You're new, so you're going to get this a lot. Let me try to save other people the trouble and beat it into your head now. USE THE SEARCH FUNCTION BEFORE POSTING! Common question, common answer. There are too many variables involved to calculate yield. I will say, however, that you're going to be limiting your plant if you plan to keep it in a one-gallon container its entire life. The general rule of thumb is one gallon per one foot of plant growth... As in, if you only have four feet above the plant to grow, a container any bigger than four gallons would be a waste. Your limiting plant size by means of limiting root growth. So, a 250W HPS over a plant in a one gallon container will have horrendously deficient yield. Instead of asking how much you should be able to pull, you should set benchmarks for yourself. You'll have to do a bit of searching yourself if you want to look into this, but IIRC an "average" yield will be somewhere around half a gram per watt per every thirty days in growth. As in [(grams harvested/watts used)/(#days from sprout to harvest/30)] should be greater than or equal to 0.5. Like I said, the specifics of the benchmark are fuzzy and you should be able to find this info yourself searching around (Mar 2002 issue of High Times I think), but the fact is calculating yield given the conditions is impossible. It's better to benchmark the grow and try to outdo that the next time around. And welcome to the City!