How to size battery and panels to run my well pump.

mike_littlerock

I have a pretty basic system thats been in place for about 2 years.. Four 100 Watt panels, 40 amp MPPT charger and 4 50Ah 12 v batteries (in 24v configuration. I have a Franklin Electric well pump with the following specs:
230 v, 3450 rpm. 3/4 HP, .55 KW 6.8 amp, S.F. Max amp =8.

I need to run the pump at the present time for a max of about 20-30 minutes a day. actually 30 min every 2-3 days will give me enough water pumped into my holding tank to take care of the irrigation needs i have. (gravity fed irrigation controlled via arduino.. opens/closes zones based on time -GPS based clocking.. the valve control is all powered on a different system as it is on a different part of the property. I have never had the nerve to just "give it a shot" and wire up the generator and turn on the inverter as I would rather not have to buy a new well pump if i calculated it wrong.. 

This is a rural area, so the power co-op wants to charge me a min fee of $3k to put in a pole, and then commit to a month minimum number of kilowatt hours.. If i can invest in my own system instead of paying the utility for the privilege to buy their power that would bring me great joy.. 

Help!! (thanks in advance)

mike95490

2 wire or 3 wire (capacitor in box above ground?) ?
Extrapolating my experience, your pump should consume about 1,500W from the inverter, and need at least 5x that for starting
 What is your Locked Rotor Amps ?  Your batteries and inverter have to supply that for about 1 to 1.5 seconds, then drop down to 1500w

40.5A x 230V = 9,315 watts for starting.  at least a 24V system and likely a 48v would work best,

mcgivor

When you factor in battery and inverter costs to support the 3/4 HP230V pump, remembering batteries have to be replaced every 3 to 5 years, perhaps a solar pumping system, without batteries, designed to fit your requirements, head, location, volume etcetera may be a viable solution, keeping the existing pump as a backup with generator for periods of no sun. The link has a sizing calculator, have no idea of prices as a submission with all relevant data is required but worth a look if merely for informational purposes. 
https://www.rpssolarpumps.com/?gclid=EAIaIQobChMIw9r3zfKy1gIVU5VoCh2qZQ9zEAAYAiAAEgKB-fD_BwE

mike_littlerock

3 wire. 

mike_littlerock

mike95490 so how does 9315 watts translate into number of batteries of what type? I picked 12v sealed deep cycle marine batteries when i first built it because i knew absolutely nothing about solar. Now that I have built my first system i know .0001% more than nothing lol. What is the middle ground "honda accord" level of battery banks in terms of price and performance. I did look at the solar pumping solution and that seems pretty neat. would be good to know the overall costs so i can make the best decisions. also, what are the general rules for sizing panel count/type to charge the battery bank once you get that properly sized? 

any good books you recommend? 

BB.

Well pumps (and refrigerators) are what drive off grid solar power systems from "small" to medium/larger systems. Standard induction motors consume high starting current and are not really that efficient. And larger motors simply draw a lot of power/energy.

The motor you have shown is (I believe) a 2 wire (2x hots + ground) motor. Those have the highest starting loads. Three wire (2x hots, capacitor/starting wire, ground) have lower starting current.

And you have not told use how many hours a day of pumping you will need.

For the sake of moving the discussion along (hopefully, somebody here with more pumping experience than I will supply the missing starting load/AC inverter recommendations), lets go with a generic full time off grid cabin/home installation.

• 2,400 Watt AC inverter (~4,800 Watt surge)
  
• 24 volt flooded cell lead acid battery bank
• Full time off grid cabin running other loads too (lighting, lights, fridge, etc.).

For a conservative system battery bank design, assume 8 hour discharge rate (maximum AC inverter output for 8 hours--A heavy, but not battery damaging load).

• 2,400 Watts * 1/0.85 AC inverter eff * 1/24 volts * 8 hour discharge rate = 941 Amp*Hour battery bank (to start/run pump)

The recommended rate of charge for an off grid flooded cell lead acid battery bank is ~5% to 13% rate of charge. 5% is good for weekend/seasonal use. 10%+ is good for full time off grid.

• 941 AH * 29.0 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 1,772 Watt minimum array
  
• 941 AH * 29.0 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 3,544 Watt nominal array
  
• 941 AH * 29.0 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 4,607 Watt "cost effective" maximum array
  

And then we can look at how much energy such an array will generate. Say you are near Little Rock Ark, fixed array, facing south:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Little Rock
Average Solar Insolation figures

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

Jan	Feb	Mar	Apr	May	Jun
3.45	3.88	4.56	5.24	5.10	5.26
Jul	Aug	Sep	Oct	Nov	Dec
5.44	5.36	5.20	4.70	3.66	3.21

Toss out the bottom 3 months (generator support, less water pumping in winter) and:

• 3,544 Watt nominal array * 0.52 off grid system eff * 3.88 hours of sun per day (February "break even" month) = 7,150 Watt*Hours per day

That would run your pump for >7 hours per day (average sunny day in February). The battery bank would supply 2 days of "no sun" energy:

• 941 AH * 24 volts * 0.85 AC inverter eff * 1/2 days of storage * 0.50 maximum discharge (for longer battery life) = 4,629 Watt*Hours (battery power for 2 days of no sun)

Now, this is not a small system, but a medium sized solar power system that would run the well pump (and irrigation) plus full size refrigerator + lightning + clothes washer + TV + Laptop computer + other stuff very nicely... With only a (relatively little) amount of generator usage in the winter.

However, if you are only running the well pump for an hour a day and no other loads--This is way over kill.

For example, if you where running the pump to fill a cistern (cattle, irrigation, domestic water)--You might want to look at a solar powered water pump that only needs ~1,000 Watts (or less) of solar panels. Pump during the day, cistern to supply water at night/during bad weather. Unfortunately, solar powered well pumps can run $2,000+ for good quality units. Although, you could get way for ~$1,000 for weekend/seasonal usage possibly with a decent pump+controller.

And, if you need to pressurize a "cabin", then get a 12 or 24 VDC "RV" water pump + small battery bank + small solar array (2/4x 6 volt @ 200 AH "golf cart" deep cycle batteries) and ~377-753 Watt solar array. That would keep a cabin in water, and enough power LED lighting + laptop computer.

• 377 Watt array * 0.52 off grid system eff * 3.88 hours of sun = 760 WH per day

Add a small/efficient 300 Watt AC 12 volt inverter (if needed):

https://www.solar-electric.com/morningstar-si-300-115v-ul-inverter.html

And call it a day.

Questions/Answers to my (mostly wrong?) guesses about your needs?

-Bill

mike_littlerock

as noted on my OP "I need to run the pump at the present time for a max of about 20-30 minutes a day. actually 30 min every 2-3 days will give me enough water pumped into my holding tank to take care of the irrigation needs i have. "

BB.

And that is the "problem"... Pumps with higher starting surges push up the minimum battery bank and AC inverter size--And end up over sizing the system larger than needed.

AGM batteries can supply much higher surge/short term running current (C/1 discharge rate)... So, if we assume 2,400 watts of surge current (5x running load and 0.85 inverter eff):

• 550 Watts * 1/0.85 inverter eff * 5x starting load = 3,236 Watts peak
• 3,236 Watt inverter / 2x surge support (good quality inverter) = 1,617 Watt AC inverter rated
• 3,236 Watt peak * 1/0.85 AC inverter eff * 1/24 volts * 1/1 surge discharge current = 157 AH @ 24 volt rated AGM battery bank (minimum)

Battery capacity based on usage:

• 550 Watt pump * 1/0.5 hours per day * 1/0.85 AC inverter eff * 2 days storage * 1/0.50 max discharge * 1/24 volts = 54 AH @ 24 volts minimum (based on storage capacity/depth of cycling)

Solar array sizing:

• 550 Watts * 1/2 hour per day 1/0.52 system eff * 1/3.88 hours of sun = 136 Watt array minimum (based on energy usage)
• 157 AH * 29 volts charging * 1/0.77 system derating * 0.10 rate of charge = 215 Watt array "nominal" (based on 10% rate of charge to battery bank)
• And a ~1,800 to 2,400 Watt AC inverter minimum

Need an answer from somebody with Franklin 2 wire pump experience--But the above would be a starting point for a minimum system with existing pump.

Don't know how deep your well is, but an alternative would be a Grundfos pump (expensive) or possibly something like this (less expensive):

Shurflo 9300 Submersible Solar Water Pump

Shurflo 9300 Basic Pump Controller - 24 Volts
Shurflo 9300 Deluxe Pump Controller 12/24 Volts

And forget the batteries + AC inverter.

-Bill

mike_littlerock

Thanks Bill!!
Unless i'm reading this wrong, i'm fairly close with my current starter system.

1. I have 100Ah 24v batteries now. could add to get that over 157AH peak.. 
2. I have 4 100 watt panels, currently not even really used other than to run a small fan at times or charge power tools. 
3. I have a 3000 watt (6000 watt peak) AC inverter (230/240v output). 

if i read the above right then i just need to beef up my batteries to handle the surge, and inverter and solar array should be OK for now.. i plan to add to the number of panels as well as battery bank size over time anyway. plan to divert the money i would have been spending on utilities if I had shelled out the $3-5K to have the co-op install power pole on the property. 

Estragon

Maybe I missed something, but didn't it look like OP's pump took ~40a@230v (9kw) to start? How is a 6kw surge inverter and a 157ah@24v going to work?

Estragon

Also, are the existing batteries AGMs rated to deliver C/1?

mike_littlerock

Estragon.. yes.. my pump is 6.8amp at 230v.. peak of 8 amps.  (pic of the label is in this thread) .. what is C/1? 

mike_littlerock

found the pump manual. it notes:
MOTOR RATING 
3/4 HP
KW 0.55 

 MINIMUM RATING OF GENERATOR:
EXTERNALLY REGULATED KW 3 KVA 3.8 
INTERNALLY REGULATED  KW 2  KVA 2.5 

Estragon

C/1 means capacity in amp hours of the battery ÷ 1hr. Typically, a deep cycle battery is rated at a C/20 rate - a capacity when discharged over a 20hr period. C/1 would be when discharged over a 1 hr period. Discharging over a shorter period reduces the apparent capacity. The magnitude of the reduction depends on the battery construction (and chemistry). Some, like flooded lead acid deep cycle, lose a lot of their apparent capacity at high discharge rates. They're designed to be cycled deeply (~50%), but slowly. Others, like a starting AGM, are designed to be discharged quickly without too much voltage sag, but typically need to be recharged fairly quickly. It's a comprimise.

I'd be surprised if the surge current on a normal well pump that draws 6.8a is really 8a. My guess is 8a is the running draw after power factor is considered. I stand to be corrected though, and would be interested to know if I'm wrong.

If you already have the gear, I don't think there's much downside to giving it a try if the inverter has overload protection. If the draw is too high, what I think will happen is the DC voltage will sag to below the inverter's low voltage cut-off, or the inverter surge output protection will trigger. Just be sure the wire from inverter to battery is heavy enough, and breaker/fused to the inverter spec. I don't think it would hurt the pump. It may stall, a state you don't want to leave it in, but just a trial shouldn't hurt? Others may have different thoughts on the stall issue.

mike95490

How about looking at a new pump?  Grunfos makes some solar pumps, run on a couple solar panels, and a fancy new variable speed pump, that needs no batteries ?

40A locked rotor (start-up) @230V means the inverter has to supply 9,200watts and then there's going to be some power factor monkey business too.  Can't do that at 12V .

jonr

Supposedly Franklin now has competition for Grundfos - the Fhoton HR SolarPAK.   Also the "SubDrive Utility Variable Frequency Drive" for existing two-wire pumps.

Marc Kurth

jonr said:

Supposedly Franklin now has competition for Grundfos - the Fhoton HR SolarPAK.   Also the "SubDrive Utility Variable Frequency Drive" for existing two-wire pumps.

Good info. 

Looking at a VFD is exactly where I would start. Eliminating the high inrush current is a game changer for inverter sizing.
Plus, having the ability to use a well pump as a modulating opportunity load can be a huge benefit. Of course the pump curves would need to be looked at.

Marc

Mangas

I installed a Grundfos SQflex DC/AC pump in one of our ranch wells.

Will eventually use them exclusively given their application multi voltage flexibility with panels or even a small 2000 w generator. 

6 gpm over 8 hours into storage is a lot of water!

mike_littlerock

as far as replacement, i guess its an option but this pump is only about a year old and was not inexpensive.. not out of the question though if its right for the long run.. as far as 9200 pull I KNOW i have run the pump many times off my generator that is 6200 watts with 8000 watt surge. I tried to find a "kill-a-watt" or some like tool that would be able to sit in-line and record peak draw when i turn on the well pump.. i would think that would eliminate the guess work but i have not been able to locate a meter that would tell me peak and typical running draw. 

Estragon

Getting the running draw is fairly simple. Odds are that the rated amps will be close, and confirming that with a meter isn't hard. Getting a peak is harder. A locked rotor amps rating should be close. Actually measuring it can be hard
If a generator with an 8kw surge can run the pump, that's indicative, but not definitive, that an battery/inverter will too. The generator doesn't have to deal with 10x the current that an inverter at 24v does, and the generator has inertia to help overcome surge.

Personally, I would give it a shot with what you have, with previous caveats.

peakbaggger

If it is three wire Franklin pump with an external capacitor, switch it to variable speed drive. I think they can it monodrive. That's will cut your startup surge watts way down. I expect the cost for a Variable speed conversion is far less than the cost to upsize the system for surge watts. Once installed then set the pump at very low pressure setting and let it run for an extended period to fill the tank.