Preheating the oven rarely makes sense - but it is absolutely necessary for these dishes. According to the federal government, if you forego preheating, you can save around eight percent on electricity.
Heating a baking tray or the rack also requires additional energy. The federal government also suggests baking with the convection function. This saves 15 percent compared to top and bottom heat. Five minutes should be enough to bring oven to the specified temperature. For cakes you can also use a stick test to determine how long the pastry should remain in the oven.