One unit of electricity is one kilowatt hour, ie 1000 watts for 1 hour, which is the same as 100 watts for 10 hours (or 10 watts for 100 hours).
I don't know what the wattage is for an incubator, but it will be marked on the label. Multiply the wattage by the total time in use and you will get the total number of units of electricity.
Electricity costs about 11p per unit, after the initial tariff.
John