Most reliable sources with suggest both ( depending the climate). the Reasoning is that the PV array works better at a lower temp. typ. and the green roof helps to moderate temp.

If you have to pick one then you have to bae the final call on the climate the relative value one may provide over the other.

green roofs are less expensive, so you can cover greater areas of the fifth facade. beyond keeping you comfortable, green roofs significantly reduce stormwater runoff - which is very important for the health of our watersheds and our drinking water.

both a mix

But what if there's just not enough roof for both? Say, when you reduce the amount of PVs to make room for green, the power output becomes miniscule in comparison to the building's needs...

So for tall thin buildings (low roof area/power needs ratio), green roof is better, and for short squat buildings (high roof area/power needs ratio) it's best to use both? Then is it ever better to use just PVs?

then make the PV those translucent ones that can be a shade canopy for stuff on the green roof that needs shade to grow?, maybe.

PVs right now are only really efficient enough to provide full energy needs to single story (fairly efficient) buildings if they completely cover the roof. Of course, they're going to increase in efficiency in years go come, but it'll still be a expensive technology compared to a green roof, which can provide a broader array of environmental benefits.

PVs will cost somewhere in excess of $200 per square foot in up-front cost, while a green roof can be implemented for $10-20 per square foot. I'd say go with a green roof and replace all the light bulbs.

i think everybody is basically saying there is no definitive solution. Where is the building? This impacts hours of daylight + irrigation requirements. Also, there may be other locations for PV's, could be used in the facade, or in active shading devices, etc. Or perhaps you move the greenery to the facade, turn it into a living machine, catch rain water, basically yeang-ify it. Also, what is the use? If it's a laundry facility or something demanding lots of hot water, why not go the solar thermal route? Also, what are the other benefits of a green roof? Programmatic courtyards? Organic Gardens?

Perhaps there are more issues you want to consider?

Nothing keeps you from maxing out the PV panels and installing a green roof. there are green things that grow in the shade aka moss. most PV arrays have a 5' aisle between rows and then every 100' - you still need to drain the water running off the PVs- so do the green roof.

fyi- solar hot water is the most efficient renewable energy source we can tap. up to 80% efficient- could be more, but you don't want to overheat the water.

wind turbines don't mix well with roofs- structurally transmitted vibrations will drive the occupants batty, and the building adds turbulence.

for estimation purpose.. PVWatts simulation could be used to determined how much energy produce

http://rredc.nrel.gov/solar/codes_algs/PVWATTS/

roughly electricity produces by PVWatts will cost 25-45 cents/kWh based on LCC

thanks guys. I don't actually have a specific building in mind, I guess I was thinking of it theoretically. This came up in a discussion at work yesterday and I thought I'd post and see what people thought.

Gee, it's sad that the most economical solution (at least to conserve energy) is still to upgrade the insulation in the attic(if you've got one), & tighten up the building envelope.

Reminds me of an Owner I was working with in the CAribbean who was so focused on installing P-V panels to reduce his energy usage. At the same time, his previous architect (note the lil' 'a') had him convinced that 'jalousie' louvers which did not seal would be 'lovely'. Besides, people that came to this particular island didn't use A/C...even though it was in every villa.

Get the easy stuff working first, because face it - we've got some work to do to before we can put Green & affordable in the same context.

there's definitely no one good answer. we're learning that, here in ky, neither is particularly great.