Learn Croatian: Moramo zvati električara zato što nemamo struje. - We must call the electrician because we don't have electricity.

After the negative verb nemati (to not have), Croatian typically uses the genitive case instead of the accusative. This is called the genitive of negation. It is especially common with mass nouns or abstract concepts, such as struja (electricity), voda (water), or vrijeme (time).

Is there a difference between using zato što and jer in this sentence?

They both mean "because" and are practically interchangeable here. Zato što is slightly more emphatic, literally meaning "for that reason that". You could perfectly well say ...jer nemamo struje and the meaning would stay the same.

Why does the direct object električara end in -a?

Električar is a masculine noun. In Croatian, masculine nouns that represent living beings (animate nouns) take the same ending in the accusative case as they do in the genitive case, which is -a. If you were calling an inanimate object, like a taxi (taksi), the accusative ending would just be the base word.

Could we use the perfective verb pozvati instead of the imperfective zvati?

Yes. Zvati is very commonly used for the general everyday act of calling someone on the phone or asking for their services. Pozvati (perfective) would focus slightly more on the completed action of summoning the electrician to come to your location. Both work perfectly in this context.