There is not a difference between powder and liquid pectin as far as gelling is concerned. You'll get a good set either way as long as you don't double your batches.
However, for jellies, liquid pectin is recomended because it redily mixes with hot liquids and it's relatively clear.
Powder pectin is used with jams because there is enough texture in the hot product to break up lumps of gluey powder.
You can use powder in place of liquid
But I would suggest vigorous wisking when adding it to the boiling liquid, but be careful!!! There also may be a difference in boiling time post add. I'm not sure cause I don't keep the liquid on hand. You may even be able to mix it in with your sugar to prevent clumping.
(There may actually be a recipe in the pectin box)
Hope this helps!