How much does panel direction matter?

[LVLAaron]

I have 5x400 watt bifacial panels to install. I have large (giant cottonwood) trees on the east and west side of my property, so I don't get much early morning or late evening light.

I do, however, have a metal roof that's 36 feet long, but it faces NW.


Option 1: Ground mount panels that face south
Option 2: Roof mount NW facing panels.

As a matter of convenience I'd much rather have the panels on the roof, than on the ground.

Option 2 nets me a an hour or so longer light in the summer, and probably a little more in the winter when the leaves are gone.

How much, if any, efficiency do I lose not facing the panels south?

 

[ChrisG]

I just did a calc using pvwatts calculator (https://pvwatts.nrel.gov/) to see what the impact is for my location in the Carolinas for a 2KW array (assuming no obstructions):
1. Ground Mount facing South (180º): 2950 kwh/year
2. Roof Mount facing NW (330º): 1608kwh/year

You will almost double your power facing panels south.

 

[Brett V]

Option 1

 

[LVLAaron]

I just did a calc using pvwatts calculator (https://pvwatts.nrel.gov/) to see what the impact is for my location in the Carolinas for a 2KW array (assuming no obstructions):
1. Ground Mount facing South (180º): 2950 kwh/year
2. Roof Mount facing NW (330º): 1608kwh/year

You will almost double your power facing panels south.

What all goes into that calculation? I understand by facing them NW they aren't getting morning sun... In my yard, they aren't getting morning sun no matter what, as an example. My roof (option 2) has much better light over the course of a day.