Sizing DC solar system for booster pump

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.

https://backwoodssolar.com/product/aquatec-550-24v-2/

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:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Albuquerque
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)

Jan	Feb	Mar	Apr	May	Jun
4.94	5.32	6.23	6.58	6.51	6.27
Jul	Aug	Sep	Oct	Nov	Dec
5.74	5.57	5.85	5.81	5.12	4.76

• 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?

-Bill

-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:

https://backwoodssolar.com/product/aquatec-550-ac-2/
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:

https://www.amazon.com/s?k=12+24+vdc+led&ref=nb_sb_noss

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.

-Bill

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.

-Bill

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.

https://www.solar-electric.com/fullriver-dc220-6-agm-sealed-battery-6v-220ah.html (6 volt @ 220 AH AGM)
https://www.solar-electric.com/fullriver-dc210-12-agm-sealed-battery-12v-210ah.html (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:

http://www.smartgauge.co.uk/batt_con.html

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.

-Bill