Posted by T on January 14, 2004, 3:29 am
Am interested in hearing from others who use Maui Solar's PV Design-Pro
I recently encountered a bug in its wiring loss calculations. The result is
an approximate tripling of wire losses, creating roughly a systematic 4-5%
underprediction bias in annual output. The % error is the same regardless of
array size, but does change a bit depending on climate and orientation.
For example, if a user selects a wire length and size in the dialog box
prompts such that the losses at the peak design condition are a very
customary 2.5%, the 'hidden' losses in the simulation results are more like
7%. This can be verified by re-running the simulation with a short length of
large wire in order to reduce the losses to near-zero. The difference
between the two cases represents the wire losses, even though the user
thought she was probably designing for no worse than 2% or so annual energy
While conservative modeling is preferable to overly optimistic modeling, the
designer should know about it, and know why. In this case, an otherwise
highly regarded program is essentially making a product that's already
expensive look 5% less financially attractive!
After multiple attempts to resolve this directly with the software vendor,
I'm convinced he doesn't seem interested in investigating this and doesn't
think a bug exists.
Perhaps the power of the marketplace will help expedite a fix. I believe
I've determined the nature of the calculation flaw, but don't know how to
read the program code to verify my assumption.
Posted by Solar Guppy on January 14, 2004, 10:00 pm
Wire is the cheapest part of the system.
I have never heard of 2.5% losses as "customary" in solar systems and in my
systems I use wire that keeps the losses under 1%.
You can down load a spreadsheet from Home Power for free that will assist
you in choosing the correct wire gauge.
Posted by T on January 15, 2004, 7:25 pm
I think that's good you design for 1% or less losses.
I try to do so as well. It's not as easy as you claim, especially for
low-voltage systems. (Consider a modest 50-foot one-way run in a 48 volt
array. A fat #6 wire carrying just 10 amps has more than a 1% voltage drop).
Keep in mind that PV Design-Pro (and most other simulation programs) treat
wire losses as a single lumped quantity. they of course are the sum of
module-to-module interconnecting wire resistance, plus the dc home runs that
you are probably referring to, plus the ac run.
The subject I raised was not how to choose wire, it was how this simulation
program proceeds to estimate the annual wire losses once you've made your
wiring selection. In your case, if you designed for 1% wire loss under peak
conditions, the program would reduce the predicted output by 3% without you
Posted by Solar Guppy on January 15, 2004, 9:34 pm
According to the Home-Power Spread Sheet , its 0.41% not 1% as you claim,
you can go 120 feet using 6 Gauge and will be at 1% losses
To get 1% losses at 50 feet total length using your 48 volts and 10 amps
would require using 10 Gauge wire
I disagree , it is "easy" to keep the losses under 1% ... its just usually a
home-run for each string making up the nominal 48 volts
Typical mppt voltage for hot climates will be in the mid-50's in voltage for
a nominal 48 volt Si string , more margin in wire size.
I don't know of any panels that push 10 amps , most panels above 120 watts
are 24 volt not 12 volt , so the KC-120 , pushing 7.2 amps would be near the
upper limit in current
My point is the tools needed for solar system "design" are free , home-power
for the losses in the wiring and PVwatts for the power output vs. mounting
angle or tracking choices.
Posted by T on January 16, 2004, 1:20 am
Some reply notes:
1. Please re-read my example. I said 50 feet one-way (the wiring tables I'm
familiar with are usually based on one-way distance). I did look at the Home
Power worksheet as you suggested. Note it asks for the TOTAL distance. In my
example, that'd be 100 feet. The Home Power worksheet would indicate 0.83%
voltage drop for a 50-foot one-way run, not 0.41%. So we're not that far
2. I did assert the voltage drop in my example would exceed 1% compared to
the 0.83% from the Home Power worksheet. At first I wasn't sure why.
However, the resistance used in the Home Power worksheet is 20% lower than
the resistance I used from a table in a Ferraz-Shawmut handbook. Ferraz
lists #6 AWG, 7-strand copper wire at 0.000491 ohms/ft, while the Home Power
sheet calculates it to be 0.000395 ohms/ft, or 20% less. I'm not sure which
is more accurate, but at least that explains the difference in our voltage
drop estimates. Take your pick. Neither resistance estimate is adjusted for
wire temperatures above 25 C, but a more representative wire temperature
might be more like 40 degrees C or even hotter for conduits exposed to
direct sun. Even at 40 degrees C, the wire resistance would be 5% more than
at 25 degrees C. Yeah, the higher voltage drop caused by increased wire
resistance at elevated temperatures would be offset by the reduced current
needed at the mid-50's operating voltages as you note. So overall, 1% losses
are achievable here but only if you use #6 wire and you have a run of 50
feet or less.
3. PVWATTS is fine as long as the user understands the system size is based
on a net ac output at 25 C module temperature. That is, the user has to do
their homework and later scale up the actual system size to adjust for
mismatch, initial light-induced degradation, shortfalls in average module
rated power, wire losses, inverter/transformer/MPPT losses, shading,
proximity to roofline, etc...but not dust. Though not obvious to the casual
user, PVWATTS has a built-in 3% monthly loss for dust.