

05072018, 09:40

#1

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Join Date: Mar 2012
Location: Boston, MA
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Calculation of Solar Output from PV
SP125 Solbian Specs
Peak Power (+/ 5%)  Pmax 128.00 w
Rated Voltage  Vmp 22.50 v
Rated Current  Imp 5.70 a
Open Circuit Voltage  Voc 26.90 v
Short circuit Current  Isc 6.00 a
Genasun GV MMPT
Peak Power (+/ 5%)  Pmax 128.00 w
Genasun efficiency in conversion 0.94 Factor
Output after Genasun 120.06 w
Into batteries 120.06 w
CEF Charge Efficiency 0.94 Factor
Charge in Batteries 112.86 w
Battery Voltage (Bulk) 14.5 v or should it be absorption voltage?
Battery Amps 7.78 a
Hours per day 4.5 h
Amps per day 35.02 ah
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05072018, 09:58

#2

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Join Date: Oct 2015
Posts: 1,397

Re: Calculation of Solar Output from PV
Quote:
Originally Posted by rgleason
SP125 Solbian Specs
Peak Power (+/ 5%)  Pmax 128.00 w
Rated Voltage  Vmp 22.50 v
Rated Current  Imp 5.70 a
Open Circuit Voltage  Voc 26.90 v
Short circuit Current  Isc 6.00 a
Genasun GV MMPT
Peak Power (+/ 5%)  Pmax 128.00 w
Genasun efficiency in conversion 0.94 Factor
Output after Genasun 120.06 w
Into batteries 120.06 w
CEF Charge Efficiency 0.94 Factor
Charge in Batteries 112.86 w
Battery Voltage (Bulk) 14.5 v or should it be absorption voltage?
Battery Amps 7.78 a
Hours per day 4.5 h
Amps per day 35.02 ah

Yeah, all very nice.
You could have just used the standard rule of thumb:
(Panel Watt rating) * 0.3 = (Daily AmpHrs into a 12 volt system)
128*0.3=38
and been within 10% of all that fussing. And trust me, your day to day variability will be WAY more than 10% so reporting the result of your calculation to the nearest 0.01 AmHr is just silly.
The correct answer is between 30 and 40, no more precise than that.
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05072018, 14:09

#3

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Re: Calculation of Solar Output from PV
That bilkinny says. Have lived on alternate energy both on boat and home. Find actual output over time was way less than advertised by the panel manufacturers.
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05072018, 14:56

#4

Senior Cruiser
Join Date: Nov 2013
Location: Port Moresby,Papua New Guinea
Boat: FP Belize Maestro 43 and OPB
Posts: 8,895

Re: Calculation of Solar Output from PV
Quote:
Originally Posted by rgleason
SP125 Solbian Specs
Peak Power (+/ 5%)  Pmax 128.00 w
...
Amps Amp hours per day 35.02 ah

The amusing part is an initial rating figure with +/ 5% and a final solution to 0.03%. Even is all the other figures and assumptions are taken as accurate, the best we can say is "somewhere around 33 to 37 Amp hours" on an "average day for a certain latitude and time of year"



05072018, 15:24

#5

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Join Date: Sep 2006
Posts: 4,599

Re: Calculation of Solar Output from PV
Guys, don't be so hard on rgleason. Occasionally I will present my calculations using spurious precision just so if others want to check my math they will get the same results. It's a form of errorchecking.
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05072018, 17:40

#6

Marine service provider
Join Date: Sep 2014
Location: puget sound washington
Boat: 1968 Islander bahama 24 hull 182, 1963 columbia 29 defender. hull # 60
Posts: 7,518

Re: Calculation of Solar Output from PV
The part nobody is thinking of are the 3 or so fuel hours of charging .
I have 400 watts for a total of just over 20 amps at peak but today by 0730 local (utc7) at world 48° north with no clouds I was already getting 5.5amps . So I would figure to get between 5 and 20 amps from then in till solar noon and dwindling from then in till about 2100 local.
For almost 100ah for the day.
His 4.5 hours at 5 amps plus the 6 that will vary from 0 to 5 and back I would have to figure he will get more likely around 50ah for the day .( that is if his batteries and loads can use that much power.)
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05072018, 17:59

#7

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Join Date: Nov 2013
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Posts: 8,895

Re: Calculation of Solar Output from PV
Quote:
Originally Posted by newhaul
The part nobody is thinking of are the 3 or so fuel hours of charging .
I have 400 watts for a total of just over 20 amps at peak but today by 0730 local (utc7) at world 48° north with no clouds I was already getting 5.5amps . So I would figure to get between 5 and 20 amps from then in till solar noon and dwindling from then in till about 2100 local.
For almost 100ah for the day.
His 4.5 hours at 5 amps plus the 6 that will vary from 0 to 5 and back I would have to figure he will get more likely around 50ah for the day .( that is if his batteries and loads can use that much power.)

You miss the point that he will most probably NOT get his 128W at any time of day at most latitudes and certainly not for 3 hours or so. His 4.5 hours is in the middle of the usual 4  5 hours "full sun equivalent" estimate for solar. a full days panel output will be equivalent to 45 hours of full insolation ramping up to some percentage of pMax (percentage dependent on latitude and time of year) at solar noon and then ramping down again.
Billknny's rule of thumb of Ah/day = 30% of rated Watts (i.e around 128 * 0.3 = approx 38Ah) is the simplest one and a good conservative estimate.
The other common estimate is to figure on the equivalent of 4 to 5 hours of rated output i,e, 128 x 4 = 512Wh, 128 * 5 = 640Wh. At 13.5V, that's about 38  47 Ah.
(In your case at 48° North, you're going to get a maximum sun altitude of about 64° so in full sun you are looking at horizontal panels being 90% efficient at most today)



05072018, 20:43

#8

Marine service provider
Join Date: Sep 2014
Location: puget sound washington
Boat: 1968 Islander bahama 24 hull 182, 1963 columbia 29 defender. hull # 60
Posts: 7,518

Re: Calculation of Solar Output from PV
Quote:
Originally Posted by StuM
You miss the point that he will most probably NOT get his 128W at any time of day at most latitudes and certainly not for 3 hours or so. His 4.5 hours is in the middle of the usual 4  5 hours "full sun equivalent" estimate for solar. a full days panel output will be equivalent to 45 hours of full insolation ramping up to some percentage of pMax (percentage dependent on latitude and time of year) at solar noon and then ramping down again.
Billknny's rule of thumb of Ah/day = 30% of rated Watts (i.e around 128 * 0.3 = approx 38Ah) is the simplest one and a good conservative estimate.
The other common estimate is to figure on the equivalent of 4 to 5 hours of rated output i,e, 128 x 4 = 512Wh, 128 * 5 = 640Wh. At 13.5V, that's about 38  47 Ah.
(In your case at 48° North, you're going to get a maximum sun altitude of about 64° so in full sun you are looking at horizontal panels being 90% efficient at most today)

they are adjustable so I was getting better than most.
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06072018, 00:01

#9

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Join Date: Oct 2017
Location: No home port, full time liveaboard
Boat: Beneteau Oceanis 50 (aka 49)
Posts: 292

Re: Calculation of Solar Output from PV
If you want to remain detailed then you also need to add the loss of your electric wire length which is up to 2% (but probably less) if you chose the correct gauge for the distance.
Account for a near permanent film of dirt on the panels too.
And the killer is of course that on most installations they won’t be properly oriented for most of the day. When I first looked into solar power and went through detailed analysis I came to conservatively rate this as a 20% loss.
Then it can be cloudy or rainy but that is out of your own control obviously.



06072018, 00:50

#10

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Join Date: Jul 2007
Boat: Bestevaer.
Posts: 11,589

Re: Calculation of Solar Output from PV
Quote:
Originally Posted by StuM
(In your case at 48° North, you're going to get a maximum sun altitude of about 64° so in full sun you are looking at horizontal panels being 90% efficient at most today)

Stu, this method of calculating peak solar output is often quoted, not only by yourself but many others, but it does not work in practice.
Peak outputs for flat mounted panels are typically much higher than this calculation would suggest. I think this is mainly because the simple formula does not take into account atmospheric scattering, especially with any cloud, but it also worth noting that 1000 w/m2 is not an absolute maximum.
A panel’s peak output is less important than the average output in terms of production, but peak output does give an indication of the system’s health. If the peak output is low it is sign that there is something wrong (usually a poor connection) so it is worth having some idea of the correct numbers. A simple calculation based on sun's elevation as you suggest usually very significantly underestimates the peak output of flat panels.



06072018, 02:26

#11

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Join Date: Jan 2014
Location: Miami Beach Fl
Boat: Colombia Cc 11.8
Posts: 1,609

Re: Calculation of Solar Output from PV
I've always taken maximum amperage and multiply by 5 to get estimated amp hours. Of course you'll get more in the summer and less in the winter. 128/17.5=7.3 ×5=36.6ah on average. if you have a controller with readout or use a meter you can be even more accurate with real world Peak amp output because not all companies use 17.5 volts to estimate their wattage. But most do.



06072018, 11:59

#12

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Join Date: Mar 2018
Location: www.mvmojo.com
Boat: Robt Beebe Passagemaker 4910 in steel
Posts: 206

Re: Calculation of Solar Output from PV
MOJO has 1,000 watts of solar panels divided into 10 panels, 100 watts each. They're wired in parallel feeding a Outback FLEXMax FM80 MPPT controller. The panels are mounted flat on the deckhouse roof. We routinely see ~300 amphrs per day into our 12v system. Our experience (over the past 16 yrs) empirically validates the 30% rule...



06072018, 13:08

#13

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Join Date: Jul 2007
Boat: Bestevaer.
Posts: 11,589

Re: Calculation of Solar Output from PV
Quote:
Originally Posted by mvmojo
Our experience (over the past 16 yrs) empirically validates the 30% rule...

Rules such as this are useful, but it is important to realise that solar insolation has considerable variability depending on the location and season.
At my current location in winter the maximum peak sun angle is only around 6° above the horizon, but in summer daylight is almost 24 hours day.
If you want to work out how much solar power your system will produce rather than use a general 3x approximation, take into account your latitude and season. Even in locations closer to the equator the 3x rule is very a rough approximation and you can expect considerable variation depending on your location and season.
Solar power is great, but uniform output is not one of the strong points.



06072018, 17:27

#14

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Join Date: Mar 2012
Location: Boston, MA
Boat: 1981 Bristol 32 Sloop
Posts: 13,417

Re: Calculation of Solar Output from PV
Everyone,
Thank you all for adding your methods, suggesting other critical factors and criting my method. Without this one does not learn, do they? Stu so sorry for being too accurate and not writing amphr when the value had ah (You must be making some kind of point here, but quite frankly I don't know what it is. perhaps that I don't know what I am doing?) Because with regard to Solar, that is true. That is why I posted. So I am hoping the I'll learn more about the correct way to calculate Solar PV.
Regarding the selection of 4.5 hours. Use is during the summer, when there is generally good weather, isolation is high and sun angles are good in the middle of the day. If someone has an actual calculation which can account for the reduction of output across those 4.5 hours that would be wonderful. Perhaps they can also properly account for the output during early morning or late afternoon of the typical sunny summer day at 41.33 latitude. (Perhaps I'll try to figure that out using architectural shadow study techniques somehow.)
Many of these calculations have been worked out before by the solar industry over the 30 years of development. I was trying to establish a reasonable way to determine output to meet the 80%SOC to 100%SOC of my batteries and then after that try to offset the use of my refrigeration, all from a mooring.
It gets more complicated. The panels were going to be zipped or double velcroed to canvas panels made of carbon fiber tubing. The two bimini panels (split by my backstay) might be moderately adjustable and reasonably flat. The third panel would be on the dodger and would be interrupted by the boom shadow.
I did ask a question.
Quote:
Battery Voltage (Bulk) 14.5 v or should it be absorption voltage?

Should I be using Absorption Voltage?
Thanks again for all the training and good information. It does appear that my calc is just about on target, although there may be easier ways of reaching the answer, and hopefully there are more quantitative calculation techniques that will help with accuracy.



07072018, 20:36

#15

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Join Date: Jan 2017
Posts: 11,811

Re: Calculation of Solar Output from PV
Usually Absorb is the only charging setpoint.
Bulk is the stage striving to get there, aka CC constant current.
Some sources use the term Bulk incorrectly, as a higher earlier stage setpoint.
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