Теплопотери выросли зимой.

Because теплопотери means heat losses and is commonly used as a plural noun in technical or everyday discussion.

• теплопотеря = a heat loss / heat loss in the singular
• теплопотери = heat losses

In this sentence, the speaker is talking about heat loss as a general measurable quantity made up of multiple losses, so plural is natural.

Also, теплопотери is the subject of the sentence, so it is in the nominative plural.

Because the subject теплопотери is plural.

Russian past-tense verbs agree with the subject in gender and number:

• вырос = masculine singular
• выросла = feminine singular
• выросло = neuter singular
• выросли = plural

Since теплопотери is plural, the verb must also be plural: выросли.

It is past tense.

The infinitive is вырасти, which means to grow, to increase, or to rise.
So выросли means grew, rose, or increased.

In this sentence, it describes something that happened in the past: the heat losses increased.

Вырасти is perfective.

That means it presents the increase as a completed event or result:

• расти = imperfective, to be growing / to grow
• вырасти = perfective, to grow / increase to a new level

So теплопотери выросли зимой focuses on the fact that the heat losses ended up higher in winter.

If you used the imperfective росли, it would sound more like the process itself was ongoing:

• Теплопотери росли зимой = the heat losses were increasing during winter

But выросли emphasizes the completed increase.

Зимой is the instrumental case form of зима.

Here it is used in a very common Russian pattern where a season in the instrumental case means in that season:

• зимой = in winter
• летом = in summer
• осенью = in autumn
• весной = in spring

So зимой functions almost like an adverb of time: in winter.

Because Russian normally uses the season words by themselves in the instrumental to mean during that season.

The natural standard forms are:

• зимой = in winter
• летом = in summer
• осенью = in autumn
• весной = in spring

You can also say в зиму or в зимний период in some specific contexts, but for a normal statement like this, зимой is the most natural choice.

В зиме is generally not how Russian expresses in winter.

Yes. Russian word order is flexible, because case endings show the grammatical roles.

These are all possible:

• Теплопотери выросли зимой.
• Зимой теплопотери выросли.
• Выросли зимой теплопотери.

The most neutral version here is probably Теплопотери выросли зимой or Зимой теплопотери выросли.

Changing the order usually changes emphasis, not the core meaning.

For example:

• Зимой теплопотери выросли puts a little more focus on in winter
• Теплопотери выросли зимой starts with the topic heat losses

It depends on context.

By itself, зимой can mean:

• in winter in general
• during the winter
• this winter, if the context makes that clear

Russian often leaves that kind of detail to context, where English might use the winter, this winter, or in winter.

So the sentence could mean any of these, depending on the situation being discussed.

Yes, especially in technical, scientific, economic, or practical contexts.

It sounds natural if you are talking about:

• buildings
• insulation
• heating systems
• energy use
• utility costs
• engineering reports

For example, in a discussion about a house or a factory, Теплопотери выросли зимой sounds completely normal.

It is a compound noun:

• тепло- = heat
• потери = losses

So literally it is something like heat-losses.

This kind of compound is common in technical Russian. It is useful to recognize the second part:

• потеря = loss
• потери = losses

That can help you guess the meaning of similar words.

The usual stress is:

• теплопоте́ри
• вы́росли
• зимо́й

So the sentence is pronounced approximately:

теплопоте́ри вы́росли зимо́й

Stress matters in Russian, so it is good to learn each word together with its stress.