كفاءة الطاقة مهمة للغاية.

Because this is a present-tense nominal sentence in Arabic. In sentences like this, Arabic usually leaves out the verb to be in the present.

So:

كفاءة الطاقة مهمة للغاية
literally looks like Energy efficiency very important

but naturally means Energy efficiency is extremely important.

If you wanted the past, you would usually add كان or كانت: كانت كفاءة الطاقة مهمة للغاية = Energy efficiency was extremely important.

It is a nominal sentence (جملة اسمية).

Its two main parts are:

• كفاءة الطاقة = the topic/subject
• مهمة للغاية = the comment/predicate

In traditional grammar terms:

• كفاءة الطاقة = مبتدأ
• مهمة للغاية = خبر

So the sentence structure is basically:

[subject] + [predicate]

It is an iḍāfa construction (a noun + noun relationship), which often corresponds to X of Y in English.

So:

• كفاءة = efficiency
• الطاقة = the energy

Together:

• literally: efficiency of energy
• natural English: energy efficiency

Arabic uses this structure very often where English might use of or a noun-noun compound.

Because it is the first word in an iḍāfa construction.

In an iḍāfa:

• the first noun does not take الـ
• the first noun also does not take tanwīn
• definiteness is determined by the second noun

So in:

كفاءة الطاقة

the second noun, الطاقة, has الـ, which makes the whole phrase definite. That is why the whole expression can mean energy efficiency as a specific concept, even though كفاءة itself has no الـ.

Because it agrees with كفاءة, and كفاءة is a feminine noun.

In Arabic, adjectives agree with the noun they describe or refer back to in:

• gender
• number
• definiteness (in attributive use)

Here, مهمة is the predicate adjective referring to كفاءة الطاقة, whose head noun is كفاءة. Since كفاءة is feminine, the adjective must also be feminine:

• masculine: مهم
• feminine: مهمة

It is an adjective, but it is being used predicatively.

That means it is not directly attached to a noun the way an attributive adjective would be. Instead, it is telling us something about the subject:

• كفاءة الطاقة مهمة = Energy efficiency is important

Compare:

• كفاءة الطاقة المهمة = the important energy efficiency or the important efficiency of energy
  This would be an attributive adjective, directly modifying the noun phrase.

So in your sentence, مهمة is the predicate adjective: important.

للغاية means something like:

• extremely
• very
• to a great extent
• to the utmost

So:

مهمة للغاية = extremely important / very important

It is a common formal MSA intensifier. It often sounds a bit more formal or elevated than جداً.

Yes. That would also be correct and very natural:

كفاءة الطاقة مهمة جداً

Both mean very important.

A rough difference is:

• جداً = very common, everyday, straightforward
• للغاية = often a bit more formal or emphatic in MSA writing

So your original sentence sounds polished and formal.

With full case endings, it would be:

كَفَاءَةُ الطَّاقَةِ مُهِمَّةٌ لِلْغَايَةِ.

Here is why:

• كَفَاءَةُ: nominative, because it is the subject of the nominal sentence
• الطَّاقَةِ: genitive, because it is the second term of the iḍāfa
• مُهِمَّةٌ: nominative, because it is the predicate
• لِلْغَايَةِ: after the preposition لِـ, so الغاية is genitive

In normal Arabic writing, these short vowels are usually omitted.

It is pronounced roughly kafā’a(h) in pause.

A few important points:

• the ء is a real hamza sound, like a brief catch in the throat
• the final ة is a tāʾ marbūṭa
• in pause, ة is usually pronounced like -a(h)
• when linked to a following word, especially in an iḍāfa, it is pronounced -t

So in isolation:

• كفاءة ≈ kafā’a(h)

In this sentence, when linked to the next word:

• كفاءة الطاقة ≈ kafā’at aṭ-ṭāqa

Because ط is a sun letter.

With sun letters, the ل of الـ is not pronounced separately. Instead, the following consonant is doubled.

So:

• written: الطاقة
• pronounced: aṭ-ṭāqa

This is very common in Arabic. The spelling stays the same, but the pronunciation changes because of assimilation.

Because in Arabic, an iḍāfa forms a tight noun phrase. The two words belong together:

• كفاءة = efficiency
• الطاقة = energy

Together they make a single concept:

energy efficiency

That whole phrase then functions as the subject of the sentence. So Arabic speakers hear:

• كفاءة الطاقة = one complete noun phrase
• مهمة للغاية = what is being said about it

This is why the sentence feels very compact and natural in Arabic.