Si vuelve a haber un apagón, guardaré el alargador al lado de la regleta.

This is the structure volver a + infinitive, which means to do something again.

So:

• haber = to be / to exist in the impersonal sense, as in hay = there is/there are
• volver a haber = to be again / to happen again

So Si vuelve a haber un apagón means:

• If there is another blackout
• more literally, If there happens to be a blackout again

It is a very common Spanish pattern:

• Volvió a llamar = He/She called again
• Va a volver a llover = It’s going to rain again

Because volver a is a fixed verbal construction.

The pattern is:

• volver a + infinitive = to do again

Examples:

• volver a intentarlo = to try again
• volver a empezar = to start again
• volver a haber = to be / happen again

So the a does not belong specially to haber here. It belongs to volver a.

Because Spanish uses haber for saying that something exists or occurs.

Here, un apagón is not being described; it is being introduced as an event that may happen. So Spanish uses the impersonal haber:

• Hay un problema = There is a problem
• Hubo un accidente = There was an accident
• Si vuelve a haber un apagón = If there is another blackout

You would not normally say si vuelve a estar un apagón or si vuelve a ser un apagón, because that is not how Spanish expresses there is/there happens.

After si meaning if, Spanish normally uses the present indicative for real future conditions, not the future tense.

So Spanish says:

• Si llueve, me quedo en casa = If it rains, I’ll stay home
• Si vuelve a haber un apagón, guardaré... = If there is another blackout, I’ll put away/store...

This is similar to English, which also says:

• If it rains, not If it will rain

So:

• si vuelve a haber = correct
• si volverá a haber = normally wrong in this kind of sentence

Because it is the result or consequence of the if clause.

The structure is:

• Si + present indicative, future

So:

• Si vuelve a haber un apagón, guardaré...
• If there is another blackout, I will...

Guardaré is the first-person singular future of guardar:

• yo guardaré = I will store / I will put away / I will keep

Spanish often drops yo because the verb ending already shows who the subject is.

Guardar often means:

• to keep
• to store
• to put away
• sometimes to save

In this sentence, it most likely means to put away / store the extension lead.

So guardaré el alargador al lado de la regleta suggests something like:

• I’ll put the extension lead away next to the power strip
• I’ll store the extension lead beside the power strip

Depending on context, guardar can sound a bit more intentional than simply put. If you only wanted to say I’ll leave it there or I’ll place it there, verbs like dejar or poner might also appear in other sentences.

Both are possible, but they are not exactly the same.

• un apagón = a blackout
• otro apagón = another blackout

Because the sentence already has vuelve a haber = there is again, the idea of repetition is already present. So un apagón works naturally.

Compare:

• Si hay un apagón = If there is a blackout
• Si vuelve a haber un apagón = If there is a blackout again
• Si hay otro apagón = If there is another blackout
• Si vuelve a haber otro apagón = very emphatic: If there is yet another blackout again

So the original sentence is perfectly normal.

Apagón usually means a power outage, blackout, or electricity cut.

It comes from apagar = to switch off / turn off, but apagón is a noun referring to a large-scale loss of electricity, not just turning off one lamp.

In Spain, apagón is a very common word for a blackout.

In Spain, alargador usually means an extension lead or extension cord.

It comes from alargar = to make longer / extend, so an alargador is literally something that extends the reach of the cable.

A learner should know this is regional vocabulary. In other Spanish-speaking places, other terms may be more common.

In Spain, regleta often means a power strip or multi-socket extension block.

So:

• alargador = extension lead / extension cord
• regleta = power strip

This is a very Spain-specific everyday word, and it is extremely common in that sense.

Al lado de means next to or beside.

It is a fixed expression:

• al lado de la mesa = next to the table
• al lado de la regleta = next to the power strip

Literally, it comes from:

• a + el lado de = al lado de

That is why you see al.

Because the full expression is estar / poner / guardar algo al lado de...

The a is part of the locational phrase, and a + el contracts to al:

• a el lado becomes al lado

So:

• al lado de la regleta = at the side of the power strip = next to the power strip

You should learn al lado de as one chunk.

Because Spanish often leaves out subject pronouns when the verb ending makes the subject clear.

• guardaré already means I will store / put away
• so yo guardaré is possible, but not necessary

Spanish does this all the time:

• Voy mañana = I’m going tomorrow
• Tenemos hambre = We’re hungry
• Guardaré el alargador = I’ll put away the extension lead

You would add yo mainly for emphasis, contrast, or clarity.

The original word order is very natural, but Spanish does allow some flexibility.

Original:

• Si vuelve a haber un apagón, guardaré el alargador al lado de la regleta.

Possible variations:

• Guardaré el alargador al lado de la regleta si vuelve a haber un apagón.
• Si vuelve a haber un apagón, al lado de la regleta guardaré el alargador.
  This is possible, but less neutral and more marked.

The original version sounds the most standard and natural in everyday speech.

Possibly, depending on the exact context.

If the meaning is I’ll store/put away the extension lead, then guardar is fine.

But if the speaker only means I’ll place it next to the power strip, some speakers might more naturally say:

• dejaré el alargador al lado de la regleta = I’ll leave the extension lead next to the power strip
• pondré el alargador al lado de la regleta = I’ll put the extension lead next to the power strip

So guardar suggests a bit more than just placing it: it can imply putting it away, keeping it somewhere, or stowing it.

Yes, it sounds strongly associated with Spain because of the vocabulary:

• alargador
• regleta

The grammar itself is standard Spanish everywhere, but those everyday object words are especially typical of Spain.

A learner of Spanish from Spain should definitely know them.