Newbie needs help with designing off-grid system

RossRoss Registered Users Posts: 20 ✭✭
Hello: I am new to solar and am trying to figure out an off-grid system for my cabin in the Eastern Sierra mountains of California. There is no grid power available. The cabin is located at 9700 feet. The Latitude: 38.07; Longitude: -119.23 are approximate. The temperatures range from around 5-10 F in the winter to around 75-80F in the summer. Snow levels get to around 5-6 feet on the average. My load is very minimal but I would like to have a fairly decent sized system to run a TV, computer, small microwave, a few lights with potential for more. My main concern is not to over build but still have room for future expansion Currently I use a Honda EU2000 to provide power at night time, turning it off at bedtime.

So far I have 6 – 205w 24 volt Evergreen ES-E 205 panels. The STC ratings are as follows:

Pmp – 205 watts
Pptc2 – 185.3 watts
Vmp – 28.5 volts
Imp – 7.21 amps
Voc – 35.2 volts
Isc – 7.91 amps
Series rate fuse max is 15 amps
Rated system voltage is 600 volts
Panels have MC4 connections

I will have to mount the array on a pole top mount to get above the snow level during winter months.

The run from the array pole combiner to the controller location would be approximately 80 feet.

I can not decide on whether to connect the panels in parallel (6 panels), or 3 in series x 2 or 2 in series x 3 and then connect them in parallel at the combiner.

So depending on which configuration is best, the appropriate wire size would have to be determined.

I have been thinking about a MPPT controller either the Outback FM 60 or FM 80 or possibly the Midnite Classic 150.

I have quite a few house circuits in my house sub panel that share a common neutral (multi wire branch circuits) that would be very difficult to correct. Based on this, I have been considering the Magnum MS4024PAE inverter that would provide me with 120 volt for each side of the sub panel buss work or 240 volt phase to phase on the busses.

For batteries, I would like to consider AMG as I have a very limited area to put the batteries – mainly in a closet. I don’t know if NEC (or other codes) would allow them to be placed where my direct-vent water heater is or not. I would also like to place the inverter/controller in the same area.

I was considering going 24 volts with the batteries and approximately 400 AH of batteries to start with.

Any suggestions on how to design this system would be greatly appreciated.

Ross
«1

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,235 admin
    Re: Newbie needs help with designing off-grid system

    Welcome to the Forum Ross,

    To not over/under design your system, knowing your peak watts, daily Watt*Hours/kWH, and any seasonal power usage/changes--i.e., winter vs summer+irrigation, etc.).

    For 120 VAC 15 amp circuits, a Kill-a-Watt meter or similar is a great starting point.

    Given that you are running on a Honda eu2000i, you probably are not drawing more than 1,600 watts peak. Plug your Kill-a-Watt meter on your Honda (and get a reading before you shut it down--some newer K-a-W do include a backup battery--but you can still only read it with AC power on).

    As a starting point, you probably are looking at 500 WH to ~3,300 WH per day... 500 WH pretty much bare bones, and 3,300 WH (3.3 kWH) per day should pretty much power a reasonably efficient and power conserving cabin/small home (gas/propane cooking+heating+hot water) with a small well and even a clothes washer.

    Perhaps Dave Sparks will be able to help too with some recommendations... He does off grid solar installs and maintenance in the Sierras.

    Basically, the more you know about your power needs (and you have taken lots of conservation measures), the better you will be able to design your system.

    If you have special needs, such as 120/240 VAC split phase power, then making that choice up front will help avoid confusion too. If you do not need 120/240 VAC power, then not making that a requirement will probably give you more options too (although, there are some really good split phase inverter/chargers out there too, Magnum having good reviews here too).

    Also, if you have 24 VDC (such as a pump for home water pressurization) requirement--Let us know too.

    It is difficult (and expensive) to add anything over 2x worth of power (say solar panels) without usually have to make other changes too (more batteries, possibly changing from 24 to 48 volt inverters, changing solar array wiring, etc.).

    We can work the other way (400 AH @ 24 volt battery bank and a 4kW inverter) and XXXX watts of solar PV array--Then predict how much useful power (by season) you could expect. But--That is our second choice on how to proceed.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • TnAndyTnAndy Solar Expert Posts: 249 ✭✭✭✭✭
    Re: Newbie needs help with designing off-grid system
    Ross wrote: »
    Currently I use a Honda EU2000 to provide power at night time.......

    I have quite a few house circuits in my house sub panel that share a common neutral (multi wire branch circuits) that would be very difficult to correct. Based on this, I have been considering the Magnum MS4024PAE inverter that would provide me with 120 volt for each side of the sub panel buss work or 240 volt phase to phase on the busses.

    If I understand the Honda specs, it only puts out 120v, so how are you using it NOW to feed a dual buss panel ?
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: Newbie needs help with designing off-grid system
    TnAndy wrote: »
    If I understand the Honda specs, it only puts out 120v, so how are you using it NOW to feed a dual buss panel ?

    Most folks who only have/use 120 volts, and don't need 120/240, just put a jumper across from one buss bar to the other, so both sides are fed with the same 120 VAC supply. Works perfectly well that way as long as 240 isn't needed.
  • TnAndyTnAndy Solar Expert Posts: 249 ✭✭✭✭✭
    Re: Newbie needs help with designing off-grid system

    Yep....I know how to do it....I just wondered how he was currently doing it, and why the need to go to a 240v inverter if the Honda 2000 is working for him now.
  • RossRoss Registered Users Posts: 20 ✭✭
    Re: Newbie needs help with designing off-grid system

    Thanks for the many replys.

    The cabin is mainly a seasonal use site and used from June to October on and off for perhaps a week at a time. Winter use means getting in by snowcat, snowmobile, skis, or foot.The water is feed from springs and pumped (gas) to gravity tanks for the small community. The water system is shut down in the winter months due to freezing temps. I have been considering installing an underground storage tank (650 gallons size) for winter use and using a small RV pump. The current electrical use is limited due to running the generator only at night and during a few occassions in the day for vaccum or watching a movie or grand kids games. Currently all lights are 14w CFLs with only 1-3 on at a given time for perhaps 5-6 hours a days. The micro (750 watt) is used a couple of times for 3-6 minutes a day. All other appliances, stove, refgrig, water heater are all propane.
    We have intertained the thought of installing a washer/dryer combo 120v unit which would really be nice to have.

    The main reason to switch over to solar would be the convenience of not having to manually start/stop the generator, reduce the noise factor, have lights available after generator shutdown, leave a porch light on during the night, install security cameras for remote wireless access when we are not at the cabin, possibly Direct TV dish, continuous cell phone charging, and so on.Generally, it would make the cabin more user friendly.

    There is really no need for 240v other than the multi wire branch circuits that share the common neutral. That has not been a concern in the past (no overloading) due to the very limited electrical use in the past. My main concern with that would be the electrical inspector when the install is done and ready for final inspection etc.

    I already have the 6 - 205 watt panels - the remaining equipment is not purchased.

    PS: I know that putting solar in will more than likely never be cost effective as I could run my generator for another 10 years and not get anywhere close to just paying for the solar panels. It's just the idea of doing it for doing its sake.


    Thanks
  • icarusicarus Solar Expert Posts: 5,129 ✭✭✭✭
    Re: Newbie needs help with designing off-grid system

    A couple of thoughts,,


    Your loads are not inconsequential. Remember, all loads grow with time. At the same time, people over estimate the amount of solar they can actually generate, while at the same time under estimating thier loads.

    Base all design based on the loads.

    Second, if you are interested in doing a reliable winter water system, IM me and I can discuss some details of how we do a year round system with little energy use.

    Tony
  • RossRoss Registered Users Posts: 20 ✭✭
    Re: Newbie needs help with designing off-grid system

    Tony:

    Thanks for your reply and further pumping info.

    What would you recommend as far as the panel array set-up. Is it best to go with the higher voltage (3 panels in series and then 2 of these strings in parallel) to have smaller wire size. What about the heat factor for the controller to convert back down to the 24 voltage batteries? One friend said to go 24v from the panels and just use bigger wire - then less concerns about heat issues. Also, how do I figue in the temperature factor for this cold climate during the winter to ensure I don't over voltage etc.

    Ross
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Newbie needs help with designing off-grid system

    Ross;

    I'm going to butt in here because it sounds like you could use a little "ask the man who owns one" advice.

    I have an off-grid cabin with solar-electric and back-up generator. It runs an electric refrigerator, as well as computer/satellite Internet/office set-up, full-size water pump (and septic pump), occasional microwave zapping, and of course lights. This is how it works:

    700 Watts of panels configured as a 48 Volt array. Outback MX60 charge controller charging 320 Amp hours of 24 Volt battery. They power an Outback VFX3524 inverter. There's a step-up transformer for the septic pump, but everything else is 120 VAC.

    There is no problem with heat from the down-conversion, btw. The higher Voltage array overcomes issues with V-drop from the panels to the controller.

    The panel temperature factor is significant in this climate. I have to take a 1.3X superconducting factor into account. Three of the 175 Watts (24 Volt) panels in series would exceed the controller's V-max in of 150 Volts when the thermometer drops (-40 is not impossible here). Generally a good controller and keeping the array Voltage around 2X system Voltage works out well. If you have long wire runs and need an even higher Voltage array the Midnite Classic controller line is your best bet.
  • icarusicarus Solar Expert Posts: 5,129 ✭✭✭✭
    Re: Newbie needs help with designing off-grid system

    Ross, As Marc ('Coot) suggests, cold temps do have an effect, but in the real world over STC conditions only happen very rarely, and then for only short period. When the sun hits the PV, even when really cold, they will heat up quite rapidly, but cold mornings, edge of cloud events can spike the output. A good MPPT controller will just limit it's output when over powered (with in reason)

    Given the choice, I would run my PV and my battery voltage as high as practicable to reduce wiring loses as much as possible. A good MPPT controller will convert more efficiently the bigger the differential between input and output voltage.

    That said, I run 12 vdc panels and 12 vdc battery bank! I run 12vdc batteries because I use a number of 12 volt loads, including phone, fans, lights etc. I run the PV at 12 because of the way my panels are arrayed. It allows me to be flexible over the course of the year as to placement of PV.

    I would wire the panels to 48 VDC, 3 strings of 2, which would be a IMP max of under 30 amps into 48 volts,, ~60into 24 vdc (or ~120into 12 vdc) and use a controller that can handle those currents.

    Tony

    Please note the previous typo had the ucurrent number wrong for 24/12 vdc,NEVER trust my math!
  • RossRoss Registered Users Posts: 20 ✭✭
    Re: Newbie needs help with designing off-grid system

    Okay - Tony and Bill: Help me out here.

    I have run a few numbers and think my peak would be 1500 watt max with approx a 1000 watt hours daily. I was thinking of a 24 volt battery set up with perhaps 225 AH to start with but perhaps 12v would work too. The only 12v load would be a Wilson cell phone amp - but can also go 120v. I like the idea of the Magnum MS4024 120v or 4024PAE 120/240 with either the OB FM60; FM80; or MN Classic 150due to my limited horizontal space - but there may be other obtions. I already have 6 - evergreen 205 w - 24v panels. I may not need all of those but I have them if needed. I'd like to be able to expand in the future if needed. Again my distance from array to Controller/inverter location is 80 feet or so. Tony recommended wiring the panels to 48v with 3 strings of 2.

    Panel info:

    Pmp – 205 watts
    Pptc2 – 185.3 watts
    Vmp – 28.5 volts
    Imp – 7.21 amps
    Voc – 35.2 volts
    Isc – 7.91 amps
    Series rate fuse max is 15 amps
    Rated system voltage is 600 volts
    Panels have MC4 connections


    Based on the above - how do you think my system should be layed out.

    Thanks again.

    Ross
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Newbie needs help with designing off-grid system

    ross, doing this calculation quick and dirty,
    205w x 6 = 1230w in pvs. 1230w/12v=102.5a. that would be 2 of the high end mppt controllers for a 12v battery bank as the current is beyond the abilities of one controller.

    going to 24v for the batteries would make 1 controller viable, but would be pushing it for some of the 60a models. expansions may then be limited to the actual controller used, but a 48v battery bank may be the real solution for expandability and keeping wire resistance losses down. strings with 3 or 4 of those pvs in series will charge a 48v battery bank fine. 2 of them in series is too small as the vmp would not be sufficient for 48v output, but would suffice for a 24v battery bank. the advantage of 3 pvs in series would be that expansions would be less costly being in groups of 3 and the advantage of 4 in series is an even higher voltage to lessen v drops in long wire runs or at least allow for a smaller wire gauge reducing wire costs. of course, expanding the pvs presents more current on the combined wire run and should be accounted for before hand to head off having to tear out the smaller wire that worked for your 6 present pvs to allow for the higher current present after the expansion.

    now a 48v battery bank would present a higher cost in batteries, but i think you are going to need more of them anyhow.
  • RossRoss Registered Users Posts: 20 ✭✭
    Re: Newbie needs help with designing off-grid system

    Niel: I am a little confused on you first line of your info on the panel (205w x 6 = 1230w in pvs. 1230w/12v=102.5a). These are 24v panels so would that make the amperage 51.25a and better to work your calcs with?

    Ross
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Newbie needs help with designing off-grid system
    Ross wrote: »
    Niel: I am a little confused on you first line of your info on the panel (205w x 6 = 1230w in pvs. 1230w/12v=102.5a). These are 24v panels so would that make the amperage 51.25a and better to work your calcs with?

    Ross

    Niel's reference is to the peak current output of all those panels on a 12 Volt system. What matters there is what the Wattage divided by the system Voltage comes to as you are looking at the output side of an MPPT controller. The input doesn't matter. In other words that many panels is too much for one controller on a 12 Volt system. Even if you factor in typical panel/controller efficiency it gets dicey for an 80 Amp controller on 12 Volts (about 78 Amps); the controller could easily be maxed out.

    With those panels and a 24 Volt system it all becomes easier, as the current drops and the Voltage goes up for the same amount of over-all power. You would have to do as Tony suggested and put two panels in series, as the individual Vmp of 28.5 is too low for charging a 24 Volt system.

    You'll probably find you use more than your expected 1 kW hour per day; everybody does. If not now then later. With a 12 Volt system that's easy to come up with as it's less than 100 Amp hours and the 225 Amp hour batteries could do it at 50% DOD. You would only need four of the 205 Watt panels for that small of a battery bank. Again since you are using an MPPT controller you can configure them as two strings of two (nominal 48 Volts) which pretty much eliminates concerns with Voltage drop and fusing (none needed). This might be a little small for your 1 kW hour per day AC, depending on sun conditions.

    If you go for the 24 Volt system, same panel configuration. You could add the other two panels if you go for a bigger battery bank. 1230 Watts will support about 400 Amp hours @ 24 Volts (you could run parallel 225 Amp hour batteries for 450 @ 24 Volts). That would give you better than 2 kW hours AC per day under minimal full sun conditions. Also you would be reducing the DOD for the battery bank to achieve that power (25% of 400 = 100 * 24 Volts = 2400 Watt hours roughly) which will result in longer battery life.
  • RossRoss Registered Users Posts: 20 ✭✭
    Re: Newbie needs help with designing off-grid system

    okay - so if I have this correct for 2000 w/hrs -

    3 strings of 2 panels

    and a 24 volt battery system - I could either go with 4 - 6v 450 AH batteries in series ending up with 24 volt and 450AH

    or

    2 - 12v 225AH batteries in serres for 24v and another 2-12v 225AH in series and then connected two two strings in parallel to obtain 450AH at 24v

    What would be the wire size for the array to controller for the 80 feet?

    Thanks

    Ross
  • mike95490mike95490 Solar Expert Posts: 8,289 ✭✭✭✭✭
    Re: Newbie needs help with designing off-grid system
    Ross wrote: »

    and a 24 volt battery system - I could either go with 4 - 6v 450 AH batteries in series ending up with 24 volt and 450AH

    or

    2 - 12v 225AH batteries in serres for 24v and another 2-12v 225AH in series and then connected two two strings in parallel to obtain 450AH at 24v

    Take the 4 - 6v 450 AH batteries in series, over the parallel setup anyday.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • westbranchwestbranch Solar Expert Posts: 5,140 ✭✭✭✭
    Re: Newbie needs help with designing off-grid system

    you will need to use this little tool to determine wire size. the answer you seek is not written in stone, you can have several options, as you will find. It all depends what you can accept for line losses.

    hth

    http://forum.solar-electric.com/showthread.php?t=29
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Newbie needs help with designing off-grid system

    With 80 feet of wire between the panels and controller you should consider wiring the panels as two parallel strings of three in series. The Voc should remain under the 150 max of the controller and it will greatly reduce the problem of V-drop between the two.
  • RossRoss Registered Users Posts: 20 ✭✭
    Re: Newbie needs help with designing off-grid system

    Okay - great info!

    2 strings of 3 in series with the two strings connected in parallel.

    Based on that the VOC (35.2 ea x 3) should be around 105.6
    Vdc?

    Then the ISC (7.91 x 2) should be 15.82 Adc?

    Based on the above (using Wire Sizer by Solarray) it looks like - #10 provides a voltage drop of 2.6% and #8 VD of 1.6%

    I tried using the Xantrex string calculator and it gave me these figures:

    2 string of 3 modules = Array watts = 1230 wdc and charger amps = 44 Adc
    Max VOC at Mn Temp (10F and 80F high ambient temp) = 121 Vdc and
    Min VMP at Max Temp o = 72 Vdc.

    Based on this (using Wire Sizer by Solarray) it looks like - #4 provides a voltage drop of 1.6%; #8 VD of 3.9%; and #10 VD of 6.3%.

    Which one of the above is more real world correct etc?

    Preferably 4 - 6v 450AH batteries in series.

    Any thoughts?

    Ross
  • mikeomikeo Solar Expert Posts: 386 ✭✭✭
    Re: Newbie needs help with designing off-grid system
    Preferably 4 - 6v 450AH batteries in series.
    Sounds like it would work with the L16 style 420 amp-hour batterys of 4 x 6v batteries in series. Your array of around 1200 watts /28 volts float =42 amps. This can be handled by a 60 amp MPPT charge controller and gives you a 10% charge rate, 42 amps X 10 = 420 amp battery bank. Perfect! You may elect for an 80 amp charge controller if you every expect to expand the system.
  • RossRoss Registered Users Posts: 20 ✭✭
    Re: Newbie needs help with designing off-grid system

    Mikeo: Thanks for the info. Looks like the 4 x 6v batteries in series would work the best. Probably go with the 80a controller for future expansion if needed.

    Ross
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Newbie needs help with designing off-grid system
    Ross wrote: »
    Okay - great info!

    2 strings of 3 in series with the two strings connected in parallel.

    Based on that the VOC (35.2 ea x 3) should be around 105.6
    Vdc?

    Then the ISC (7.91 x 2) should be 15.82 Adc?

    Based on the above (using Wire Sizer by Solarray) it looks like - #10 provides a voltage drop of 2.6% and #8 VD of 1.6%

    I tried using the Xantrex string calculator and it gave me these figures:

    2 string of 3 modules = Array watts = 1230 wdc and charger amps = 44 Adc
    Max VOC at Mn Temp (10F and 80F high ambient temp) = 121 Vdc and
    Min VMP at Max Temp o = 72 Vdc.

    Based on this (using Wire Sizer by Solarray) it looks like - #4 provides a voltage drop of 1.6%; #8 VD of 3.9%; and #10 VD of 6.3%.

    Which one of the above is more real world correct etc?

    Preferably 4 - 6v 450AH batteries in series.

    Any thoughts?

    Ross

    ross,
    i should mention that there is an advantage with only 2 strings being combined as a fuse or circuit breaker will not be required for each string.

    that said, i now have to correct you that you don't go by the voc or isc as these are not real world conditions for operating. i like to use the nominal values of pv voltage as a worst case scenario type thing, but using the vmp and imp should suffice as many times the nominal value is not easily determined.

    what you have is,
    3 pvs in series for a vmp of 85.5v per string at 7.21a imp.
    2 strings in parallel is 14.42a.
    the distance is 80ft for the wire run so there's 160ft of wire total. 2 wires remember?;)
    i used a 48v battery bank voltage for this example.
    i calculate this for 90 degrees c as it is an nec specification wire in solar is meant to withstand and can be considered a worst case scenario, but do understand that wire can get very hot being in the sun and if confined with other wire especially if in conduit while producing some heat of its own due to the wire resistance.

    i did not account for the wire from the pvs to the combiner and i only illustrated the v drop % for the 80ft run itself. other sections of wire like the cc to the battery or from the combiner to the pvs will add to the v drop % presented here.
    #10> 3.44%
    #8> 2.16%
    #6> 1.36%
    #4> .86%
    #2> .54%
    now adding say another 6 identical pvs for another 2 parallel strings of 3 in series would double the current the wire has and will double the v drop %.

    if you configured this for a 24v battery bank the figures for the wire v drops would be double for this too.

    what this means is that for a 24v battery bank and doubling the pvs to 12 would in essence multiply the v drop seen by the combiner to cc by 4x for each of the wire gauge listed %s.

    some say to shoot for a total v drop % of 3% and under while others may say 2%. the smaller the % the better as it is lower losses and lower heat produced in the wires and is up to you as wire is costly these days.
  • RossRoss Registered Users Posts: 20 ✭✭
    Re: Newbie needs help with designing off-grid system

    Niel: Thanks for the great info.

    I used the voltage drop calc that you referenced. I came up with the %s that you listed . But when I use 24 or 48 v batteries and either 6 or 12 panels in the calcs, I do not come up with the 2 or 4 times the volatage drops. I do see increases but nothing along those lines.

    Here is an example of the 2 strings of 3 using #8 wire.:

    2 strings of 3 - 85.5v and 7.21a ea of 85.5v & 14.42 total
    Combiner to breaker - #8 and 160 feet = 2.16% VD (24 & 48 battery bank)
    breaker to to controller - #8 and 12 feet = 0.16% VD (24 & 48 battery bank)
    Controller to battery - 2/0 and 16 feet = 0.34% (24v bat) & 0.085% (48 bat)
    VD from array to controller =2.43% (24 & 48 Battery bank)
    VD from array to battery = 2.77% (24v bat ) & 2.52% (48v bat)

    Am I looking at this incorrectly?

    Thanks

    Ross
  • icarusicarus Solar Expert Posts: 5,129 ✭✭✭✭
    Re: Newbie needs help with designing off-grid system

    So here is a question I should know the answer to, but don't

    We know that voltage drop is a function of voltage and wire size, but does voltage drop percentage mean a similar drop in net power output? For example, using Niel's voltage drop number, of #10 wire of 3.44%: 24 vdc would then drop .082 volts. (assuming my suspect math is correct. It you put 10 amps through a wire with no drop at 24 vdc, you would get 240 watts of power out, right? But if you put the same 10 amps, but at only 23.91 volts, you would get 239.17 watts out? That works out to a throughput of 99.65%.

    The question then becomes (assuming I am right!) is less than one percent loss enough to worry about?

    Change the numbers for different voltages and wire loses and the question remains the same. How much net power is lost in the net/net with any given percentage of voltage drop?

    Tony
  • BB.BB. Super Moderators, Administrators Posts: 29,235 admin
    Re: Newbie needs help with designing off-grid system

    Depends on the circuit/source/load characteristics.

    Power=volts x current = volts^2 / resistance = current^2 x resistance

    So, for solar panels, they are mostly fixed current, so in this case, a 10% voltage drop is a 10% loss of power.

    If you double the current on a fixed wire resistance, the the losses go up by 4x.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarusicarus Solar Expert Posts: 5,129 ✭✭✭✭
    Re: Newbie needs help with designing off-grid system

    Bill,

    I confess I don't understand the function of the symbol "^" used in the second part of your equation. Can you clarify for me?

    Tony
  • westbranchwestbranch Solar Expert Posts: 5,140 ✭✭✭✭
    Re: Newbie needs help with designing off-grid system

    Tony, I may be wrong but I'll bet BB means that ^2 = "to the power of" 2 or 'volts squared' in this case

    Eric
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • BB.BB. Super Moderators, Administrators Posts: 29,235 admin
    Re: Newbie needs help with designing off-grid system

    Yep... I have seen ^ and ** both used as text for exponents... Too bad we don't have super scripts here in the forum.

    -Bill

    PS: I did the above on my phone--and I see it filled in "write" instead of "wire" in the previous post--Now fixed.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RossRoss Registered Users Posts: 20 ✭✭
    Re: Newbie needs help with designing off-grid system

    Bill, Niel, Tony and others:

    I have been reviewing all the comments that everyone has made. There have been some great suggessions on how to design my new system. There was some discussion on batteries as to whether to use 24v or 48v. Would there be an advantage of going with the Magnum MS4048PAE versus the MS4024PAE inverter other than some wire size reduction between the battery and inverter?

    Thanks

    PS: Happy Holidays to everyone.
    Ross
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Newbie needs help with designing off-grid system

    Ross;

    The reasons I would choose a higher system Voltage would be if the expected current draws were high and/or the needed Watt hour capacity was great. Exactly what point in power requirements you decide to switch is debatable. Technically the higher Voltage system is ever so slightly more efficient.
  • BB.BB. Super Moderators, Administrators Posts: 29,235 admin
    Re: Newbie needs help with designing off-grid system

    Yipee!

    For Tony's sake, I can now use exponents with the new forum software:
    • Power = Volts*Amps = Volts2 / Resistance = I2 * Resistance :D
    Regarding the question of 24 vs 48 volt inverters (note your 48 volt inverter is actually a 4448, not a 4048--so it is 10% higher rated)...

    First, always want to confirm you need 4kWatts of load... These inverters use around 22 to 27 watts just to turn on (interestingly the 48 volt inverters use a bit less power and are a bit more efficient 93% vs 90% for 48:24 volt models).

    You need a lot of solar panels + backup genset to recharge after you have supported the loads these guys are capable of. Also, note that if your loads tend towards the 400 watt or less range (few lights, laptop/TV receiver, etc.), your inverter efficiency is less (i.e., a small inverter to supply your many hour a day smaller loads and a larger inverter for washer/drier/well pumps, etc.).

    That all said, I would tend towards 48 volt inverter. You are looking at 2/O wires instead of the 4/O needed to run the 24 volt model.

    Don't get me wrong, the Magnums you are looking at are very nice inverters and quite a few people use them. Especially nice if you need 120/240 VAC output. No additional transformers required.

    However--Start looking at your Balance of System stuff... For example you can get some pretty nice 12 and 24 volt water pumps; 48 volt may be more difficult (i.e., 24 VDC marine and diesel truck appliances).

    A backup AC to DC battery charger--If you need a backup charger, can you find one in the ratings you need to recharge your battery bank with your genset (power factor correction, etc.).

    And, wind turbines... They tend to be designed to be more efficient when charging 12 volt battery banks (in low average winds). Finding turbines that will produce well directly at 24 and 48 volts are a bit more difficult.

    Also, building out your battery bank. I am in favor of single string banks (if possible) and recommend against more than 2 or 3 strings in parallel (current sharing, number of cells to check, difficult to catch various failures without closely monitoring bank, etc.).

    When picking batteries--If you do not have easy access (flat concrete floor, lift/crane/etc.) to move cells/banks around--You have to look at the maximum battery/cell you can move with one or two people... So, you have to choose between 12/6/4/2 volt cells.

    Note that a 6 volt @ 220 AH battery vs a 2 volt 660 AH cell have exactly the same stored energy (E=AH*Voltage) and have the same weight/dimensions.

    You could build a 24 volt 660 AH battery bank with either 3 strings * four [email protected] AH batteries in series (12x 6 [email protected] AH batteries) or a single string of 12x [email protected] AH cells in series.

    The 6 volt batteries should have 3 fuses/breaker (one per string) and will have 36 cells to check electrolyte levels in.

    The 2 volt batteries will need one fuse/breaker and have only 12 cells to check.

    Also remember that MPPT solar charge controllers are rated based on their output current. As you increase the bank voltage, the same controller can manage a larger array... For example, a 60 amp controller with 0.77 derating:
    • 60 amps * 14.5 volts charging * 1/0.77 derating = 1,130 watt nominal maximum solar array
    • 60 amps * 14.5 volts charging * 1/0.77 derating = 2,260 watt array
    • 60 amps * 14.5 volts charging * 1/0.77 derating = 4,519 watt array
    So, you need to buy fewer MPPT charge controllers for your 48 volt vs 24 volt array.

    In the end, you probably will need to make a couple paper designs to see which will work best for your installation (and what you can buy locally/get shipped to your address).

    Until you have made the decision for almost every piece of equipment--you don't know what will be "optimum" for your needs.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Sign In or Register to comment.