I'd go solar, it'll cost you more but definitely do a better job of reducing your carbon-footprint. That said, you can probably have both, half-and-half, solar on poles in the backyard etc... If you have a flat roof it would be really neat to put a patio garden up there with a solar-gazebo.

Why not do both, solar panels placed for optimum sun exposure and green roof on the rest?

I don't know the feasibility or drawbacks to this arrangement, but what about solar on the equator facing side of the roof, and green material on the other?

Actually, solar PV and Green Roofs work well together. The cooling effect of the Green Roofs helps the PV panels perform more efficiently. Consider mounting the PV on the south edge of the roof attached above the green roof.

Good question! There's a lot to weigh, but one thing to note is that the two can work in concert. Solar panels that share the roof with foliage have been shown to have increased efficiency due to decreased operating temperatures. I've seen case studies that show quite significant improvements in conversion efficiency. Anyone know of any good examples to cite?

As it was pointed out in the other debate, if you have good southern exposure on the ground, you could put the PV panels on a pole mount. However, it sounds like (from the posts above), that doing both on the roof may be best. Haven't heard of the efficiency gains before, I'll have to look into that.

Something that shouldn't be underestimated is the educational power of having a residential green roof.

Showing your neighbors and people passing by that it exists and works is gotta be worth something!

I think that the Centre for the Advancement of Green Roof Technology at BCIT (http://commons.bcit.ca/greenroof/) has done research on the efficiency gains of Green Roofs with PV (or at least their staff is familiar with it). But I can’t find anything on their website.

The fact that cooling helps PV is well established.

I recently put in a proposal for a roof that will combine both. I am not sure if the project will go ahead but if it does I will let TreeHugger know.

Go for maximum solar panels that you can, cover the rest green. That way if the USA ever switches over entirely to green energy (we can all dream, can't we?) then you can scrap the solar panels once they wear out and go all green roof.

Two questions:
Solar panels and green roofs work well together. How do you physically combine them on one roof? The vegetation underneath the panels, which then have to be raised from the roof a bit? Or simply next to eachother on the same level?

Also, if solar panels operate so much better when cooled, why not combine them with solar boilers? Has a device combining the two like that been invented yet? Or would that reduce efficiency for the boiler-part of it?

Interesting stuff...I wish I had my own roof to start experimenting.

I agree with most of the comments. PV and green roofs are compatible and complementary. They help each other. The green roof helps slightly reduce PV temperatures, which in turn lead to greater PV efficiency (in hot weather). The PVs provide partial shade throughout the day to the green roof, thereby reducing evaporation rates and amount of needed watering (again in hot weather). One thing that I would caution though is that PVs mounted on a flat roof at a 45 degree angle must be very well mounted, with either an extremely strong metal structure and strong roof or a good wind block behind the PVs, preferably both, in order to prevent wind damage.

Why not use solar and harvest your rainwater off your roof?

Chris - I read about experiments with combined PV/thermal panels in Home Power, and the result is a very poor thermal output for the cost. Much better thermal output - and much cheaper - to just keep the two system seperate. The cooling for the PV side wasn't effective, either.

Scott,
Harvesting rainwater to re-use to flush toliets, wash clothes, car washing and irrigation is an excellent idea which I feel is about to become the norm for North American homes. Europe currently uses this technology extensively and it pasy for itself in less than 10 years.
I am currently working on getting this installed on my new home and I will keep updates coming as to my successes and challenges. I do know that re-suing the rainwater from a green roof makes the water less than ideal to wash clothes due to the turbidity of the water harvested.