Need more feedback on costs per usable watts with 3 potential systems

eajonesyk2eajonesyk2 Registered Users Posts: 14
Hi,

I have put together 3 planned pv systems that I feel are with in my budget. the following systems are based on components I already own and components I can afford to buy. I would appreciate it if I could get some more feedback on what the following systems would provide in usable watts after accounting for component efficiencies and wiring related losses. I will put anticipated cost next to items I need to purchase. If you are aware of an available item at a much lower cost please let me know. Also if you have an idea of a alternative system that utilizes the components I own and items that are with in my budget then please feel free to share it. I will include my estimates of cost per useable watt but I won't be shocked if they are way off.

I have decided on using the Midnite Classic 200 cc because I think it will provide the most flexibility for future mods/upgrades to the starting system. The array will be mounted approx 100ft from the cc and batts. I was planning on direct burial of the wire (except for option 3 which would require conduit) but could run conduit with option 1 and 2 for about $30. I am trying to keep the cost of items I need to purchase as close to a $1000 as possible.



Proposed System 1. Estimated watts coming out of controller 407 watts/component cost $836=$2.05 per useable watt.
Calculated useable watts as follows: Name plate panel watts 580x.77=446watts Wire loss 446x.97=433 watts CC efficiency 433x.94=407watts
4 dmsolar 145 watt panels wired in series.
100ft of two-conductor 8awg wire ($226 plus shipping)
Midnite Classic 200 charge controller ($610)



Proposed System 2. Estimated watts coming out of controller 510 watts/component cost $1050= $2.06 per usable watt
Useable watts calculated as follows: Name plate panel watts 870x.77=584 Wire loss 584x.97=567watts CC efficiency 567x.90=510 watts
6 dmsolar 145 watt panels wired in series. ($320)
100ft of two-conductor 10 awg wire ($120 plus shipping)
Midnite Classic 200 charge controller ($610)



Proposed System 3 Estimated watts coming out of controller 420 watts/component cost $1260= $3.00 per useable watt
Useable watts calculated as follows: Name plate watts 580x.77=446 watts Wire loss 446x.97=433 watts CC efficiency 433x.97=420 watts
4 dmsolar 145 watt panels wired in two strings of two panels
200ft of 2 awg welding cable ($600 plus shipping)
pvc 1 1/4 inch conduit 100ft ($50)
Midnite classic 200 charge controller ($610)



Please note that I am completely guessing on the charge controller efficiency numbers because there are no published numbers or calculators to provide these numbers. Word on the street is, higher voltage arrays going into lower voltage systems equals lower controller efficiency. So i guessed at what the numbers might be.

Thanks for any input. Eric

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Need more feedback on costs per usable watts with 3 potential systems

    Because I am a glutton for punishment ... :p

    "higher voltage arrays going into lower voltage systems equals lower controller efficiency"

    That is true. But you are looking at a difference of 97% vs 93% efficiency kind of thing. Not some enormous fall-off like 25%.
    Normally nominal array Voltage shouldn't be more than 2X nominal system Voltage. But if you need the higher array Voltage to overcome V-drop between panels and controller you have to put up with a bit of loss.

    I'm assuming this is an off-grid system your "usable Watts" don't mean much. You really have to plan the system the other-way 'round. You need so many Watt hours of power, the battery bank is therefore 'X' Amp hours at 'Y' Volts, that necessitates 'Z' Watt array. When you get to that point the cost effectiveness is simply a matter of what they are selling comparable panels for as all other components are the same.

    And there is every possibility I'm confused about what you are asking. :blush:
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Need more feedback on costs per usable watts with 3 potential systems

    This might give you some insight into array configurations which may help with your decision: http://forum.solar-electric.com/showthread.php?16241-Different-Panel-Configurations-on-an-MPPT-Controller
  • eajonesyk2eajonesyk2 Registered Users Posts: 14
    Re: Need more feedback on costs per usable watts with 3 potential systems

    Hi Cariboocoot, You helped me before when we were looking at the tristar controllers but in the end the tristar controllers 150 volt max limit was just too close for comfort.

    MY goal was a system that provides 1kwh a day in the summer and maybe 500 watt hours or so in the winter. If I can add some potential room to add a couple of panels later with out spending a bunch now that would be swell.

    I have 4 180 amp hour at c/20 rate (198amp hour c/100 rate) 6 volt batteries which I need to wire in 12 volt.

    I have four of the dmsolar 145 panels.

    I would like to use the Classic 200 charge controller.


    So based on a 5 to 10% charging amps ratio I need the controller to put out around 36 amps. dmsolar panels are nameplate rate at 7.75 amps each. So we should be close to good there. About 2.5 hours of sun should give me my 1kwh. This will be no problem the majority of the year but I could come up a little short on the 10% charging rate in the winter. When the batteries need to be replaced I will buy larger capacity batts and could add a couple panels at that time.

    So since i have decided on the Classic 200 charge controller my main dilemma now is what wire to use. That is the main focus of my 3 options i needed to have vetted.
  • eajonesyk2eajonesyk2 Registered Users Posts: 14
    Re: Need more feedback on costs per usable watts with 3 potential systems

    yes i read that post and paid special attention the option number 4. That led me to search out the effects on controller efficiency with the higher array voltages and and low system voltages. I have to say I am still confused about how much difference there would be. I would like to think your most recent suggestion of a few percent is is right on. Just seems that there isn't a lot of consensus on the numbers. That is why in my post, I used a 90% efficiency rate in option 2. My bet is it would be better than that but I wanted to be conservative in my numbers. What ever system config I ultimately end up with hopefully the real numbers will be better than I expected.

    So after all that, what do you think about my numbers for options 1,2 and 3? Close, way off?
  • PhotowhitPhotowhit Solar Expert Posts: 5,657 ✭✭✭✭✭
    Re: Need more feedback on costs per usable watts with 3 potential systems

    I will often suggest using a PWM charge controller and more panels, with a small system. If your doing the math the cost per watt tends to be less up to 12-1500watt system. You do have the long wire run, Where are you located? your batteries and inverter may live just fine outside (well in a small box) depending on where you live and you can send 120 AC across the 100 feet.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Need more feedback on costs per usable watts with 3 potential systems

    When you're using an MPPT controller the panels' Volt and Amp rating don't enter in to the output power: it take the Watts available from the panel at what the controller thinks is the best V*A and turns it into the maximum output power for the battery at that stage.

    So instead of 4 * 7.75 = 31 Amps being your maximum charge current it's more like 4 * 145 = 580 * 0.77 / 12 = 37 Amps. The 0.77 is a typical efficiency factor for panels & controller. Your actual number may be higher or lower due to a number of factors. Mine's 0.82. :D

    You have two efficiency losses: the long wiring and the conversion in the controller. They tend to trade off against one another; you can lose power as heat in the wiring or in the controller. Most people pick the wiring because it doesn't suffer as many ill effects from heat, being able to dissipate it better than electronic components sealed in a box.

    If your distance is 100 feet from array to controller, you would need to use 4 AWG to get the V-drop down to around 3% at maximum power using a nominal 24 Volt array. You could tolerate possibly 8 AWG which would give a whopping 8% drop but would still have enough Voltage to charge the batteries.

    Using all four in series you get quite different numbers: 10 AWG would bring the V-drop to around 3%. Feeding the controller 4X 'normal' Voltage is no place near as bad as feeding it 10X as is sometimes necessary. The Voc is not in the danger zone even for a 150 Volt controller in cold temps.

    Were it me, I would go with option #1. Others may have different ideas/suggestions. I know vtMaps worked out this same problem for his system where it was nearly an even trade off between wire loss and controller efficiency loss. Somewhere on here he's included a graph of the efficiency ration from MidNite I think. (I'd look, but it keeps signing me out when I open a second windows today :confused: )
  • eajonesyk2eajonesyk2 Registered Users Posts: 14
    Re: Need more feedback on costs per usable watts with 3 potential systems

    Hi Photowhit,

    Gave the pwm cc some thought as well as locating the cc and batts next to the array. It gets pretty cold here in the winter. High desert area 5200 ele southern oregon east of cascades, and lot's of snow in winter. Security is also an issue and I just decided having the cc inside and batts closer to me was the way to go. I may give it some more thought though. thanks for the input. Somebody else suggested putting the batteries in the ground (next to array) which might take care of the temp problem for the batteries. We get about 4 feet of snow on average over the winter usually at least a foot of that is left to melt off when spring rolls around. I have dug holes in April near my burn areas and they filled with water which i used to extinguish the fires before evening. For best/most hours of sun the array is going to be close to one of these areas. So i am not sure putting batteries below ground would work very well. Won't completely rule it out though. Thanks for the input.
  • eajonesyk2eajonesyk2 Registered Users Posts: 14
    Re: Need more feedback on costs per usable watts with 3 potential systems

    Cariboocoot, funny you mentioned vtMaps,

    I heard from him over at the midnite forum, I believe he was fond of splitting the array up in to 2 strings (3 panels each) to make it easier on the controller. I thought the wire cost wouldn't make it worth it. But then again I miscalculated wire cost in the beginning. Option one is my first choice too because it's the least expensive up front and still allows for easily adding a couple of panels later on either all in series for a less than 2% v-drop or even two strings of three panels if I wanted to suffer a 6% v-drop. I also think that since the Classic 200 is rated for a max 200 volts, even if i ran six panels in series it would be well with in it's comfort range on the coldest days.
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