A few misconceptions on what the "potencia" and upgrade is. Firstly, all properties from 2002 onward should be more than capable of handling the 5.75 kw required (it was 5.5 until Spain and other EU countries standardised their voltage to 230v - Spain went up, UK went down). 3.3 kw is insufficient to handle the modern appliances required, dishwasher/washing machine/big TV/kettle/possibly aircon etc and should be upgraded to 5.75.
Firstly you will need a boletin. This is certificate of electrical safety and must be completed by a qualified electrician. The power company will charge you around 600 euros for this but a good, qualified electrician will be able to do this for around 250 euros. Anyone on the S Costa Blanca who needs one, give me a nod and I will PM you a very good electrician who can do this. Once the fuse box has been checked to ensure all circuits are correctly wired (and they will be if you have more than 5 breaker circuits in your fuse box) the electrician can fit the ICP for around 30 euros.
Once this is done and you have the boletin and ICP, you need to take out a new contract with Iberdrola. Their charges are as follows:
Just over 17 euros per extra kilowatt. Access rights charge (no, I don't know what that means) 19 euros per extra kilowatt. Once this is done, Iberdrola will inspect and seal the ICP and charge 9 euros for the privelege. Of course, VAT is then added.
If you have a property over 20 years old, you may need the cabling to the property changed for a minimum 6mm cable.
If your property is 10 years old or less, it will probably cost about 350 euros all told.
Of course, if you are quite happy to lose the TV every time you switch the kettle on or lose your heating when you want to run the washing machine, keep to the 3.3.