You can use anything for the veg stage, including LED's.

I have yet to see anything worthwile come out of flowering with anything other than HPS. The possible exception might be flouros, but a shitload of them instead of one.

Because I have not seen it does not mean that it can't happen (are triple negatives OK?). I have a feeling that you could equal the yield produced by one 600 watt bulb with several thousand LED's. I haven't done the math but there is a formula to figure this out. How much would this cost? Much more I am sure than 1 HPS bulb and the additional ventalation necessary. How long until the LED's would pay for themselves? The cost to power thousands of LED's - would that be similar to the cost to power one 600 watt bulb? And if it is less, how long until it pays for itself in electricity.

And do you have to replace them? HPS bulbs need replacing every year or so due to decrease in intensity. My guess is that LED's last much longer. I Think the real cost we are talking about is the electricity to power HPS and it's ventalation.

I am pulling for LED's. I hope some day they will be economically viable. Right now they are way too expensive for flowering.

Good luck, and keep us posted.