Hi Jeannette. Three phase... that means almost certainly it is more than 2.5kW. How long do you run the kiln for? As an example, 3 hours at 5kW makes 15kWhours, and on a 24V battery system than means 208A for 3 hours, so at least 1500Amphours of battery power 1t 12V would be needed.
Next question is how often is the kiln used - twice a day, once a week? That would determine the size of the solar panels and wind generator needed to recharge the batteries. Becasue of the economic risks of running out of power half way through a 'baking' it seems like you would need a generator capable of running the kiln as a backup in case the sun doesn't shine and the wind doesn't blow.