Razred energetske učinkovitosti je visok.

Here razred means class, category, or rating.

So razred energetske učinkovitosti is the energy efficiency class/rating of something, such as an appliance or building.

A useful thing to remember is that razred can also mean school class in other contexts, but here it is clearly a technical label or category.

Because after razred Croatian uses a structure that works like class of energy efficiency.

That means energetska učinkovitost changes into the genitive case:

• nominative: energetska učinkovitost
• genitive: energetske učinkovitosti

So:

• razred = class
• energetske učinkovitosti = of energy efficiency

This is very common in Croatian: one noun is followed by another noun in the genitive to show of.

Because energetske is an adjective modifying učinkovitosti, so it has to match that noun in case, gender, and number.

The base phrase is:

• energetska učinkovitost = energy efficiency

Here:

• energetska is feminine singular nominative
• učinkovitost is feminine singular nominative

After razred, the whole phrase goes into the genitive:

• energetske = feminine singular genitive
• učinkovitosti = feminine singular genitive

So both words change together.

Because visok agrees with the subject razred, not with učinkovitost.

The subject of the sentence is:

• Razred energetske učinkovitosti = the energy efficiency class

The main noun there is razred, and razred is masculine singular.
So the predicate adjective must also be masculine singular:

• visok

Compare:

• Razred je visok. — masculine singular
• Učinkovitost je visoka. — feminine singular

English speakers often look at the nearest noun, but in Croatian the adjective agrees with the actual grammatical subject.

Je is the 3rd person singular present tense of biti = to be.

So the sentence structure is:

• Razred energetske učinkovitosti = subject
• je = is
• visok = high

Literally: The energy efficiency class is high.

Croatian often needs je in this kind of sentence, just like English needs is.

Because Croatian does not have articles like a and the.

So Razred energetske učinkovitosti je visok can mean:

• The energy efficiency class is high
• or, depending on context, An energy efficiency class is high

In real use, context tells you whether it is definite or not.
In many technical or descriptive sentences, English will naturally translate it with the.

Yes. Visok normally means high or tall, depending on context.

In this sentence it means high in the sense of at a high level or good/high-rated.

Croatian often uses physical adjectives like this in broader meanings too, just as English does:

• visoka temperatura = high temperature
• visoka cijena = high price
• visok standard = high standard

So razred ... je visok is understandable as the rating/class is high.

Yes, Croatian word order is fairly flexible, although the given sentence is a normal neutral order.

You may also see variations such as:

• Visok je razred energetske učinkovitosti.
• Razred energetske učinkovitosti visok je.

But Razred energetske učinkovitosti je visok is the most straightforward and neutral for a learner.

A useful note: je is a clitic, so it often appears near the beginning of the sentence, typically after the first phrase.

Yes, it is a common technical/administrative expression.

It is used especially for:

• appliances
• buildings
• official product descriptions
• energy labels

So learners should treat it as a useful set phrase meaning energy efficiency class/rating.

You could, and it would help you understand the grammar, but in natural English it sounds a bit awkward.

More natural translations would be:

• The energy efficiency rating is high.
• The energy efficiency class is high.
• It has a high energy efficiency rating.

So the literal structure is useful for learning Croatian grammar, but the best English wording may be slightly different.

The dictionary forms are:

• razred = class, category, grade
• energetski / energetska / energetsko = energy-related
• učinkovitost = efficiency
• biti = to be
• visok / visoka / visoko = high, tall

In the sentence, some of these appear in changed forms:

• energetske učinkovitosti instead of energetska učinkovitost
• je instead of the full infinitive biti
• visok stays in masculine singular because it agrees with razred