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I am extremely confused as to which bulb i.e a 250w or 400w should i use for an excellent image considering the hot indian environment and electricity bill (electricity is costly out here).Thirdly the PJ would be used primarily to show powerpoint presentations.So please guide me as to what is the largest image size that can be produced with sufficient brightness?The bulb i am planning to use has the following specs:

How bright you need the image to be depends entirely on how well you can control ambient light within the presentation room.

The lower wattage bulb has been proven to provide adequate brightness by others.

In the end you must decide precisely how bright you need your image to be and how large as well.

Thanks for answering first
I want an image size of around 100".Lighting conditions would be like having a 60w bulb glowing.

Based on what I've seen that should be acceptable.

if I am going to be totally honest, why not just get a cheap commercial. For under 500 you could probably get something to suit your presentation needs.

Cheap commercial? That's a great point there! How about an eVo?

Thanks for answering first
I could have taken a commercial Pj but that would be too costly to operate .Some of my friends told me that cheap PJs like Benq are useless as they can only last for 2-3 years + the Bulb cost is too high.We in india can get a benq for around 800$ whereas this DIY would be around 350$

What size lcd will you be using? Since you probably need portability I imagine it must be a small panel.

Many Chinese premade pj's like the LL one use 150 watt slightly overdriven lamps and a 7" inch lcd. Mine is more than adequate for video in a mostly darkened room especially if any room light is not directly on the screen. With text and simple graphics you can crank up the brightness and/or contrast on the lcd or laptop and the image will still look pretty good so it's more forgiving than with video where dark scenes can make things look muddy.

I've come to the conclusion that a smaller arc size can compensate for the smaller wattage lamp. It's entirely possible that a 250 watt lamp at 20000 lumen with a small arc can outperform a 400 watt lamp of 30000+ lumen but with a large light arc by having a greater percentage of the light hitting the sweet spot of the rear fresnel. I'm sure some here have done the math and know precisely where the arc size efficiency lies.

If I was starting over again with what I know today, my choice would be to use a smaller lcd not greater than 10.6 and a 200 - 250 watt lamp.