Sizing DC solar system for booster pump

DesertDwellerDesertDweller Registered Users Posts: 13 ✭✭
Hi All,

Looking for some help as solar calculations really confuse me!

I'm trying to setup an off-grid well house. My well pump will be powered directly from panels. Then water will get stored in a cistern, then a booster pump to pressurize water into the pressure tank. I'm trying to gather my equipment and just want to be sure everything is sized right!

Here's the plan for the second leg of the system:
I would like the set up the Aquatec 550 24vDC to pull water out of my 1275 gal. cistern and into my 86 gal. pressure tank. I am trying to size a DC solar system to power this. Can you advice me on size of PV panels and size of battery to get?

I am thinking of a 260W PV panel connected to an mppt charge controller and then 12V 200aH battery. Can anybody confirm whether this will work or not? Any advise would be greatly appreciated!


  • BB.BB. Super Moderators, Administrators Posts: 31,618 admin
    Welcome to the forum DesertDweller,

    One thing you did not tell us was how much water (hours per day) you will be running the pump or the water pressure... Just making some guesses (feel free to alter the math below to fit your actual conditions). You did not tell us that you have other loads for this system (lights, etc.)--If just water pumping, that will be the calculations below.

    The pump seems to be rated at 3 amps (no head pressure) to 8.5 amps @ 70 PSI. And is rated for for a max of 25 minutes at 25 PSI (not for many hours of irrigation use per day). So guess at 6 amps @ 24 volts @ 2 hours per day usage around Albuquerque New Mexico.

    Sizing the battery bank... A good optimum seems to be 2 days of energy storage and a maximum of 50% planned discharge (bad weather).
    • 6 amps * 24 volts * 2 hours per day = 288 Watt*Hours per day
    • 288 WH per day * 2 days storage * 1/0.50 max discharge * 1/24 volt battery bank = 48 AH @ 24 volt battery bank minimum
    Next, double check battery's ability to supply the needed current. Battery should be discharge at no more than an 8 hour rate (for flooded cell lead acid battery bank for long life):
    • 6 amps * 8 hour discharge rage = 48 AH @ 24 volt minimum battery bank
    Next, two solar array sizing steps. One based on the size (AH and voltage) of battery bank--Large batteries need more charging current. And second based on location, season, and energy usage by load.

    A good starting point for charging from solar... 5% of bank AH capacity can work for a weekend/sunny weather system. For full time off grid, typically 10% to 13% is better for longer battery life and less day to day "energy management" (less water use during bad weather).
    • 48 AH * 29.0 volts charging * 1/0.77 solar panel+controller deratings * 0.05 rate of charge = 90 Watt array minimum
    • 48 AH * 29.0 volts charging * 1/0.77 solar panel+controller deratings * 0.10 rate of charge = 181 Watt array nominal
    • 48 AH * 29.0 volts charging * 1/0.77 solar panel+controller deratings * 0.13 rate of charge = 235 Watt array "typical" cost effective maximum
    You are looking at 2x 12 volt @ 100 AH batteries in series for 24 volt volts. Or 100 AH @ 24 volt battery bank... The calculations for the larger bank (not going to hurt anything with larger bank):
    • 100 AH * 29.0 volts charging * 1/0.77 solar panel+controller deratings * 0.05 rate of charge = 188 Watt array minimum
    • 100 AH * 29.0 volts charging * 1/0.77 solar panel+controller deratings * 0.10 rate of charge = 377 Watt array nominal
    • 100 AH * 29.0 volts charging * 1/0.77 solar panel+controller deratings * 0.13 rate of charge = 490 Watt array "typical" cost effective maximum
    So--The larger battery bank does come at a cost of a larger solar array to keep the batteries "happly charged"... But that will certainly give you a long and relatively maintenance free battery bank life (if flooded cell, you will need to check electrolyte levels and once a month, and add distilled water every month or six).

    And sizing for daily loads and hours of sun at your location. Assuming Alburquerque New Mexico, fixed array facing south:

    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 55° angle from vertical:
    (For best year-round performance)

    • 288 WH per day * 1/0.61 DC off grid solar system eff * 1/4.76 hours of sun per day (Dec) = 99 Watt array "December break even"
    You should plan on using ~65% to 50% of your predicted daily harvest for "base loads"--Those loads that you have to run every day (lights, water pumping for home, etc.) that cannot be put off for sunny weather (such as pumping to a large cistern):
    • 99 Watt array * 1/0.65 base load fudge factor = 152 Watt array minimum @ 65% base load factor
    • 99 Watt array * 1/0.50 base load fudge factor = 192 Watt array minimum @ 50% base load factor
    So--Your paper design of 24 volts @ 100 AH battery bank looks good.

    I would suggest a larger solar array at ~377 Watts for full time off grid for a "happier" battery bank (10% rate of charge). However, 260 Watt array is >5% rate of charge--So stil not bad (relatively light loads on your battery bank).

    The 86 gallon pressure tank--Talk with your pump supplier... That may be on the large size for your selected pump (no thermal protection. If running at >>25 PSI, large tank + water usage may take more than 10 minutes of "full pump loading" maximum suggested runtime (just a concern--I am not a pump engineer).

    Last question/issue... The 260 Watt solar panel... You need to look at the specifications for that panel. See if the Vmp is ~30 volts or ~36 volts. To charge a 24 volt battery bank, you need >=35 Volts Vmp to account for losses (hot panels, Vmp can fall by up to 20%--I.e., 30 volts Vmp-standard-conditions * 0.80 = 24 volts Vmp-array-hot... And you need ~30 volts to charge a 24 volt lead acid battery bank).

    And that leads to the MPPT controller question... for Vmp-array of ~35-40 volts, PWM controller will be just about as efficient and cheaper than an MPPT controller... If you are interested in MPPT (many good reasons for that)--The Vmp-array should be >~1.3x the Vbatt voltage:
    • 30 volts Vbatt charging * 1.3 MPPT array voltage = 39 volts Vmp-array-std 
    So--For Vmp of 30-36 volts (typical for large format solar panels), you really should be looking at 2x panels in series for Vmp-array in the 60-72 Volt range. So 2x 260 Watt panels in series would be OK (520 Watts vs 490 Watts suggested maximum "cost effective" array for 100 AH @ 24 volt battery bank--That would be just fine)... Of course, you could use 2x lower wattage panels in series too...

    My two cents... Your thoughts?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • DesertDwellerDesertDweller Registered Users Posts: 13 ✭✭
    edited March 28 #3
    Thank you Bill, I really appreciate all your calculations! Not sure if I understand them all right now (it will take some studying) But I'll tell you what I know so far:
    -I want to minimize roof space because I need to leave room for the panels I need to run the well pump (Grundfos SQF-2). That is why I want to just use ideally 1 panel. At tech support they suggested (1) 250-275 W panel.
    -My local solar supplier has a 310W panel with Max Power Voltage (VMPP)-36.8 Do you think this will be better?
    -Can I run the Aquatec 550, 24vDC pump on a 12V battery? (seems like these are more readily available). You were mentioning a 24V battery @48AH. Can I use a 12V battery at 100AH? Or should I get a battery with more AH?
    -As for Charge Controller, I'm thinking the Renogy Rover Elite 40A mppt CC.
    -Also, I would ideally like to run a few DC LED lights and 2 fans for this wellhouse/greenhouse I built to house all this. Not sure if that complicated things. If it steps up all the equipment a lot more, I might just wire those things to the grid, since I do have grid power access.
    -I will have to research how to wire all this together. I am wondering how to connect at least the fans/lights to switches that I can turn on/off. I'm thinking something like this:
    Any other suggestions I'd greatly appreciate it!!
  • BB.BB. Super Moderators, Administrators Posts: 31,618 admin
    -My local solar supplier has a 310W panel with Max Power Voltage (VMPP)-36.8 Do you think this will be better?

    That would be perfect for a PWM controller, and will work with an MPPT (MPPT will not really increase harvest much)

    -Can I run the Aquatec 550, 24vDC pump on a 12V battery? (seems like these are more readily available). You were mentioning a 24V battery @ 48AH. Can I use a 12V battery at 100AH? Or should I get a battery with more AH?

    A clarification... A lead acid "battery" is just a collection of 6x 2 volt cells connected in series. A 12 volt @ 100 AH battery is just 6x 2 volt @ 100 AH cells.

    A 24 volt battery is just 12x 2 volt cells in series... That could be:
    • 12x 2 volt @ 100 AH cells = 24 volts @ 100 AH battery bank
    • 2x 12 volt @ 100 AH batteries in series = 24 volts @ 100 AH
    • 4x 6 volt @ 200 AH batteries in series (aka "Golf Cart" batteries) = 24 volts @ 200 AH
    • etc...
    Note.. When you add batteries in series, the voltage adds.

    When you add batteries (or strings of batteries in parallel), then they add AH capacity.

    For example... Say you have 2x 12 volt @ 100 AH batteries available... You can wire them as:
    • 1 series x 2 parallel = 12 volts @ 200 AH battery bank
    • 2 series x 1 parallel = 24 volts @ 100 AH battery bank
    Both are still 2x 12 volt batteries. And they both are the same amount of "stored energy":
    • 2 x 12 volts * 100 AH = 24 volts * 100 AH (2s x 1p) = 12 volts * 2x 100 AH (1s x 2p) = 2,400 Watt*hours (either configuration)
    There are advantages and disadvantages to series and parallel configurations, vs just 1 string of larger batteries...

    12 volt vs 24 volt water pump... As far as I can tell, both 12 and 24 volt A-550 pumps are identical except for their operating voltage. The 24 volt pump costs ~$7 more.

    A bit of electrical math: Power = Volts * Current

    The 12 volt pump: Power draw at no head is 5.5 amps, up to 18 amps at 70 psi.
    The 24 volt pump: Power draw at no head is 3.0 amps, up to 8.5 amps at 70 psi.
    • 12 volts * 18 amps = 216 Watts
    • 24 volts * 8.5 amps = 204 Watts
    The 24 volt pump is slighting more efficient (might even be the same Wattage--Just roundoff errors in measurements/specifications).

    But note that the 12 volt pump uses 2x more current than the 24 volt pump. That means you need heavier copper wiring and generally the 12 volt pump needs to be closer to the battery bank vs the 24 volt pump (if that is an issue).

    So, higher voltage wiring and equipment tends to be more efficient and cheaper to install vs lower DC Voltage wiring (i.e., 14 AWG wiring for 24 volt pump and 12 or 10 AWG wiring for 12 volt pump--Just a quick guess--Details always matter in solar/off grid power).

    And you also need to look at the battery bank... 6 volt @ 200 AH "golf cart batteries" tend be cheap and available "everywhere). Say we decided on the 24 volt @ 100 AH or 12 volt @ 200 AH battery bank as the "correct size" for you needs (certainly seems fine).

    Your options are:
    • 2 x 12 volt @ 100 AH batteries: 2s x 1p = 24 volts @ 100 AH
    • 2 x 12 volt @ 100 AH batteries in parallel: 1s x 2p = 12 volts @ 200 AH
    • 2 x 6 volt @ 200 AH batteries in series: 2s x 1 = 12 volts @ 200 AH
    Note that a 6 volt @ 200 AH battery stores the same amount of energy (Watt*Hours) as a 12 volt @ 100 AH (about the same weight/size). Remember Power=Voltage*Current (or AH capacity).

    If you wanted a 2x larger (Watt*Hour) capacity bank (just as an example)--Here are 4x 6 volt @ 200 AH batteries for 12 and 24 volt banks
    • 4 x 6 volt @ 200 AH batteries in series/parallel = 2s x 2p = 12 volts @400 AH
    • 4 x 6 volt @ 200 AH batteries in series = 4s x 1p = 24 volts at 200 AH
    I like 24 VDC for pump (lower current draw). But a 12 volt with 2x 6 volt "GC" batteries (2s x 1p) has its avatages (possibly cheaper "golf cart" batteries... Only 6 cells to check specific gravity/electrolyte levels vs the 12 volt batteries with 12 cells to check, and more wiring to interconnect.

    And your 310 Watt Vmp~36 volt panel with a 12 volt battery bank would be a very nice fit. A PWM controller should not be used for 310 Watt panel with a 12 volt battery bank (need MPPT function).

    With a 24 volt battery bank and 310 Watt panel, a PWM controller would work well and be cheaper than a MPPT type charge controller (MPPT will also work, but will harvest little "extra energy" over PWM in this setup).

    Now you bring up "the grid"--If you can bury some cable from the home to the pump house--That could save you money (solar+batteries are almost always more expensive than the grid--Plus more maintenance).

    The A-550 is also available in 120 VAC:
    Power draw at no head is 0.89 amps, up to 2.10 amps at 70 psi.

    Regarding LED lighting... There are lamps/"bulbs" that run on either 12 or 24 VDC... So would work on ether 12/24 VDC system:

    And you can get 12 or 24 VDC fans... Fans are a bit of a mixed issue... DC fans tend to be smaller and move less air (and use less power). Mechanical Air Handling (and bright lights) can take a lot of energy... And while the pump may run 2 hours a day... The lights/fans may run 12 hours per day/night--Or 6x longer than the pump, and using as much, or more energy per day vs the water pump.

    And that is the big issue with solar--You design the solar for the loads you know... Or you design for future (undefined loads) which costs you more money up front.

    Switches--Another bag of worms... DC current is "more difficult" to switch vs AC current... The switch you linked to does not list current rating--But others that look the same are rated to 4.8 amps. Your DC water pump draws much more current.. Lighting???

    Suggest not using cigarette lighter plugs for DC power--Unless you have specific needs (like a DC car charger for cell phone). The lighter plugs just are not very stable (I have to work to find ones where the plugs don't walk out by themselves).

    There are various DC plugs around--If you need to get some, we can talk more.

    Anyway--What you need. But if you only need to trench 100-200 feet of wiring for the small booster pump--I would bring AC from home panel... I would only go with solar/battery if you really needed it (backup power, unreliable AC power, etc.). Many DC pump motors are "brushed" (universal) motors (like a simple electric drill)... Brushes (and the commutators) have limited life. AC motors (and more expensive DC PM motors) will last many times longer. Maybe 1,000 to 5,000 Hours for a set of brushes (if these are brushed motors--I cannot tell).

    Lots of choices here... Setting up a DC or AC "slow pump" to cistern with boost pump is a good start... The whole AC vs DC choices (or even 12/24 VDC battery bus) is a lot of personal choices and what you feel comfortable with.

    Making your own power system--You will need to maintain and replace parts down the road--Design an efficient and reliable system may cost more up front--But can save a lot of hassles down the road.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • DesertDwellerDesertDweller Registered Users Posts: 13 ✭✭
    Thank you so much for your time and advice! I'm sure I will have more questions down the road. I will ponder your guidelines for now...
  • Wheelman55Wheelman55 Registered Users Posts: 139 ✭✭✭
    edited March 30 #6
    Guys..if this is the 86 gallon pressure tank you are looking at, it has a draw down of 23 gallons. 

    The pump is 5.6 to 2.9 GPM so you are only looking at a few minutes run time to fill the pressure tank up.  At 5 GPM that's 300 gallons an hour out of a 1,200 gallon tank...hard to imagine that your house will require that much water :)

    How many GPM does your well produce?  Have you done a back of napkin estimate on your daily water usage?
    Building Off-Grid in Terlingua, TX
    14 CS 370 watt modules. HZLA horizontal tracker. Schneider: XW6048, Mini PDP, MPPT 80-600, SCP. 1 Discover AES 48 volt LiFePO4 battery 130 ah
  • BB.BB. Super Moderators, Administrators Posts: 31,618 admin
    edited March 30 #7
    They do recommend at full pressure, 10 minute run time 2.9 gpm... that would be 29 gallons.
    Since these pumps can overheat if the limit is exceeded, just need to confirm nominal pumping time and pressure.
    Secondary issues like pump wear (2-5 years) does reduce gpm and lengthen run times.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • DesertDwellerDesertDweller Registered Users Posts: 13 ✭✭
    edited March 30 #8
    The pressure tank is already chosen and I went with a larger capacity in order to reduce the pump run-time.
    The pump is the missing link, so I am trying to choose the right one so everything is sized right. I'm thinking the Aquatec 550 series. Originally I thought the 24 V would give me more power, but now I'm considering the 12V thinking it will be more compatible with equipment. I have a Goal Zero backup generator (Yeti 1500X), but I think the output ports only support 12V (?). Also, would this reduce my battery/panel size requirements?
    I'm trying to keep the system simple, but not underpowered. But I also don't have room for more than 1 panel, so I don't want to overpower it unnecessarily. And I would ideally want to run off 1 battery that is sufficient.
  • BB.BB. Super Moderators, Administrators Posts: 31,618 admin
    I don't see any specific reason that 24 vs 12 volts has any great advantages for this size system... If you have 12 volt equipment, that is one check mark for 12 volt battery bank. And, personally, I would suggest 2x 6 volt @ 210 AH (+/-) in series vs 2x 12 volt @ 100 AH batteries in parallel. Fewer cells to water. No parallel wiring (batteries and wiring tend to need less maintenance--And hopefully the 6 volt batteries are cheaper and easier to get).

    If  you choose 12 volts, then you will need heavier 12 volt wiring for loads (such as the pump), and longer wire runs @ 12 volts will require much heavier wire to keep the voltage drop down (suggest 0.5 volt max planned drop)--And practical distances from battery bank to 12 VDC loads will be shorter (higher currents, low voltage drop requirements).

    And with your panel, you will need an MPPT type charge controller to match the >30 Vmp panel with the 12 volt (~15 volt charging) battery bank (MPPT controllers take the higher voltage/lower current from the array and efficiently down convert to the lower battery bank voltage and higher charging current--Power=V*I). The single ~300 Watt panel should be cheaper/less wire/mounting hardware than (for example) 2x 1XX Watt "12 volt" (aka Vmp~18 volt) solar panels in parallel.

    You should be happy with the results (assuming you don't pile on too many other 12 volt loads/run the pump more hours per day).

    Only major down side--A 12 volt system cannot be expanded as much in the future (if needed) vs a 24 volt system... Suggest a 12 volt system is around 1,200 to 1,800 Watts max... And a 24 volt system around 2,400 to 3,600 Watts max. And for either, ~800 AH battery bank before stepping up to the next higher voltage bank.

    And, if you do go higher bank voltage, then your 12 volt loads have to be changed to 24 volts (and/or you may need a 24 to 12 VDC buck down converter--Do not pull "12 volts" from a 24 volt battery bank--It unbalances the the upper vs lower 12 volt battery sets).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • DesertDwellerDesertDweller Registered Users Posts: 13 ✭✭
    Ok, I've gone ahead and acquired a 40A MPPT charge controller by Renogy:

    I also got a 12V 100aH AGM battery:

    I decided to go with a battery made for solar instead of golf cart or deep cycle car battery. It just seems better suited for the application.

    I'm guessing I will need a second battery of this same size...? But I wanted to try with 1 first. Not sure if that will do any damage.

    Now I'm still deciding whether to get this 12V or 24V  Aquatec booster pump. I'm trying to get some clarification since I don't want the 24V to be incompatible with anything. I am getting a Goal Zero Yeti 1500x solar generator to use as a backup for anything. I want to make sure I don't run into problems if I ever have to run this pump with this.

    Goal Zero customer service keeps assuring me that I can simply put a regular plug on my 24vDC pump and plug it into the 120V AC port.

    This sounds very unsafe, although I know nothing of electrical. What are your thoughts?

    This is what the customer service replied and also a graphic of the ports included on the Yeti:

    If the 24 v pump has an AC adapter (wall charger), then you would be able to safely plug that into the 120V plug. At the same time, the Yeti 1500x would be able to provide enough power to run your pump. 

    The 120 AC does have an inverter on it, so it would be able to provide power to your pump. The port that you would want to use, is the one that looks like a standard wall plug.


  • Wheelman55Wheelman55 Registered Users Posts: 139 ✭✭✭
    edited March 31 #11 do realize that the Yeti 1500 setup is a Lithium battery, a MPPT charge controller, an inverter and several power ports?  Instead of buying a second system, why not use the Yeti, charge it with the solar panel, and use a 120 to 24 volt transformer to run the pump?  
    Building Off-Grid in Terlingua, TX
    14 CS 370 watt modules. HZLA horizontal tracker. Schneider: XW6048, Mini PDP, MPPT 80-600, SCP. 1 Discover AES 48 volt LiFePO4 battery 130 ah
  • BB.BB. Super Moderators, Administrators Posts: 31,618 admin
    Goal Zero customer service keeps assuring me that I can simply put a regular plug on my 24vDC pump and plug it into the 120V AC port.

    NO. The 120 VAC pump could plug into the AC inverter 120 VAC output...

    The 24 VDC pump requires 24 VDC power... Either directly from a 24 volt battery bank, or some sort of converter (whatever in => 24 VDC out).

    You can find converters from 12vdc to 24vdc, 24vdc to 12vdc... 12vdc to 120vac (AC inverter), etc. Generally, you want to avoid converters... Extra costs, and more things to "figure out" to make work, some extra lost power, etc...

    The Goal Zero 1500x has a 1,500 Watt*Hour Li Ion battery internally (LiFePO4?). Your 1,2 volt @ 100 AH battery stores:
    • 12 volts * 100 AH = 1,200 WH
    That is just a bit less than the GZ 1500... But the 1500X costs something like $2,000 vs something like $340 for the AGM battery. And very roughly nearly $1,000 for a 12 volt @ 100 AH Lithium Battery.

    Using the GZ 1500X for your boost pump--Probably not a good use of $$$. Adding another battery and solar panel(s) would give you more power (daily harvest) vs that $$$ for a one time hit of energy (and still have to pull it out and recharge elsewhere).

    Another way of looking at short term/backup power... A small genset is around 3-4 kWH per gallon of fuel:
    • 1.5 kWH lithium * 1/3kWH per gallon = ~1/2 gallon of fuel worth of energy...
    A small genset + battery charger or AC powered pump + 5 gallons of fuel will give you more than a week of backup power (bad weather, failed solar panels, etc.).

    Regarding mixing old and new batteries... Ideally, you would like "matched" sets in a battery bank. Buying one and using for a short period of time, then adding a 2nd battery is not going to be a big issue... If you put two batteries in series--They should be balanced/charged equally before connecting in series and exercising... You don't want one full battery and one discharged battery in series... The empty one will take a very long time to recharge to full (especially if AGM or Li Ion), and if you discharge, the "low battery" will go dead first and be damaged while the other battery still has lots of energy (worst case example).

    Deep Cycle Lead Acid batteries (flooded, AGM, etc.) are all designed for solar (more or less). The batteries you want to avoid are those that ARE NOT Deep Cycle type... For example, there are "Float Service" batteries (UPS use) or SLI (starting/lighting/ignition for car batteries)--And those are not good for daily solar cycling.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • DesertDwellerDesertDweller Registered Users Posts: 13 ✭✭
    edited March 31 #13
    Great, thanks for all your help. You're really the only one giving me straight answers!
    I'm leaning towards a 12V DC system for my booster pump. It seems small enough and I don't want trouble with converting DC. I'd like to find some 12V lights and 2 fans to go off the batteries as well. My equipment so far:
    -12V 100aH AGM
    -310 W 72 cell PV panel
    -40A mppt charge controller
    -I'm just not sure if the 310 panel is 12V and how to check for that.
    -Should I just go ahead and get a second 12V battery before I hook it all up?
    -Now I'm leaning towards the Aquatec 550 12V booster pump.
    Hope that all works and thanks for your advice!
  • BB.BB. Super Moderators, Administrators Posts: 31,618 admin
    Think of the MPPT (maximum power point tracking) solar charge controller the equivalent of an "automatic transmission"... It takes the energy of the solar panel and automatically changes it to match the requirements of the battery bank.
    • Power = Voltage * Current
    • 310 Watt panel @ ~36 volts Vmp
    • Current = Power / Voltage
    • Current (Imp--current maximum power point) = 310 Watts / 36 volts = 8.61 amps (under full noon time sun/perfect conditions) from solar panel
    • Battery Current = Power / Voltage = 310 Watts / 14.4 volts battery charging = 36 Amps charging to battery bank
    That is the "Magic" of MPPT charge controllers... They take high voltage/low current from the solar array and efficiently "down convert" to the lower voltage/higher current needed by the battery bank.

    Vpanel input for your controller--15 - 100 volts (from spec sheet--as I recall). So any solar panel that is (roughly) Vmp~20 volts to 66 volts will work fine (there are a whole bunch of "engineering reasons" why not 15-100 volts Vmp-std--For the moment--just a "real life" of matching solar panel Vmp to Vpanel input of the charge controller).

    Also, in real life, you will not get much over 77% of 310 Watts Pmp into your battery bank... Vmp falls as the solar panel gets hot (under full sun) and some electrical losses inside the charge controller.

    So, in this case, your 310 Watt panel appears to be "perfect" for your needs charging a 12 volt battery bank.

    Assuming that my initial guesses about your pump usage are correct, and the hours of sun per day, etc... are also correct--Then get the 2x 12 volt @ 100 AH AGM batteries and connect them in parallel for 12 volts @ 200 AH. (6 volt @ 220 AH AGM) (12 volt @ 210 AH AGM)

    The above are ~$600 worth of AGM deep cycle batteries... Vs less than $200 for 2x 6 volt @ 200 AH "golf cart" type batteries from Costco/Walmart/etc... 

    I would highly suggest that you also look at 6 volt @ ~200 AH "Golf Cart" deep cycle batteries... They may only last you 3-5 years ("good' AGMs properly charged should last around 5-7 years in moderate climates), and you have to check electrolyte levels in an FLA battery once a month (and keep terminals clean). But they are usually much cheaper than AGM batteries. And you spend a little money to setup your system, and see how it works... If you need to change it (larger/smaller/agm/LiFePO4/etc.) batteries--You have not dump a bunch of $$$ into AGM.

    Also, if you are looking to parallel connect the batteries, here is an explanation on how to parallel properly:

    Personally, I prefer not to parallel batteries, and 2-3 parallel strings is my suggested maximum if you have no other option.

    And with the AGM batteries, you might look around for 2x 6 volt @ 200 AH (2 in series), or even a 12 volt @ 200 AH single battery. There are usually lots of options out there.

    Just be very careful about monitoring your battery(ies). Many of us have "murdered" our first bank or two of batteries (under charging, over discharging, etc.)--And another reason that we suggest "cheap" Golf Cart FLA deep cycle batteries" for the first bank or two... If you do kill them, your bank account is not hit so hard.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 31,618 admin
    And you are very welcome on the advice... We all try here to give you the information needed for you to make your own design/component selections.

    That way you can make the choices that make sense for you and your needs... Not based on our guesses of what your needs and wallet size are.

    I don't see any reason to not use a 12 VDC pump... There are lots of them out there, and 12 VDC is handy for power other small loads.

    And, as always, this is based on your power needs. If you start adding more loads--Then may need to revisit your system design. Fans tend to take a lot of energy--So if you are looking for fans on a green house that may run 12 hours per day--That could easily be more loading than your 12 volt water pump.

    With fans, providing enough vent area (air inlet and air outlet) with possibly a "chimney" of some sort (hot climate, moving more air).

    If you need a fan(s)--Their power requirements are not small. Figuring out the ventilation (passive/active), etc.--May be a good reason to bring utility power to the site:

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • DesertDwellerDesertDweller Registered Users Posts: 13 ✭✭
    Ok, new question. An electrician friend brought up the possibility of tying in both my systems into 1. He's wondering if the well pump (being powered by 3 panels) will fill my cistern and be done working for a while. Can I then use the excess energy from these panels to charge my batteries that will power the booster pump? I can step up the system to 4- 300W panels in this case.
  • BB.BB. Super Moderators, Administrators Posts: 31,618 admin
    edited April 9 #17
    In general, you should not "share" a single array between two or more MPPT type charge controllers (or active SQflex type pumps)... For the most part, each MPPT/SQE assumes it is 100% in control of "its" array. Could you "switch" the pump array over to the solar battery system once you are done pumping--Yes probably (need to match arrays for voltage, etc.).

    When you share the array between the two devices, the Pmp=Vmp*Imp search software can get all confused (one MPPT controller doing search, the second "sees" clouds going by and/or may need to do its own search, etc.).

    Yes--The idea of one large system vs several small systems--I like. That way the total power harvest can be used where needed... Vs each solar system is its own "island" (no power/harvest sharing).

    But you need to look at your (for example) pump needs... Will you want to change from DC SQFlex to AC SQ (I think SQ pumps are cheaper?)-- I.e., now adding an AC inverter, larger battery bank to support loads, monitor bank for extra power usage (i.e., the pump does not drain battery bank dry as it tries to fill cistern on a cloudy day, etc.).

    The solar panel => DC powered pump--It the definition of "low maintenance". No sun, no pumping, no dead batteries, no battery maintenance.

    When you combine into one big system--Assuming the other system(s) need battery bank, and now you need AC inverter (24 VDC may not be high enough to run "your" DC well pump, for full power, something like >=90v Vmp, etc.).

    This is the point where you spend your time on research and paper designs (i.e,. the cost/benefit of 2x separate vs 1x larger system). Paper design both systems and see what works best for you.

    The solar panels these days are about the cheapest part of the solar power system these days (i.e., $0.50 a watt for large format panels deals can be found). If you only need to pump water when the sun is up/clear days... It is pretty compelling to have a pumping system you don't need to worry about--Adding more batteries, AC inverter, more panels (battery+inverter losses for off grid battery system), etc. to run the pump--Just may not be worth it (to share X00 Watts of solar panels).

    Originally, your DC/solar power needs were quite small... Adding the well pump and other loads certainly make a "redesign" for the original "small" system a necessity--If not simply "another" system design (small RV type water pump; medium system +lights+fans; large system +AC inverter+well pump).

    I certainly don't have enough experience to give you an answer without doing the detailed calculations and designs either (I am no expert--Just trying to guide)--And you would still need to look at the time and $$$ vs your needs.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • DesertDwellerDesertDweller Registered Users Posts: 13 ✭✭
    OK, thanks for your thoughts! This was an idea an electrician had who did solar years ago and is not caught up. I will still keep the 2 systems separate. I already have the Grundfos well pump (SQ Flex-F2). It can be run on any source of power (DC or AC). I plan on running on grid at first, until I get my panels. It was suggested I get 4- 300W panels  wired in series. I thought it overkill and will take up all of my roof space.  But the specs for this pump recommend running it around 1000W.

    I've been holding off on my booster pump, but I think I will get the 12V DC and figure out where the panels for this will go somehow!
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