Learn Portuguese (Portugal): Queremos comprar um gerador caso haja uma falha de eletricidade. - We want to buy a generator in case there is a power failure.

The word caso (in case) is a conditional conjunction that always requires the subjunctive mood. It sets up a hypothetical situation since we do not know for sure if a power failure will actually happen. Because of this uncertainty, we must use the present subjunctive form haja instead of the indicative há.

What verb does haja come from?

It comes from the verb haver. You likely already know its present indicative form, há (there is / there are). Haja is simply the present subjunctive version of that exact same word, carrying the same meaning.

Could I just use se (if) instead of caso?

You can, but it changes the grammar rules. If you use se to talk about a possible future event, you have to use the future subjunctive instead: se houver uma falha. The conjunction caso is unique because it forces us to use the present subjunctive even when talking about future possibilities.

Are there other common ways to say 'power failure' in Portuguese?

Yes! While uma falha de eletricidade is perfectly correct, saying uma falha de energia (a power or energy failure) or um corte de energia (a power cut) is just as common and natural in Portugal.