There are two types of conventional oven. Fan assisted and non fan assisted.
In a fan assisted oven the air at the top shelf is the same temperature as the bottom because the air is circulated.
You can put a pizza in the top or the bottom shelf and it will be the same.
Non fan assisted ovens are slightly more power efficient when you are only using the top shelf because they don't power a fan and the heat is focused on the top shelf so the food cooks more quickly. There is also less openings for the heat to escape out of.
For cooking on more than one shelf it is better to use a fan assisted oven because the heat is redistributed evenly.
On some fan assisted ovens you can turn the fan off.
