Like I said, aim for at least 4000 lumens per plant. It might help to use mylar and make sides for that shelf so all the light can be used. 150 watt hps won't make your power jump hardly at all plus if you use a digital ballast you can reduce your bill. If you don't have enough light for one plant then I don't see the point of doing 5-10. You will not get much bud from a plant with only 2000 lumens on it. a 600 watt hps might only use 40 bucks worth of electricity a month and supply you with 95,000 lumens (if you use a nice bulb). It is the most efficient light out there. if you have 10 plants then they will receive around 9,000 lumens each and your yield will thank you. If you want less then do a 250 or 400 watt hps. You will need to make sure you have proper ventilation as well because those lights get hot.