L'indice di prestazione energetica dipende da questa superficie utile.

L' is the shortened form of the definite article il before a masculine singular noun that begins with a vowel.

• il libro = the book
• l'indice = the index

Here, indice is masculine singular and starts with i, so il indice becomes l'indice.

It is a noun phrase built around indice.

• indice = index
• di = of
• prestazione energetica = energy performance

So literally it is index of energy performance. In more natural English, this is often rendered as energy performance index.

This is a very common Italian structure:
noun + di + noun phrase

For example:

• centro di città = city center
• livello di qualità = quality level

The preposition di often corresponds to of in English and is widely used to connect nouns.

Here it links indice to what kind of index it is:

• indice = index
• di prestazione energetica = of energy performance

Italian uses this structure much more often than English, where English may prefer a noun used as an adjective, as in energy performance index.

Because energetica agrees with prestazione, not with indice.

• prestazione is feminine singular
• so the adjective must also be feminine singular: energetica

The structure is:

• prestazione energetica = energy performance

Even though the whole phrase depends on indice, the adjective stays with the noun it directly describes, which is prestazione.

Because dipendere normally takes the preposition da in Italian.

So:

• dipendere da qualcosa = to depend on something
• dipendere da qualcuno = to depend on someone

This is just the standard pattern of the verb. You should learn it as a unit:

• dipendere da

Examples:

• Tutto dipende da te. = Everything depends on you.
• Il risultato dipende dal tempo. = The result depends on the weather/time.

Because contractions such as dal, dalla, dai, dagli, dalle happen when da is followed by a definite article.

For example:

• da + il = dal
• da + la = dalla

But questa is a demonstrative adjective, not an article, so there is no contraction:

• da questa superficie
• not dquesta or anything similar

Because questa must agree with superficie, which is feminine singular.

• superficie = feminine singular
• so the demonstrative is questa = this

Also, utile describes superficie:

• superficie utile = usable surface / usable area

So the whole phrase means this usable area or this useful/usable surface, depending on context. In technical language, superficie utile often refers to usable floor area or net usable area.

Some Italian adjectives have the same form for masculine and feminine singular. Utile is one of them.

Compare:

• un dato utile = a useful piece of data
• una superficie utile = a useful/usable surface

The singular form stays utile for both genders. Only the plural changes:

• utili

So:

• masculine singular: utile
• feminine singular: utile
• masculine plural: utili
• feminine plural: utili

Because the subject is l'indice, which is singular.

Structure:

• L'indice ... dipende ...
• The index ... depends ...

Even though there are several words after indice, the main subject is still just indice, so the verb must be third person singular: dipende.

Yes, the sentence uses a very natural and standard Italian order:

• L'indice di prestazione energetica = subject
• dipende = verb
• da questa superficie utile = complement

Italian can sometimes move elements around for emphasis, but this version is the most neutral and normal. It is especially typical of formal or technical Italian.

You could word-for-word, but in many contexts that would sound unnatural in English.

In technical Italian, superficie utile usually means something like:

• usable area
• usable floor area
• net usable area

So although utile often means useful, in this kind of phrase it is better understood as usable. This is a good example of why technical phrases should often be learned as a whole, not word by word.