On some level, area damage spells can't be balanced. If they're at the point where they're even useful against a single opponent, then they're completely overwhelming against a large group of opponents.
As a base assumption i would say a fireball should be balanced assuming it is hitting at least two creatures.
Isn't one of the goals of 5e that those of low level can still tackle those of high level? So a Fireball would be just the ticket to clear out the mooks.
So how about Fighter damage? When the Fighter can strike for, say, 1d10 + 7 (between STR, magic and specialization), an average of 12.5 per hit, and hit twice a round, a Wizard with a Fireball that averages 21 hp, save for half and apply resistances, doesn't seem like he has a comparable level of power. He's better off, as posited above, to Haste the fighter.