تصنيف الطاقة جيد لتوفير الطاقة.

Because this is a present-tense nominal sentence in Arabic.

The structure is:

• تصنيف الطاقة = the subject
• جيد = the predicate

So Arabic simply says:

• تصنيف الطاقة جيد
• literally: Energy rating good

In English, you need is, but in Arabic the present-tense to be is usually left out.

If you wanted a past meaning, then Arabic would use a form of كان.

This is an iḍāfa construction, often called a possessive or of construction.

It is made of:

• تصنيف = classification / rating
• الطاقة = energy

Together, it means something like:

• energy rating
• literally: rating of energy

A few important things about iḍāfa:

• The first noun usually does not take الـ
• The second noun can be definite
• If the second noun is definite, the whole phrase becomes definite

So تصنيف الطاقة is a complete noun phrase meaning the energy rating or energy rating, depending on context.

Because جيد matches the head noun of the subject phrase, which is تصنيف, not الطاقة.

In تصنيف الطاقة:

• تصنيف is the main noun
• الطاقة only completes its meaning

Since تصنيف is masculine singular, the predicate is also masculine singular:

• جيد

If the head noun were feminine, then you would expect جيدة instead.

Because it is being used as the predicate of the sentence, and in Arabic that predicate is often indefinite.

So:

• جيد = good

This is the normal way to say is good in a sentence like this.

If you said الجيد, that would sound more like the good one, which is a different idea.

The لِـ here usually means for or in order to.

So:

• لتوفير الطاقة = for saving energy / to save energy

It shows purpose.

So the sentence is saying that the energy rating is good for the purpose of saving energy, or helps with saving energy.

Because توفير is a verbal noun (called a maṣdar).

It comes from a verb meaning to save / conserve / provide, and here it works like the English noun saving in energy saving.

Arabic often uses:

• لِ + maṣdar

where English might use:

• to + verb

So:

• لتوفير الطاقة literally = for saving energy
• natural English = to save energy

This is a very common Arabic pattern.

Because the two instances do different jobs:

• تصنيف الطاقة = energy rating
• توفير الطاقة = saving energy

So the first الطاقة belongs to the idea of rating, and the second belongs to the idea of saving.

Arabic often repeats the noun when that makes the meaning clear. It does not sound unnatural here.

With full vowels, it would be:

تَصْنِيفُ الطَّاقَةِ جَيِّدٌ لِتَوْفِيرِ الطَّاقَةِ

Here is the grammar:

• تَصْنِيفُ: nominative, because it is the subject
• الطَّاقَةِ: genitive, because it is the second part of the iḍāfa
• جَيِّدٌ: nominative, because it is the predicate
• لِتَوْفِيرِ: genitive after the preposition لِـ
• الطَّاقَةِ: genitive in the phrase توفير الطاقة

In normal Arabic writing, these endings are usually not written.

It is pronounced jayyid.

In fully vocalized form, it is:

• جَيِّد

Unvocalized Arabic writing usually leaves out:

• short vowels
• shadda

So learners have to know the pronunciation from vocabulary study or context.

It is grammatical and understandable.

Depending on the exact context, native speakers might also use more specific expressions, for example:

• تصنيف كفاءة الطاقة = energy efficiency rating
• موفّر للطاقة = energy-saving / energy-efficient

But تصنيف الطاقة جيد لتوفير الطاقة is still a useful and understandable learner sentence, especially for practicing:

• iḍāfa
• nominal sentences
• لِـ of purpose
• the maṣdar توفير