"Sorry I could not reply to you yesterday; we had no electricity. ". It is possible to use "shall" as an alternative to "will" when making future-tense statements ("I shall write to you"), and in emphatic statements about things that must (or mustn't) happen ("You shall not enter"), but this can sound rather formal and stilted, even in written English.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.