You mean charcoal briquettes? The BBQ type?

Well, if you kept them dry, they would keep just about indefinitely. And they would provide a slow, even heat when burned in a wood stove. You could also cook with them of course.

In mild climates, charcoal in a fireplace would break the chill and be cheery to boot. But it would take quite a large volume to keep a house warm in a three-day blizzard.

Unless you were able to buy charcoal in bulk, I think the cost would be a killer.