You don't need 200W exactly. I placed 10 bulbs in order to increase the exposure area. (It was quite unnecessary btw). The final W will depend on the exposure area you decide. If you decide 6 bulbs, 2-3cm wide each placed with 2-3cm space between each bulb you might have an area of roughly 40x60cm. Big enough to accommodate A3 size paper. That unit will use 120W ( 6x20W ).
Originally Posted by m1tch
You can place bulbs higher, and therefore you can have a larger area to illuminate but then the exposure times would increase.
I have never effectively used the sun for exposure. I tried some cyanotype at cloudy days 1-2 hours of exposure yielded only a faint image, which eventually washed away.