Samoopskrba i vlastita potrošnja smanjuju račun za električnu energiju.

Because the subject is actually two things joined by i:

• samoopskrba
• vlastita potrošnja

Together they form a compound subject, so Croatian uses a 3rd person plural verb:

• smanjuju = they reduce

This is similar to English Self-supply and own consumption reduce...

Because vlastita has to agree with potrošnja in:

• gender
• number
• case

Here, potrošnja is:

• feminine
• singular
• nominative (because it is part of the subject)

So the adjective must also be:

• feminine singular nominative → vlastita

Compare:

• vlastit stan = one’s own apartment
• vlastita kuća = one’s own house
• vlastito vozilo = one’s own vehicle

Samoopskrba literally means something like self-supply or self-provision.

It is built from:

• samo- = self-
• opskrba = supply/provision

In an energy context, it usually refers to producing or covering your own energy needs, for example with solar panels.

So it is a fairly formal or technical word, not just an everyday household word.

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

So:

• račun can mean a bill, the bill, or just bill in a general sense

You understand which one is meant from context.

That is normal in Croatian. English speakers often expect an article, but Croatian simply does not use one.

Because račun is the direct object of smanjuju, so it is in the accusative case.

However, račun is a masculine inanimate noun, and for many masculine inanimate nouns, the:

• nominative singular and
• accusative singular

look the same.

So:

• nominative: račun
• accusative: račun

Even though the form does not change, the case function still changes.

Here za means for, as in a bill for electricity.

After za in this meaning, Croatian uses the accusative.

So:

• električna energija = nominative
• električnu energiju = accusative

Both words change because the adjective must agree with the noun:

• električnu = feminine singular accusative
• energiju = feminine singular accusative

So the phrase means for electrical energy / for electricity.

It is correct, but it sounds more formal or technical than the everyday word struja.

So:

• struja = everyday electricity
• električna energija = formal/technical electrical energy

In official, legal, or energy-sector language, električna energija is very common.

It is present tense, 3rd person plural.

So literally:

• smanjuju = they reduce / are reducing

In this sentence, it expresses a general fact or usual effect, not necessarily something happening only right now.

So the sense is:

• Self-supply and own consumption reduce the electricity bill
• or can reduce the electricity bill

The form smanjuju is from the imperfective verb smanjivati.

This matters because Croatian often uses:

• imperfective verbs for ongoing, repeated, or general actions
• perfective verbs for completed actions

Here the sentence describes a general process/effect, so the imperfective form is natural.

Very roughly:

• smanjivati = to be reducing / to reduce regularly
• smanjiti = to reduce once, to make smaller completely

Yes. Croatian word order is more flexible than English.

This sentence has a neutral, natural order:

• Samoopskrba i vlastita potrošnja smanjuju račun za električnu energiju.

But you could also say:

• Račun za električnu energiju smanjuju samoopskrba i vlastita potrošnja.

That version puts more focus on the electricity bill.

So the meaning stays basically the same, but the emphasis can shift.

Yes, that is the usual idea here.

In an energy context, vlastita potrošnja often means self-consumption: electricity that you produce and use yourself, instead of taking it from the grid.

So although the literal meaning is own consumption, the practical meaning is often closer to:

• self-consumption
• consumption of one’s own generated electricity

Often yes, because the verb ending already shows the person and number.

Here:

• smanjuju already tells you it is 3rd person plural

And the sentence also has the full subject:

• samoopskrba i vlastita potrošnja

So adding a pronoun like oni would usually be unnecessary unless you wanted special emphasis.