Energiankulutus vähenee sitä mukaa kun sisälämpötila laskee.

It is a fixed expression meaning as, in proportion as, or the more/less ... the more/less ....

So the sentence is not just saying that one thing happens when another happens. It is emphasizing a parallel change:

• Energiankulutus vähenee sitä mukaa kun sisälämpötila laskee.
• Energy consumption decreases as the indoor temperature falls.

A very natural English paraphrase is:

• The lower the indoor temperature, the lower the energy consumption.

Not very well. This is best learned as a set phrase.

The word sitä is the partitive form of se, but in this expression it does not work like a normal standalone it/that in English. The whole phrase sitä mukaa kun acts adverbially and means something like:

• to that extent as
• at the same pace as
• in step with how

So it is better to understand the whole chunk than to translate each word separately.

Also, do not confuse it with sen mukaan, which often means according to or in accordance with.

Because Finnish writes compounds as one word.

Energiankulutus = energian + kulutus

• energia = energy
• kulutus = consumption

So literally it is energy-consumption, and in Finnish that becomes one compound noun: energiankulutus.

This is very normal in Finnish. English often writes these as two words, but Finnish usually joins them.

For the same reason: it is a compound noun.

Sisälämpötila = sisä- + lämpötila

• sisä- = inside, interior, indoor
• lämpötila = temperature

So the whole word means indoor temperature or interior temperature.

Again, English often uses separate words, but Finnish usually combines them into one compound.

Vähenee is a 3rd person singular present-tense verb form.

Why 3rd person singular?

Because the subject is energiankulutus, which is singular:

• Energiankulutus vähenee.
• Energy consumption decreases.

Here vähenee means decreases, gets smaller, or becomes less.

A useful comparison:

• vähenee = decreases / becomes less
• vähentää = reduces something

So:

• Energiankulutus vähenee. = energy consumption decreases
• Joku vähentää energiankulutusta. = someone reduces energy consumption

Yes. Laskea has several meanings.

In this sentence, laskee is the 3rd person singular present form of laskea, and it means:

• falls
• drops
• goes down
• decreases

So:

• sisälämpötila laskee = the indoor temperature falls

That is completely natural Finnish.

The same verb can also mean to count, depending on context:

• Hän laskee rahat. = He/She counts the money.
• Lämpötila laskee. = The temperature falls.

Because although both can be translated as decrease, they are not used in exactly the same way.

• vähenee focuses on amount becoming smaller
• laskee focuses on level going downward

So these pair naturally with different kinds of things:

• energiankulutus vähenee = energy consumption decreases
• lämpötila laskee = temperature falls

You could sometimes use other verbs, but this sentence sounds very natural as it is.

Finnish uses the present tense for general truths, regular relationships, and scientific/technical statements, just like English often does.

This sentence is describing a general rule:

• when indoor temperature goes down, energy consumption goes down too

So the present tense is exactly what you would expect.

It is a subordinate clause introduced by kun.

The clause:

• kun sisälämpötila laskee

means:

• when/as the indoor temperature falls

Inside that clause:

• sisälämpötila is the subject
• laskee is the verb

So the sentence structure is roughly:

• [Main clause] Energiankulutus vähenee
• [Linking expression] sitä mukaa kun
• [Subordinate clause] sisälämpötila laskee

It is in the nominative, because it is the subject of the subordinate clause.

• sisälämpötila laskee
• the indoor temperature falls

There is no special case marking needed here; it is just the normal subject form.

The same is true for energiankulutus in the main clause: it is also nominative, because it is the subject of vähenee.

Yes, that would be grammatical and understandable.

But the nuance changes a little.

• kun by itself can mean when or as
• sitä mukaa kun more clearly emphasizes a gradual proportional relationship

So:

• Energiankulutus vähenee kun sisälämpötila laskee
  = Energy consumption decreases when/as indoor temperature falls.

• Energiankulutus vähenee sitä mukaa kun sisälämpötila laskee
  = Energy consumption decreases in step with the indoor temperature falling.

The version with sitä mukaa kun is more precise if you want to stress that the two changes happen together progressively.

Because Finnish has no articles.

There is no direct equivalent of English a/an or the. Finnish usually leaves that information to context.

So:

• energiankulutus can mean energy consumption or the energy consumption
• sisälämpötila can mean indoor temperature or the indoor temperature

In this sentence, the nouns are being used in a general, technical sense, so English naturally adds the or leaves it out depending on style.

Yes. A very common alternative is the mitä ..., sitä ... pattern, which corresponds closely to English the ..., the ...:

• Mitä alempi sisälämpötila on, sitä vähemmän energiaa kuluu.

This means:

• The lower the indoor temperature is, the less energy is used.

That structure is very common in Finnish when expressing proportional change.

So the original sentence and this version are very close in meaning, but the grammar is different:

• sitä mukaa kun ... = as / in proportion as ...
• mitä ..., sitä ... = the ..., the ...