Päästöt vähenivät sitä mukaa kun aurinkopaneelit alkoivat antaa enemmän sähköä.

It is a fixed expression meaning as, in step with, or to the extent that.

It is used when two changes happen alongside each other gradually. In this sentence, the idea is that emissions went down as the solar panels started providing more electricity.

So this is not just a simple when. It specifically suggests parallel development.

Because it is best learned as a whole expression.

In sitä mukaa, the words are old grammatical forms that do not map neatly onto natural English word-for-word. A learner does not usually need to analyze it deeply. The important thing is:

• sitä mukaa kun ... = as ... / in proportion as ...

So memorizing the whole chunk is the most useful approach.

Päästöt is the nominative plural of päästö, meaning emission.

Finnish often refers to emissions in the plural, just like English often does. Here päästöt is the subject of the main clause.

• päästö = emission
• päästöt = emissions

Because the subject päästöt is plural, and in standard Finnish the verb agrees with a plural subject.

• Päästö väheni = The emission decreased.
• Päästöt vähenivät = The emissions decreased.

The dictionary form of the verb is vähentyä = to decrease / to become less.

Because Finnish very often uses compound words.

Aurinkopaneelit is built from:

• aurinko = sun
• paneeli = panel

Together they make aurinkopaneeli = solar panel.
Then the plural is aurinkopaneelit = solar panels.

This is extremely normal in Finnish.

Finnish uses alkaa + infinitive to mean begin/start to do something.

So:

• alkoivat = began
• antaa = to give / to provide

Together:

• alkoivat antaa = began to give / started producing

The first verb carries the tense and agreement, and the second stays in the infinitive.

Yes, antaa literally means to give, but Finnish also uses it naturally for things like machines, devices, or energy sources that provide or output something.

So aurinkopaneelit alkoivat antaa enemmän sähköä is natural Finnish for something like:

• the solar panels began to provide more electricity
• the solar panels started producing more electricity

Another possible verb in similar contexts is tuottaa = to produce, but antaa is perfectly natural here.

On its own, kun often means when. But in this sentence, because it follows sitä mukaa, the best translation is closer to as.

So here the structure means:

• as the solar panels began to provide more electricity

It shows that the two events developed together.

Because enemmän = more is followed by the partitive, and sähkö is an uncountable or mass noun.

So the correct form is:

• enemmän sähköä = more electricity

This is a very common Finnish pattern:

• enemmän vettä = more water
• enemmän rahaa = more money
• enemmän sähköä = more electricity

It is the partitive singular of sähkö.

The partitive is used here for two reasons:

1. sähkö is a mass noun
2. it comes after enemmän

So sähköä is exactly the form you would expect in this structure.

Because the sentence describes two developments in the past:

• emissions decreased
• solar panels began to provide more electricity

Finnish uses the imperfect for both clauses here. English might sometimes use something like were decreasing in similar contexts, but Finnish does not need a separate progressive form in this sentence.

Yes. You can also say:

Sitä mukaa kun aurinkopaneelit alkoivat antaa enemmän sähköä, päästöt vähenivät.

This means the same thing. The difference is mainly emphasis:

• the original sentence starts with emissions decreased
• the reordered version starts with the as-clause

Both are natural.