Most recipes tell us to preheat the oven. This is not always needed. For items that require a strict amount of time at a particular degrees, such as cakes, desserts and the like, you will want to continue to preheat the oven. But for items, such as roasts, baking chicken, etc. put the pan in once you turn the oven on. This prevents wasting heat and resources that are needed to create the heat.
A similar way to save heat and therefore resources when cooking can be done on the back end. Just as the oven needs time to heat up it will also naturally cool off. And generally when the oven is cooling off, we are not utilizing the heat, therefore its wasted heat and resources. So to reduce the number of times this happens during a week or a month, you can plan for batch cooking. You can cook a roast, bake potatoes or other vegetables, bake chicken, etc. Depending on your family’s needs, you can cook all the meat for a week at a time and freeze the items that won’t be used right away.