Nem azért használunk LED-izzót, mert olcsó, hanem azért, mert energiahatékony.

This is a very common Hungarian structure meaning:

not because ..., but because ...

So:

• Nem azért használunk LED-izzót, mert olcsó = We do not use LED bulbs because they are cheap
• hanem azért, mert energiahatékony = but because they are energy-efficient

Literally, azért means something like for that reason, so the whole sentence is built around contrasting two possible reasons.

Here azért does not mean therefore. In this pattern, it means for that reason or for that purpose/reason.

So:

• azért, mert ... = for the reason that ... / because ...

In this sentence, azért helps introduce the reason being discussed:

• nem azért ... = not for that reason ...
• hanem azért ... = but for that reason ...

This is why the sentence sounds much more natural in Hungarian than a simple word-for-word version.

Because each half of the contrast has its own reason clause.

• nem azért ..., mert olcsó
• hanem azért, mert energiahatékony

Each mert introduces a full explanation:

• mert olcsó = because it is cheap
• mert energiahatékony = because it is energy-efficient

English also often repeats because in this kind of contrast: not because it is cheap, but because it is energy-efficient.

Because hanem is the normal Hungarian word for but rather, especially after a negation.

Since the first part contains nem:

• Nem azért ...
• the contrasting continuation naturally uses hanem

So:

• nem ..., hanem ... = not ..., but rather ...

By contrast, de usually means a more ordinary but:

• Olcsó, de nem jó. = It is cheap, but not good.

In your sentence, the meaning is not just simple contrast; it is correction/replacement of one reason with another. That is exactly what hanem is for.

Használunk is the 1st person plural indefinite form of használ = to use.

It means:

• használunk = we use

Hungarian verbs have two conjugations:

• indefinite
• definite

Here the indefinite form is used because the object is not treated as a specific, clearly identified single bulb, but more like LED bulbs in general.

If you were talking about a specific bulb, you would more likely get:

• használjuk a LED-izzót = we use the LED bulb

But in this sentence the meaning is general, so használunk is natural.

Because it is the direct object of the verb használunk.

Hungarian marks direct objects with the accusative ending -t.

So:

• LED-izzó = LED bulb
• LED-izzót = LED bulb (as the thing being used)

The sentence is saying we use an LED bulb / LED bulbs, so the noun needs the object ending.

Because here they are not direct objects. They are predicate adjectives inside clauses introduced by mert.

• mert olcsó = because it is cheap
• mert energiahatékony = because it is energy-efficient

Hungarian often leaves out the subject it and the verb is when they are obvious. So these parts are understood as:

• mert [a LED-izzó] olcsó
• mert [a LED-izzó] energiahatékony

Since the adjectives describe the subject, they stay in their basic form.

Hungarian often leaves out the indefinite article when speaking generally.

So:

• használunk LED-izzót = we use LED bulbs / we use an LED bulb

This sounds general, like talking about the category.

You could also say:

• használunk egy LED-izzót

but that would sound more like we use one LED bulb or would give more emphasis to a single item.

In this sentence, the article is omitted because the statement is general.

Hungarian often uses the singular to express a general category, especially with objects.

So:

• LED-izzót használunk can naturally mean we use LED bulbs in a general sense

English often prefers the plural for this kind of generic meaning, but Hungarian does not have to.

You could also say LED-izzókat if you specifically want to emphasize plural objects, but the singular is very normal here.

Yes, it often can.

You may also hear:

• Nem azért használunk LED-izzót, mert olcsó, hanem mert energiahatékony.

This still means the same thing: We do not use LED bulbs because they are cheap, but because they are energy-efficient.

Including the second azért makes the sentence feel a bit more balanced and explicit:

• nem azért ..., hanem azért ...

So the version in your sentence is slightly fuller and more formal/neat, but both are natural.

Hungarian word order often reflects focus and contrast rather than fixed English-style sentence structure.

Here, azért is in an important contrastive position because the sentence is about the reason for using LED bulbs.

The structure highlights:

• not for that reason
• but for this other reason

So the order helps organize the sentence around the contrast between the two reasons, which is the main point of the sentence.

Yes, it is a single adjective.

• energia = energy
• hatékony = efficient, effective

So energiahatékony literally means energy-efficient.

Hungarian often builds longer compound words like this, and they are very common in modern vocabulary.

Because LED is an abbreviation, and Hungarian commonly uses a hyphen when attaching an abbreviation to another noun.

So:

• LED-izzó = LED bulb

This is standard Hungarian spelling. The hyphen helps show clearly that LED is functioning as the first part of a compound word.