Estimating panel output of flat mounted panels?

[PNW_Steve]

Hey Everyone,

I am plugging away on my RV solar design and have run into a question that is causing me some confusion.

I will be mounting the panels flat on the roof as my parking orientation is variable. No telling, on any given day, which direction I will be pointing .

I set out to figure out how much to derate panels that are flat mounted. What I found was, depending on who you believe, it is somewhere between 10-50%.

That's a pretty broad range.

What have you all found to be "reality " regarding flat panel production Vs tilted?

Thanks.

S.

[bcroe]

Its mostly a matter of how much solar radiation is intercepted. If the suns rays are not perpendicular
to the panel surface, try the cosign of the angle difference. Bruce Roe

[PNW_Steve]

I got it.... The difference will vary depending on latitude and season. ..


Is there a "rule of thumb " for design purposes?

if I am off by 15 degrees from perpendicular then I will loose about 4% and at 30 degrees I will loose 14%.

If I am understanding correctly, the loss won't be as bad as I feared.

[J.P.M.]

I got it.... The difference will vary depending on latitude and season. ..


Is there a "rule of thumb " for design purposes?

No.

Do this: Run PVWatts for a 1 kW array in the horizontal orientation for a desired location. Get the hourly output option

Then run the same location at an orientation of your choice.

Then divide corresponding outputs for a comparison of penalties/gains one orientation to the other.

[PNW_Steve]

No.

Do this: Run PVWatts for a 1 kW array in the horizontal orientation for a desired location. Get the hourly output option

Then run the same location at an orientation of your choice.

Then divide corresponding outputs for a comparison of penalties/gains one orientation to the other.

I Goggled "pvwatts" and found an NREL site. I am assuming that is what you are referring to.

Using my current location in WA I see a 14% drop in production. That is much less than I had anticipated.

I have 1800 watts of panels on the garage. Given my energy budget, looking like I am not going to need them all

[bcroe]

Fortunately for solar, cosigns of smaller angles are not so much less than a 1 multiplier. But shade,
clouds, and reduced sun intensity will cost more in many areas. A panel putting out its rated max is
rarely seen here. Bruce Roe

[Ampster]

If there is an inexpensive way to add some slope you may enjoy slightly better production because dust will wash off more easily. It depends on how much dust accumulates and the frequency and quantity of rain in your area. I have no way to quantify the benefit, butI have one flat installation and light summer rains leave some dirt because the lip of the panel inhibits complete drainage and the dirt collects near the edges of the panels. It is very difficult for me to access those panels because it takes a 30 ft ladder. Perhaps more experienced users have more to add.

[J.P.M.]

Fortunately for solar, cosigns of smaller angles are not so much less than a 1 multiplier. But shade,
clouds, and reduced sun intensity will cost more in many areas. A panel putting out its rated max is
rarely seen here. Bruce Roe

A panel putting out its STC rating is pretty rare everywhere, and even then only for something like maybe a few minutes a year.

[J.P.M.]

I Goggled "pvwatts" and found an NREL site. I am assuming that is what you are referring to.

Using my current location in WA I see a 14% drop in production. That is much less than I had anticipated.

I have 1800 watts of panels on the garage. Given my energy budget, looking like I am not going to need them all

Yes, PVWatts. It is a pretty handy tool for preliminary design that can have other uses for the creative mind besides as a model.

Depending on orientation and location, that 10 - 50 % initial guess probably isn't bad for a 1st approx.

In Seattle, PVWatts Jan. daily output for horiz. ~ 0.99 kWh/day per STC per STC kW.
In Seattle, PVWatts Jan. daily output for tilt @ lat. ~ 2.00 kWh/day per STC kW.

In Yakama, PVWatts Jan. daily output for horiz. ~ 1.1 kWh/day per STC kW.
In Yakama, PVWatts Jan. daily output for tilt @ lat. ~ 2.14 kWh/day per STC kW.

In Albuquerque, PVWatts daily output for horiz. ~ 3.32 kWh/day per STC kW.
In Albuquerque, PVWatts daily output for horiz. ~ 5.73 kWh/day per STC kW.

Those model numbers are for grid tied systems. Off grid will be different, and any and all applications' outputs will always be different than any model will produce. Any system's output will be have like the difference between weather and climate: Climate ( or a model's output) is what you might reasonably expect over the long haul. Weather (or a system's actual output) is what you get over the short term.

Notice in the above examples, that the ratio of tilted to horiz. output tends to go down as the clearness of a location goes up. That's a general but not universal or unfailing trend, and why I includes Albuquerque in the examples as that place is about as sunny as it gets in the winter.

If you live in eastern WA, that may be one reason(but probably a small one), besides Bruce's comment about cosines, and perhaps some differences in the way you're using the model, why you may be seeing the model shows a lower output penalty for a horiz. orientation than you might have thought.

[Ho jo]

0080-b3.jpg

[bcroe]

I'll suggest that more power could be had than indicated on the above chart, by operating the cell
at MPPT. Probably near 80% of OC V. If that was done, the voltage would not vary so much.

Interesting how much power is shown for 0 degrees. That would be from dispersed light in this case,
I think 0 watts if done in a vacuum with no dispersed light allowed. Highly variable. Plenty of power
is made here from light dispersed by clouds, with a grossly over paneled array. Bruce Roe

[J.P.M.]

0080-b3.jpg

Where did these numbers come from ?

[J.P.M.]

Is there a "rule of thumb " for design purposes?

Why not just run a model like PVWatts? Real simple and more reliable for design purposes.

[Ho jo]

What was really interesting to me was on pv watts in may -july at my latitude .made very little difference flat or at 12

[Ho jo]

(Finishing last post) 12 from flat June was best flat