Potrošnja vode i potrošnja energije su niske.

Because after the noun potrošnja (consumption), Croatian normally uses the genitive case to mean consumption of something.

So:

• potrošnja vode = consumption of water
• potrošnja energije = consumption of energy

The basic dictionary forms are:

• voda → vode
• energija → energije

This is a very common Croatian pattern: a noun is followed by another noun in the genitive to show what it relates to.

Yes, it could.

Your sentence says:

• Potrošnja vode i potrošnja energije su niske.

This treats water consumption and energy consumption as two parallel items.

A very natural alternative is:

• Potrošnja vode i energije je niska.

That version has only one potrošnja, and it means something like water and energy consumption is low as one combined idea.

So the difference is roughly:

• potrošnja vode i potrošnja energije = two separate but parallel kinds of consumption
• potrošnja vode i energije = one combined phrase

Both are grammatical and natural.

Because the subject is plural in meaning.

The sentence has two coordinated noun phrases:

• potrošnja vode
• potrošnja energije

They are joined by i (and), so together they form a compound subject. In Croatian, that normally takes a plural verb:

• su = are

If you used only one head noun, like this:

• Potrošnja vode i energije je niska.

then the verb would be singular:

• je = is

So:

• X i Y su ... = X and Y are ...
• Potrošnja vode i energije je ... = one singular noun phrase, so is

Because the adjective must agree with the subject.

Here the subject is made of two coordinated feminine singular nouns:

• potrošnja vode
• potrošnja energije

Together, they are plural, so the adjective is also plural:

• niske = low (feminine plural)

If the sentence had only one singular subject, you would use niska:

• Potrošnja je niska.
• Potrošnja vode i energije je niska.

So the agreement works like this:

• singular feminine → niska
• plural feminine → niske

The dictionary form is nizak.

This can be confusing, because the forms change quite a bit:

• masculine singular: nizak
• feminine singular: niska
• neuter singular: nisko
• feminine plural: niske

So in your sentence, niske comes from nizak and agrees with the plural feminine subject.

This kind of stem change is normal in Croatian adjectives.

Yes, absolutely.

That sentence is also correct and very natural.

The difference is mostly in structure and emphasis:

• Potrošnja vode i potrošnja energije su niske.
  Emphasizes two separate types of consumption.

• Potrošnja vode i energije je niska.
  Presents them as one combined category.

In English, both might translate similarly, but Croatian makes this distinction more clearly through grammar.

I means and.

It connects:

• potrošnja vode
• potrošnja energije

Because it joins two subject elements, the whole subject becomes plural in meaning, which is why you get:

• su
• niske

So i is not just linking words mechanically; it also affects agreement in the sentence.

Yes. Croatian word order is fairly flexible.

The neutral, most straightforward order here is:

• Potrošnja vode i potrošnja energije su niske.

That is a normal subject + verb + adjective structure.

Other orders are possible for emphasis or style, for example:

• Niske su potrošnja vode i potrošnja energije.

This is grammatical, but more marked. A learner should usually stick to the original order unless there is a reason to emphasize niske.

So the given sentence is the safest and most natural default.

Because Croatian does not have articles like English a/an/the.

So:

• potrošnja vode
• potrošnja energije

can mean either a general idea or a specific one, depending on context.

Croatian speakers understand definiteness from the situation, the surrounding sentences, and sometimes word order or emphasis.

That means you often have to stop expecting a separate word for the in Croatian.

A rough pronunciation guide is:

• po-TROSH-nya

A few useful details:

• š sounds like English sh
• nj sounds like the ny in canyon or piñata
• stress is roughly on tro

So potrošnja sounds approximately like:

• po-TROSH-nya

This is only an approximation, but it is close enough for a beginner.