What he said. Also, potency is not determined by light, it's in the genetics. Plenty of light will make your buds bigger, but again this is determined by genetics. As for cost, a 400w hid will cost about $15-25 per month, $30-45/600w and $50-70/1000w depending on the cost of electricity in your area. This is on veg. cycle 18/6. So it would be a little less during flower. Hope this was helpful.Ya.....we don't know the size of your shed,is it really cold in the shed?
do you have experience growing before this?
We need to know your room size,temp,your budget,ect
my advise,without knowing anything about you or your setup I would suggest getting a electronic ballast that can do metal halide for veg and high pressure sodium for bloom.
if you shed is outdoors,the heat from the light might migh actually help you,but you might need to vent it out if it gets too hot