I have used powders that had different use rates, as AJ posted. I recently purchased a bottle of liquid from PIWine that calls for just a couple drops per batch, worked terrific on grapes. Then I went to apple and number of drops had to about triple, but better than a huge amount of powder.
The liquid is refrigerated, the powder not. Seems like the liquid has a longer shelf life, but if you use old powder it simply won't work, not spoil the wine I think.