You’re going to need a lot more information about the ovens to figure that out. https://oasisenergy.com/appliances-natural-gas-usage/ Here’s this; and once you get the info about the ovens the math is as simple as plugging it in, not time consuming or difficult at all. However, if it’s coal, not only will that require SIGNIFICANTLY less than gas since coal burns for so ridiculously long, but Germany during WWII was on of the largest coal producers on the planet and is still one of the biggest producers today. Gas would be continuously pumped in to burn, no gas, no fire. Coal can burn for a loooooooong time. A hell of a lot longer than five minutes, so you’re going to be getting several batches of cookies out of each use.