Let's cut straight to the chase. The sweet spot for any refrigerator is between 39 and 40 F. This is the temperature at which bacterial growth is inhibited but the difference between your refrigerator and the ambient temperature outside of your refrigerator is minimized.
In other words, keeping your refrigerator's internal temperature at about 39 F will keep your food safe while minimizing your energy costs.
Simple, right? Well, not quite so simple.
Your refrigerator is one of the biggest power guzzlers in your house. Over the course of a year, a modern refrigerator sucks down about 350 kWh of energy. That adds up to about $50. With a few simple one-time changes taking just a few minutes, though, you can cut that by about 20%, saving you $10 a year for as long as you own the fridge.
First of all, most refrigerators have a terrible internal dial that doesn't indicate actual temperatures. I loathe this design "feature." It doesn't tell you a thing about the actual internal temperature of your fridge. It merely compares it to other settings.
The simple solution? Get an inexpensive thermometer and stick it in your fridge. You can get a small digital thermometer at your local hardware store for a pittance. Just tape it somewhere to the inside of your refrigerator. I just rolled up a bit of duct tape and stuck it to the back of one for placement inside of the refrigerator.
Then, start playing with that dial. I suggest moving slowly downward, then checking the temperature every day. What you're looking for is a temperature close to 38.5 or 39, at which point you're going to want to stop on that setting for a while. For most refrigerators, this temperature coincides with the lowest setting or one of the lowest settings.
It's important to remember that the temperature isn't an exact thing. A refrigerator works just like your home. It tries to keep the temperature within a degree or so of the ideal temperature. This is why you should stop adjusting the temperature dial for a while if you observe a temperature around 38.5 or 39 degrees, as this may be a "low" temperature, a "high" temperature, or an average one. Do some observations over time and see what the average is.
I would shoot for an average of around 38.8 to 39 F, so that the "high" temperature doesn't break 40 F very often.
Also, pay attention to where the fan is in your refrigerator. The air coming in there is going to be cold air, so the items near it are going to be colder than items elsewhere in the refrigerator. We generally keep items that we worry the most about spoiling close to the fan (items such as milk and eggs) and other items further away. I placed the temperature gauge on the other side of the refrigerator interior from the fan to try to get more of an ambient temperature. Also, never block the fan. Always make sure the fan can blow cold air into the interior of your refrigerator. If you block it, you'll have one frozen item and a bunch of warm items in your fridge.
If your refrigerator is really full with items, lower the temperature a bit. Lots of items means poor air flow inside of your refrigerator, which is the key to keeping things cool. Lowering the temperature means you're going to be using a bit more energy, but all of your items are cold. We do this when we're prepping for a party or something similar.
What about the freezer? You want your freezer to be low enough so that temperature variation does not allow anything to melt in the freezer. I suggest keeping the freezer at a temperature around 5 F - and a bit lower than that if your freezer is jam-packed with stuff, blocking the air flow. You can check this with a thermometer taped to the side away from the fan, just like with the refrigerator.
Another useful tactic is to thaw frozen foods in the fridge instead of on the counter. As frozen foods thaw, they cool the air around them. This is a good thing in a refrigerator that you want to keep cool internally. It's a bad thing most of the time outside of the refrigerator, as it's not going to provide a significant enough effect to keep your air conditioner from running any significant amount and it's going to work in a small way against your furnace.
Once the temperature is set correctly on your refrigerator and freezer, and you're using the device sensibly (not blocking the air flow, thawing foods in the refrigerator, etc.), your refrigerator is going to use less energy. It's also not going to have to work as hard, extending its lifespan. Both of these save you money, as your monthly energy bill is reduced and the time until your next refrigerator replacement is extended.
This post is part of a yearlong series called "365 Ways to Live Cheap (Revisited)," in which I'm revisiting the entries from my book "365 Ways to Live Cheap," which is available at Amazon and at bookstores everywhere. Images courtesy of Brittany Lynne Photography, the proprietor of which is my "photography intern" for this project.