Bi-Facial Panels Watt Rating: Impacted by Placement on a Roof?

[jmoles]

Hi all,

I'm looking at doing a DIY solar panel install on my roof. My roof is a ~ 4:12 pitch with asphalt shingles.

I've come across Bi-Facial panels that are on sale - cheaper than similar panels of the same advertised wattage.

Is it apples-to-apples to compare a Bi-Facial panel , for example, rated at 400w to a standard panel at 400w once they are installed parallel to a roof?

Are bi-facial panels rated by adding each side? Like a Bi-Facial 400w panel only having 200w on each side thus leaving me with a 200w panel once I install it parallel to a roof.

Thanks for any help!

 

[pvgirl]

Bi-facial panels are rated for the power from the front only with any power from the back being a bonus. So a 400w bi-facial is equivalent to 400w standard panel, excluding any extra power from the back.

On thing to be aware of some vendors may promote panels by prominently stating a number for watts that includes the bi-facial gain, not the rated watts, be sure to check the data sheet to be sure.

 

[zanydroid]

Like a Bi-Facial 400w panel only having 200w on each side thus leaving me with a 200w panel once I install it parallel to a roof.

FWIW bifacial gain isn't 100%, maybe more like 15-20%?

And wattage rating is a function of area and efficiency, so you can sort of sanity check based on dimensions (should be accurate as long as you correct by efficiency), or on weight (less accurate b/c there is variation in how heavily a panel is built).

 

[pvgirl]

FWIW bifacial gain isn't 100%, maybe more like 15-20%?

For many panels the backside is rated to produce up to 70% or more of the rated power of the front, when the back is being exposed to STC. Something which only really matters if you are mounting panels vertically where the back will be exposed to more direct sunlight.

 

[zanydroid]

For many panels the backside is rated to produce up to 70% or more of the rated power of the front, when the back is being exposed to STC. Something which only really matters if you are mounting panels vertically where the back will be exposed to more direct sunlight.

OK, that limit makes sense and is reasonable, but are they actually advertising 170% of the front face STC rating as the wattage? That would be rather flagrant. I think the bifacial spec sheets I looked at (for better known manufacturers) had tables showing the output at various backside illumination scenarios, and stopped at 25% or so

 

[pvgirl]

OK, that limit makes sense and is reasonable, but are they actually advertising 170% of the front face STC rating as the wattage? That would be rather flagrant. I think the bifacial spec sheets I looked at (for better known manufacturers) had tables showing the output at various backside illumination scenarios, and stopped at 25% or so

You'd never get 170% power from the panel, and I doubt any one is advertising that. That is just the maximum potential for the backside compared to the front side. Max output is going to be max front side power plus 10-20 or maybe 25% bi-facial gain.

 

[zanydroid]

It's probably mathematically impossible to hit 100% STC on one side and 70% on the other side. Or if it's achieved, some terrible thing is happening with the climate.

 

[millsan1]

I put bi facials on my roof, because they were cheaper than the same non bi facial panels, from the same company (Canadian Solar), when i bought them. 400 Watt non bi facial was x and 400 watt bifacial with 100 watt bifacial gain (500 watt panel) was like 90% of x.

 

[shadowmaker]

It's probably mathematically impossible to hit 100% STC on one side and 70% on the other side. Or if it's achieved, some terrible thing is happening with the climate.

Two Suns 180 degree apart? At this time of year I'd be glad to see even one around here.

With fresh snow I've noticed ~140% performance out of my bifacials.

People should understand that while bifaciality factor can be almost 1 (at least I have seen panels claiming >0.9), sun is only shining at one side at the time, so the other side performance is dependent on reflected light and thus the albedo of the surroundings. While fresh snow and whitest paints can have albedo of ~0.9, water, soil and grass are usually around 0,05 to 0,15.

To OP: You are going to be just fine with bifacials and that's exactly what my friend did when we bought our bifacials as a group buy. You aren't going to get much bifacial gain with black roof, but if those panels are cheaper than monofacials like ours was, go for it. Just remember if those bifacials have glass on both sides (there are bifacials that have clear plastic membrane on the backside), they are almost twice as heavy. My 700W panels weight 40kg each.

Here's ideas how you can improve bifacial gain even installed on a roof.

 

[zanydroid]

Two Suns 180 degree apart? At this time of year I'd be glad to see even one around here.

Or the earth moves closer to the sun.

More realistically if you mount it high up on a mirrored stick over a really high albedo surface with diffuse reflections it’ll get quite high. It might even exceed 200%, depends on if diffuse reflection of the sun covering the entire backside field of view can exceed direct on the front side.