Potrošnja energije je na energetskoj oznaci.

Because energije is the genitive singular of energija.

In potrošnja energije, Croatian is using a very common pattern:
noun + genitive = consumption of energy, price of the product, door of the house, etc.

So:

• energija = energy
• energije = of energy

That means potrošnja energije literally has the structure consumption of energy.

Because na here means on, and after na Croatian often uses the locative case when talking about location.

So:

• oznaka = label
• energetska oznaka = energy label
• na energetskoj oznaci = on the energy label

Both words change because they are in the locative singular feminine:

• energetska → energetskoj
• oznaka → oznaci

This is normal adjective-noun agreement in Croatian.

Because oznaka is a feminine noun ending in -a, and in the locative singular many such nouns change like this:

• oznaka → oznaci
• škola → školi
• kuća → kući

So after na when it means location, oznaka becomes oznaci.

Because adjectives in Croatian must agree with the noun they describe in:

• gender
• number
• case

Here the noun is oznaci, which is:

• feminine
• singular
• locative

So the adjective must match:

• energetska oznaka = nominative
• na energetskoj oznaci = locative

That is why you see energetskoj, not energetska.

Potrošnja is a feminine noun meaning consumption or usage.

It comes from the verb trošiti / potrošiti, meaning to spend, to consume, or to use up, depending on context.

Examples:

• potrošnja vode = water consumption
• potrošnja goriva = fuel consumption
• potrošnja energije = energy consumption

So in this sentence, potrošnja is the subject.

Je is the 3rd person singular form of biti (to be), so it means is.

The sentence structure is straightforward:

• Potrošnja energije = the subject
• je = is
• na energetskoj oznaci = on the energy label

So it works like:

Energy consumption is on the energy label.

In normal standard Croatian, you would usually keep je in this kind of sentence.

So:

• Potrošnja energije je na energetskoj oznaci. = normal and correct

In some informal situations, Croatian can omit forms of to be, but not usually in a plain statement like this. For a learner, it is safest to keep je.

Because Croatian distinguishes between different kinds of location.

• na often means on, at, or sometimes on a surface / on a medium
• u usually means in / inside

A label is thought of as a surface or printed place where information appears, so na is natural:

• na etiketi = on the label
• na stranici = on the page
• na ekranu = on the screen

So na energetskoj oznaci means on the energy label, not inside it.

Yes, it is a standard and natural expression.

• energetski = energy-related, energetic in the technical sense
• oznaka = label, marking, indication

So energetska oznaka is the usual term for the kind of efficiency/energy information label found on appliances and products.

A learner might think of etiketa, which also means label, but energetska oznaka is the better fit in this context.

The neutral word order is:

Potrošnja energije je na energetskoj oznaci.

Croatian word order is flexible, so other versions are possible, for example:

• Na energetskoj oznaci je potrošnja energije.
• Potrošnja energije na energetskoj je oznaci.

But these may sound more marked or emphasize different parts of the sentence.

For learners, the original order is the safest and most neutral.

The letter š is pronounced like sh in ship.

So:

• potrošnja sounds roughly like po-TROSH-nya
• energije roughly e-NEHR-gi-ye
• oznaci roughly oz-NA-tsi

A few useful pronunciation notes:

• š = sh
• nj is like the ny sound in canyon
• c in Croatian is usually like ts

So potrošnja has that shn cluster: potrosh-nya.

Because Croatian has no articles like English a and the.

So Croatian simply says:

• potrošnja energije
• energetska oznaka

Whether English translates that as energy consumption, the energy consumption, an energy label, or the energy label depends on context.

That is completely normal in Croatian.

Yes, in many practical contexts it can.

Even though energija literally means energy, on appliances and technical labels potrošnja energije is often the natural Croatian phrase used where English might say:

• energy consumption
• power consumption
  depending on the exact context

So the Croatian wording is normal even if the English translation varies a bit.

Yes, it is grammatical and natural, especially in an informational or technical context.

It sounds like the kind of sentence you might see in:

• product documentation
• appliance instructions
• consumer information
• technical descriptions

If someone wanted to sound a little more explicit, they might also say something like:

• Podatak o potrošnji energije je na energetskoj oznaci.

But the original sentence is perfectly fine.