What is needed is an upgrade to the ICP (Interruptor control de potencia) to the Kw that you want. This can be 4.6Kw (20A) or 5.75Kw (25A).

Normally for an upgrade you just need a certificate fron a registered electrician and the change of ICP.

The certificate is 100 euros, the ICP depends on the consumption and the work is normally around 30 euros.

The change may depend on the quality of your existing installation.

Call me if you want me to take a look - 620353138

Nigel