If the battery is used at the maximum power, at least 500AH, there is no calculation efficiency. If the inverter efficiency is 0.8, the battery is 625AH. But the cookie machine I know won't always use high power. You can make a discount according to the actual use. This will give you a basis.

You mean the waffle maker that makes cookies? If this is the case, an inverter above 1200W W can be used.