I recently had a Home Depot/BP system installed with 54 panels. The panels are on the roof of my workshop which is essentially flat. The panels were installed flush with the roof, ie. parallel to the ground. How much efficiency am I sacrificing because of this orientation? I am at ~ 35 degrees latitude.

1) is there air circulation space between the panels and the roof ? (cooler panels = better production)

2) with no tilt at all, they will tend to * collect * gunk, because of the raised rim of the frame, so washing may become needed monthly - depending .

There are many solar planner sites to browse.
http://www.consumerenergycenter.org/ren ... index.html

you are taking a big hit in lost power especially in the winter. if i had to guess on the percentages i'd say about 30% minimumly in the summer and 90% in the winter. add to that any dirt or snow that would lay on the pvs and you're compounding the problem. not keeping a minimum 6-8 inch clearance will also cause the pvs to overheat and reduce pv output slightly when you do get enough solar energy.
it seems home depot needs to be taught more than just how to connect wires to nec specs.

You can check with the solar radiation data here for actual numbers based on your location (in the US).

But, as Niel says, you should also be worried that flat panels are not self cleaning/shedding of dust and debris (and lack of ventilation).

You are probably between a rock and a hard place right now on whether or not to bite the bullet and tilt your array up--if they would do it for a reasonable cost (difference in materials plus labor) and you are OK with them tilted up--it is probably worth doing the work.

-Bill

The more I read forums like this one the more I realize what a third rate job I got. I was not home when the installation was done, though my handyman was. He questioned whether the panels should be tilted and was told that they recommend just adding more panels if the customer wants more output. I guess that makes good business sense.

i would suggest you go over that contract with a fine toothed comb to see if they failed to fulfill the contract. I would complain to the store as i'm sure you probably paid a premium for their shoddy work. want ammo, copy these responces. want more opinions, then ask on another forum.
http://www.wind-sun.com/ForumVB/