A falta de energia elétrica começou à noite.

Both are possible, but a falta de is more specific and natural here.

• falta de energia elétrica = lack of electrical power / power outage in a general sense
• a falta de energia elétrica = the power outage / the lack of electrical power in this particular situation

Portuguese often uses the definite article more than English does, especially when talking about something understood in context.

Here, falta means absence, lack, or shortage.

In this sentence, falta de energia elétrica is a common way to say power outage or loss of electrical power.

Other possible words in related contexts are:

• apagão = blackout
• queda de energia = power failure / power cut
• interrupção no fornecimento de energia = interruption in the power supply

But falta de energia elétrica is very common and neutral.

You often can say just energia in everyday speech, especially when the context is clearly about electricity.

However, energia elétrica is more precise. It specifically means electrical power/electricity, not energy in a broader sense.

So:

• falta de energia = very common in everyday Brazilian Portuguese
• falta de energia elétrica = a bit more explicit or formal

In Portuguese, adjectives usually come after the noun.

So:

• energia elétrica = electrical energy / electric power
• carro vermelho = red car
• casa grande = big house

Sometimes adjectives can come before the noun, but the normal position is after it. In this sentence, elétrica is simply describing energia.

Because energia is a feminine noun, and adjectives must agree with the noun in gender and number.

• energia = feminine singular
• therefore: elétrica = feminine singular

Compare:

• carro elétrico = electric car
• energia elétrica = electrical energy

Começou is the third-person singular preterite of começar.

That means it describes a completed event in the past:

• começar = to begin / to start
• começou = began / started

It matches the subject a falta de energia elétrica, which is singular.

Because the sentence focuses on when the outage began, as a completed event.

• começou = it started / it began
  
  • a specific starting point in the past
• começava = it was starting / it used to start
  
  • more ongoing, descriptive, or habitual

In A falta de energia elétrica começou à noite, the speaker is identifying the moment or period when the outage started, so the preterite (começou) is the natural choice.

The ending -ou is the normal preterite ending for -ar verbs in the third person singular.

Examples:

• falar → falou
• chegar → chegou
• começar → começou

So começou follows a regular pattern for -ar verbs.

The ç in começou keeps the s sound before o.

Without it, c before o would sound like k.

So:

• começou is pronounced with an s sound in that part
• the cedilla ç is used before a, o, and u to represent that soft s sound

This happens because the verb is começar, and when it changes form, the spelling adjusts to preserve the pronunciation.

À is the contraction of:

• a = preposition at/on/to
• a = feminine singular definite article the

So:

• à noite = at night / in the evening / during the night

This is called crase in Portuguese.

Compare:

• à noite = at night
• a noite = the night

So the accent matters a lot here.

They are similar, but à noite is usually the more standard and expected choice in this kind of sentence.

• à noite = at night / in the evening
• de noite = at night, especially in more conversational usage in some contexts

In Brazilian Portuguese, both exist, but começou à noite sounds very natural and standard.

It can cover both, depending on context.

In Portuguese, noite often refers to the broader evening/night period after daytime. So à noite may be understood as:

• in the evening
• at night
• during the night

The exact English translation depends on the situation, but the Portuguese expression itself is very common and flexible.

The subject is A falta de energia elétrica.

The main noun is falta, and de energia elétrica explains what kind of lack it is.

So structurally:

• A falta = the lack / the outage
• de energia elétrica = of electrical power
• começou = started
• à noite = at night

Even though the phrase is long, the verb is singular because the head noun falta is singular.

Yes. Some natural alternatives are:

• A falta de energia começou à noite.
• A energia acabou à noite.
• Houve falta de energia à noite.
• Teve falta de energia à noite.
  
  • very common in spoken Brazilian Portuguese
• O apagão começou à noite.
  
  • if it was a blackout

The original sentence is correct and natural, but Portuguese offers several common alternatives depending on tone and context.