The theory is that they can then run the numbers and identify which homes and offices are being rented out without the income being declared. If an address is marked as empty for fiscal purposes, but power is being used…. around goes an inspector and a large fine issued to the owned.
Utility companies must supply Hacienda with the details of the person paying the bill, the address, castastro information and amount being consumed.
Of course, you have until March to declare on your IRPF any income from rental properties, so if worried…. pay your taxes.