Energialuokka on hyvä.

Because Finnish makes very frequent use of compound nouns.
Energialuokka is made from:

• energia = energy
• luokka = class, category, rating

When these are combined in Finnish, they are normally written as one word: energialuokka.

This is very common in Finnish:

• kahvikuppi = coffee cup
• kirjahylly = book shelf / bookshelf
• energialuokka = energy class / energy rating

The last part of the compound is usually the main noun, so energialuokka is a kind of luokka.

Finnish does not have articles like English a/an and the.

So Energialuokka on hyvä can mean:

• The energy rating is good
• An energy rating is good
• simply Energy rating is good

Which one sounds best in English depends on context, but Finnish does not mark that difference with articles.

On is the third-person singular present tense of the verb olla, which means to be.

So:

• minä olen = I am
• sinä olet = you are
• hän on = he/she is
• se on = it is

In this sentence, energialuokka is singular, so Finnish uses on:

• Energialuokka on hyvä = The energy rating is good.

Because it is the subject of the sentence, and the subject in a basic statement is usually in the nominative case, which is the dictionary form.

So here:

• energialuokka = nominative singular
• it is the thing being talked about

This is the normal pattern in simple sentences:

• Talo on suuri. = The house is big.
• Auto on kallis. = The car is expensive.
• Energialuokka on hyvä. = The energy rating is good.

Because it is a predicate adjective after the verb olla.

In a sentence like this, the adjective is usually in the nominative when it describes a singular subject:

• Talo on suuri.
• Ruoka on hyvä.
• Energialuokka on hyvä.

So hyvä is nominative singular, matching the singular idea of the sentence.

A useful comparison:

• Energialuokka on hyvä. = singular
• Energialuokat ovat hyvät. = plural

In the plural sentence, both the verb and the adjective change:

• ovat = are
• hyvät = good, plural

This is the neutral, normal word order in Finnish:

subject + verb + complement
Energialuokka + on + hyvä

Finnish word order is somewhat flexible, but changing it usually adds emphasis, contrast, or a special tone.

For example:

• Energialuokka on hyvä. = neutral statement
• Hyvä on energialuokka. = unusual in everyday speech; sounds strongly marked or literary

So for a learner, Energialuokka on hyvä is the safest and most natural version.

Not in normal standard Finnish.

In a full standard sentence, Finnish usually needs the verb:

• Energialuokka on hyvä.

Leaving out on may happen in:

• headlines
• notes
• advertisements
• very informal or stylized language

But for normal speech and writing, you should keep on.

A rough guide is HÜ-vä.

A few key points:

• y is not like English y in yes
• Finnish y is a front rounded vowel, similar to the u sound in some pronunciations of French tu or German über
• ä sounds somewhat like the vowel in cat, but cleaner and shorter

Also remember:

• stress in Finnish usually falls on the first syllable
• so HY-vä
• energialuokka is stressed on the first syllable too: EN-er-gi-a-luok-ka

It can be broader than English good, depending on context.

Hyvä can mean things like:

• good
• nice
• fine
• satisfactory
• of good quality

So in Energialuokka on hyvä, hyvä could mean:

• the energy rating is good
• the energy class is favorable
• the efficiency category is satisfactory or strong

The exact English wording depends on context, but grammatically hyvä is the straightforward adjective good.