استهلاك الطاقة كبير اليوم.

Because Arabic often uses a nominal sentence in the present tense, with no explicit verb for to be.

So:

استهلاك الطاقة كبير اليوم
literally works like: Energy consumption high today

In natural English, we add is:
Energy consumption is high today.

If you wanted the past tense, Arabic would usually add a form of كان:

كان استهلاكُ الطاقةِ كبيرًا اليوم
= Energy consumption was high today.

استهلاك is a verbal noun (often called a maṣdar). It comes from the verb استهلك = to consume.

So:

• استهلك = he consumed
• استهلاك = consumption

In this sentence, استهلاك is the main noun of the subject phrase:

استهلاك الطاقة = energy consumption / the consumption of energy

This is because the phrase is an iḍāfa construction, often translated as X of Y.

Here:

• استهلاك = consumption
• الطاقة = the energy

Together:

استهلاك الطاقة = consumption of the energy / more naturally energy consumption

In an iḍāfa:

1. the first noun usually does not take ال
2. the second noun can be definite with ال

So الاستهلاك الطاقة is not correct here.

Because the second noun in the iḍāfa can be definite, and here Arabic is referring to energy as a known/general concept.

So:

• طاقة = energy
• الطاقة = the energy

In many English translations, you would still simply say energy consumption, even though Arabic uses the definite form الطاقة.

Also, because the second noun is definite, the whole iḍāfa phrase استهلاك الطاقة becomes definite in meaning.

Because كبير agrees with استهلاك, not with الطاقة.

This is very important: in an iḍāfa like استهلاك الطاقة, the head noun is the first one, استهلاك. That is the noun the adjective relates to.

• استهلاك is masculine
• so the adjective is كبير (masculine singular)

Even though الطاقة is feminine, it does not control the adjective here.

Compare:

• استهلاكُ الطاقةِ كبيرٌ = Energy consumption is high
• الطاقةُ كبيرةٌ = The energy is large/great or The power is great

These are different structures.

Because it is the predicate of the sentence, not an adjective directly attached to the noun.

In a nominal sentence like this:

• استهلاك الطاقة = the subject
• كبير = the predicate (high / large)

Predicates are often indefinite in Arabic.

So:

استهلاك الطاقة كبير
= Energy consumption is high

But if you said:

الاستهلاك الكبير للطاقة
that would mean something like
the high consumption of energy

There, الكبير is an adjective modifying a noun directly, so it is definite.

Yes, كبير literally often means big or large, but in context it can also mean great, high, or significant.

So in this sentence:

استهلاك الطاقة كبير اليوم

the natural English translation is usually:

• Energy consumption is high today
• or Energy use is high today

A more specialized or formal Arabic word for high in this context is often مرتفع:

استهلاك الطاقة مرتفع اليوم
= Energy consumption is high today

Both are understandable, but مرتفع may sound more like the standard choice in technical or statistical contexts.

اليوم means today, and here it functions as an adverb of time.

It tells you when the statement is true:

استهلاك الطاقة كبير اليوم
= Energy consumption is high today

It is not the subject and not the predicate; it is a time expression attached to the whole sentence.

Yes. Arabic word order is often flexible, especially with adverbs of time.

You can say:

استهلاك الطاقة كبير اليوم
or

اليوم استهلاك الطاقة كبير

Both are correct. The second version gives a little more emphasis to today.

The version with اليوم at the end is very natural and straightforward.

If fully vocalized in careful MSA, it would be:

اِسْتِهْلاكُ الطّاقَةِ كَبيرٌ اليَوْمَ

Case by case:

• استهلاكُ → ḍamma because it is the subject of the nominal sentence
• الطاقةِ → kasra because it is the second term of the iḍāfa
• كبيرٌ → ḍamma / tanwīn ḍamm because it is the predicate
• اليومَ → often fatḥa as an adverb of time

In normal speech, news reading, or unvocalized text, these endings are often not pronounced clearly.

A careful pronunciation is roughly:

istihlāku ṭ-ṭāqati kabīrun al-yawma

A simpler learner-friendly approximation is:

is-tih-LAA-ku t-taa-qa-ti ka-BEE-run al-YAW-ma

A few notes:

• استهلاك = istihlāk
• الطاقة begins with ال, but because ط is a sun letter, the l sound assimilates:
  • not really al-ṭāqa
  • more like aṭ-ṭāqa
• In pause, people often drop the final case vowels, so you may hear:
  • istihlāk aṭ-ṭāqa kabīr il-yōm

Because this is a standard nominal sentence pattern:

[subject] + [predicate]

Here:

• استهلاك الطاقة = the thing being talked about
• كبير = what is being said about it

So the sentence structure is:

• Subject: energy consumption
• Predicate: high/large
• Time expression: today

This is one of the most common sentence types in Arabic.

Yes. That would mean:

Energy consumption is very high today.

Adding جدًا after the adjective is a common way to say very:

• كبير = big/high
• كبير جدًا = very big/very high

So this is completely natural.

Yes, several.

Some natural alternatives are:

• استهلاك الطاقة مرتفع اليوم = Energy consumption is high today
• استهلاك الطاقة عالٍ اليوم = Energy consumption is high today
• الطاقة المستهلكة اليوم كبيرة = The energy consumed today is large

The original sentence is correct and clear, but depending on context, مرتفع is often the most idiomatic adjective for things like prices, rates, levels, and consumption figures.