Does the dehydrator have a tech info plate or sticker somewhere on it? That should tell you the wattage it draws. Then call your electric supplier to determine the charge per kilowatt. After that, it is (machine-wattage) times (# of hours) times (kilowatt-rate). Now, you will have to do a conversion as follows: If the machine is in watts, then you will have to change machine wattage to machine-kilowattage. To do this, just divide machine wattage by 1000
An example: your electricity cost 7 cents per kilowatt hour, your dehydrator uses 600 watts, and you run it 18 hours
(600/1000) x 18 hours x .07 = $1.84