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

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

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