works better for you.

So, what should you do?

The first step I would take is to check out my energy bill. When do the rates go up and down? Considering that rates can sometimes shift as much as $0.40 per kilowatt hour (!) when you switch from prime periods to lower use periods, this can make or break any argument.

For example, Nevada Energy has a residential time-of-use package that brings the rate up to $0.50 per hour from 2 PM to 7 PM in the summer, but it’s $0.07 per hour the rest of the time. In those conditions, it makes sense to keep the house cool during the day, have the air conditioning run to get the house really cool just before 2 PM (thanks to a programmable thermostat), then switch off unless the inside temperature gets really high again.

A programmable thermostat can help with this approach, allowing you to cool the house down considerably during the low-cost hours and then switching off during the expensive hours.

What about the other factors? If the energy costs do not switch during the day, I would experiment. Try using one method for several days, then switch to a different method. Keep track of temperatures, particularly daytime highs. Work with your energy company to see what your daily use looks like during those different periods when you’re experimenting.

You should also consider lots of passive methods for cooling in addition to the general tips listed above. The Natural Building Blog has a great list of passive cooling tactics that you should really look into. A few of my favorites:

– keeping vegetation moist around the house to help cool the breezes (the yard)
– plant trees that don’t block breezes
– open plan living areas that encourage air circulation
– light roof color that reflects sunlight

Still, if you take home one message from this article, it’s that you should plan your home cooling to avoid the most expensive energy periods from your energy company. Do whatever it takes to plan around those periods and the worst sting of summer cooling goes away.

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