What about a 150 watt hps?This could save me some money.I can get mh and hps at a local hardware store.

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how big of an area and how many plants do you want to do but if i where u i would do n less the 250

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I already have the 150 watter...I just wanted to know if I could use metal halide in it as previously suggested, or if there is some cutoff on wattage for these kind of bulbs.

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oh oh k my bad it should pick one up at lowes and if dont werk take it back and your out no money

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yes. that is why it is called a "conversion" bulb.

no. you need a 150W ballast to run a 150W bulb.

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I have a 150 watt ballast.I guess I should have been clearer.I just meant...does it have to be a conversion bulb?Because the wording in a previous post suggested not.

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Do not take this as gospel, but on all the sites I've been on, one of the common schools of thought is that you can use either an HPS or a MH for both vegging and flowering. The spectrum of an HPS is more suited for flowering, the MH for vegging. From what I can gather, an HPS would be preferred over a MH, but an MH will work fine, and ideally one would use both throughout. I cannot state for certain, but this seems to be a common school of thought.
Supposedly, using an HPS for vegging results in slightly stretched out plants, and using an MH bulb for flowering reults in looser, less dense buds.
Personally, I used MH for vegging, and HPS for flower. I added an MH to the mix for the last few weeks of flower. My plany has 2 400w HPS and a 400w MH on it now, 12/12.
I thought about how convenient only one ballast would be for both types of light. (Even tho I have 4 of each) I can now just unplug the bulb/reflector from the ballast and switch out the ballast and bulb, leaving the cords and reflectors where they are. I'm still working on my system, trying to improve it.

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Ok, lots of questions, here are the answers.
1) Yes, of course you can use a conversion lamp, thats what they are designed for.
2) The confusion comes in when you try to use a "standard" (not a conversion) MH lamp with an HPS ballast. The only time this will work well is with a 1000 watt system. This is because the LAMP OPERATING VOLTAGE is nearly the same for a 1000w HPS lamp and a 1000w MH lamp. Now, don't confuse lamp operating voltage with the voltage of the power in your house or the voltage of your ballast. Lamp operating voltage is the voltage supplied FROM your ballast to the lamp. There are some great sites that you can learn all about how an HID lamp (High Intensity Discharge) like a HPS or MH works.
3) The basic difference between the lamps is that an HPS lamp requires an "ignitor" in the ballast to start the lamp, while and MH lamp does not. This is one of the main reasons you should not use a "standard" MH lamp with your HPS ballast, as the ignitor sends a 4000 volt pulse into the lamp to start it (or "ignite" it) Your standard MH lamp is not built to withstand this ignitor pulse for long, but for 1000w units, if you are running 24 hour cycles with your veg anyway, it only gets this ignitor pulse the first time it is turned on. So you should be ok for the life of the lamp.
With any MH lamp, you will want to change it out after 6 months of "use" because the "lumen depriciation" starts to fall off pretty quickly after that. Lumen depreciation is the amount of light that is "lost" after a lamp is used for a certain amount of hours. For example, most 400w MH lamps have an "initial" lumen rating of 36,000 lumens, that means, the first week you are using it, it will be cranking out 36,000 wonderful lumens, but if you look at the charts available from the manufacturers like GE and Philips and others, you will see that most MH lamps will "lose" about 35%-45% of the lumens with time. So that means, the 400 watt lamp, that is still sucking down 400 plus watts, is now cranking out only 25,000 lumens.

i dont think they make a 150 mh conversion lamp