teiden no toki ha, sumaho dake de naku razio mo aru to ansin desu.

Here, の links the noun 停電 (power outage) to とき (time / when).

So:

• 停電 = power outage
• とき = time, when
• 停電のとき = when there is a power outage / during a power outage

A noun often uses の to modify another noun in Japanese. This is very common.

Examples:

• 雨のとき = when it rains / in rainy situations
• 旅行のとき = when traveling / during a trip

The は marks 停電のとき as the topic or situation being talked about.

So 停電のときは、... means something like:

• As for when there is a power outage, ...
• In a power outage, ...

It sets the scene for the rest of the sentence.

Without は, the phrase would still make sense, but は makes it clearer that this is the context being discussed.

This is a very common pattern:

• X だけでなく Y も
• not only X, but also Y

So here:

• スマホだけでなく = not only a smartphone
• ラジオも = but also a radio

Together:

• not only a smartphone, but also a radio

This pattern is a bit more formal/written than casual speech. In everyday conversation, people also say:

• スマホだけじゃなく、ラジオも
• スマホだけじゃなくて、ラジオも

These mean basically the same thing.

Because スマホ is directly attached to だけでなく as part of the fixed pattern Xだけでなく.

So the structure is:

• スマホだけでなく = not only a smartphone

You do not need a particle between スマホ and だけ.

This is normal in Japanese. The noun goes straight before だけ.

The も means also and matches the earlier だけでなく.

So:

• スマホだけでなく = not only a smartphone
• ラジオも = a radio too / also a radio

This pairing is very natural:

• Xだけでなく、Yも

Without も, the sentence would sound less balanced.

It is basically to exist / to be available, but in English we often translate it more naturally as to have.

So ラジオもある can feel like:

• there is also a radio
• you also have a radio
• having a radio too

In this sentence, the idea is not just abstract existence. It means that in a power outage, it is reassuring if a radio is available as well.

Japanese ある often covers both there is and have, depending on context.

Because スマホ and ラジオ are inanimate things.

Japanese uses:

• ある for inanimate things
• いる for animate beings like people and animals

So:

• ラジオがある = there is a radio
• 猫がいる = there is a cat

Here と is a conditional marker meaning something like:

• if
• when
• if you have
• when there is

So ラジオもあると安心です means:

• It is reassuring if you also have a radio
• You can feel at ease when there is also a radio

This is not the quotation と. It is the conditional と.

A simple way to think of it is:

• V-dictionary form + と + adjective / result

Examples:

• これを押すと開きます。 = If you press this, it opens.
• 水があると便利です。 = It is convenient if there is water.
• ラジオもあると安心です。 = It is reassuring if there is also a radio.

Good question. Both can be possible in Japanese, but they feel a little different.

• 安心です = it is reassuring / it brings peace of mind / one can feel at ease
• 安心します = I feel relieved / people feel relieved

In this sentence, 安心です sounds like a general statement about what is reassuring in that situation.

So:

• ラジオもあると安心です = Having a radio too is reassuring.

If you said:

• ラジオもあると安心します it would sound more like:
• I feel relieved if I also have a radio
• People feel relieved when there is also a radio

The original sentence is a little more neutral and general.

Yes, you can think of the subject as omitted.

Japanese often leaves out subjects when they are obvious or not important. This sentence does not explicitly say:

• you
• we
• people

But the meaning is general, like:

• During a power outage, it is reassuring to have not only a smartphone but also a radio.

So the sentence is giving general advice, not talking about one specific person.

Yes, it is natural. It sounds like standard, clear Japanese, the kind you might see in advice, safety information, or a textbook.

A more casual version might be:

• 停電のときは、スマホだけじゃなくてラジオもあると安心。

Differences:

• だけでなく → more formal / neat
• だけじゃなくて → more conversational
• 安心です → polite
• 安心 at the end without です → casual

So the original sentence is polite and very natural for general advice.