How cold are your freezers? I mean: not just in principle, but really?
Having your freezer set at -20ºC doesn´t necessarily mean that your freezer IS -20ºC. In fact, most freezers are NOT -20ºC when they are supposed to be!
Freezers need a lot of energy to stay cool -- and the colder they are, the more energy they consume.
A freezer at -24ºC generally uses 20% more energy than a freezer at -20ºC. And if your freezer is -28ºC, it's using 50% more energy than a freezer at -20ºC!
Checking that your freezer REALLY is -20ºC thus has great potential for the planet and your energy bill.
Remember to make sure that the display is showing THE ACTUAL TEMPERATURE, not the set temperature, before making any conclusions. And in case your freezer doesn´t have an accurate temperature display: use a portable thermometer to check.