The difference 20 degrees makes

For energy bills and healthy AC systems

It’s 100 degrees, and you’re wondering – how did my energy bill go up when I haven’t changed my thermostat. The answer is simple and yet complex. Your thermostat is still at the same setting, but the weather outside is not. It’s much hotter. Still, you ask, how can that possibly affect my bill – the thermostat hasn’t been changed to make it colder inside. It’s on the same setting.

This is where it gets a little more complicated. Air conditioning systems are not designed for extreme heat. They are made to cool indoor air to about 20 degrees from outdoors. Let’s call it the 20-degree differential. So, let’s say the thermostat is on 76 degrees. A couple of months ago when temperatures averaged about 92 to 95 degrees, your AC system – if well maintained – worked efficiently. Then, July brought us some intense heat. Except for two days, every day last month was 97 or higher with one reaching 107. Nearly half the month recorded triple degree weather and now we’ve seen nearly 30 days of this intense heat.  

Suppose it’s 101 degrees outside, and your thermostat is still set on 76, that’s a difference of 25 degrees. And, that’s before factoring in the heat index, which means there’s more humidity in the air for your system to remove, which is one of its four functions. It’s more than an AC system can handle without causing the compressor to work hard, resulting in the system running for longer periods and shorter off-cycle times, or breathers. The result is greater energy use and higher bills because cooling makes up the majority of the bill.

AC units are designed to: cool the home, filter the air, circulate the air, remove humidity

Want proof? Try a simple experiment. Before 10 a.m., start logging when your AC system comes on and for how long, noting your thermostat setting. Log it at the top of every hour throughout the day. You’ll notice that as the outside temperature increases, the system stays on longer, and the rest periods between cycles are much shorter.

Looking for additional third-party expertise? There’s an abundance of blog posts from AC repair companies (based in hot climates like ours) and we’ve included some here to check out:

Why Your Air Conditioner Will Not Cool Well When the Temperature is 100°

Why Your AC Can’t Keep Up With The 100 Degree Heat Wave

How Many Degrees Cooler Can Your Air Conditioner Actually Make Your Home?

Why Won’t My AC Cool Below 75 Degrees?

Top 14 Things You Should Know About Your AC

Why is the compressor working so hard? Part of the air conditioning process involves the system taking the hot air from your home and sending it outside. As it does that, there’s a heat exchange that happens. To simplify, that exchange works best when the outside air being exchanged is cooler than the air being released. Once we climb into 100-degree weather, the air exchange is not as efficient. If you’ve ever wondered why so many AC systems break down in the heat of summer when needed most, weather, your thermostat setting, and the simple mechanics of how the system works are the culprits. Poor maintenance increases the risk.

Unfortunately, we didn’t get much relief this month. Temperatures of 98 or triple degrees have smothered us as we wrap up what’s typically the hottest month of the year.

But all is not lost. Now that you know about the 20-degree differential, there are some simple steps you can take to better manage your bill and keep your AC from breaking down. Check out these tips.

Tips to keep AC bill down

The good news? Fall starts officially in less than a month on Sept. 21.

Christine Patmon

Christine is part of the Corporate Communications team at CPS Energy.

Christine Patmon