New member, old question on converting existing pump to solar

james9001

Hello all, new member here. 

I am researching for my first solar project, which will be to migrate my house well pump from grid to off-grid solar.  The primary goal is for water supply independence in the event of extended power outage. I have read several threads on this topic, but most of them end up with the recommendation to install a Grundfos.  Since this is our home water supply, I really don't want to muck-up a working system (not to mention the cost) by replacing the pump. 

One of the most informative threads is here : https://forum.solar-electric.com/discussion/352536/how-to-size-battery-and-panels-to-run-my-well-pump , however, that goes way over my head.

The specs on my pump are (not sure of the manufacturer, but Franklin does sound familiar):  3/4 HP, 230V, 1 phase, 60 Hz, 7.2A (8.3 SF Max), 3450 RPM, KVA code M, .55Kw

There is a pump control box in the pumphouse, so I assume it is a 3 wire pump.

The well is 260-270 ft.   I estimate total run time per day at 3 hrs maximum.  Best I can tell, the run time per fill to top off the storage tank is maybe 3-5 minutes.

I have been looking at a Magnum MS4024PAE MS-PAE Series 4000W 24VDC inverter / charger, but want to get other's experience on what they are using in a similar setup. 

I have already purchased 2 100Ah 12v Battle Born batteries, but that is it so far.  I am still trying to size the array, charge controller, etc.  I don't mind over-engineering the system, unless it would be harmful to my well pump.

My location is central Texas.

Any input from other members that have also gone through this process would be much appreciated.

Dave Angelini

You are going the wrong way in my opinion. Buying before the design is researched. What is the cost to replace the pump with the right one for the application? Hard to beat Grundfos SQ's in the ground. Array direct above the pump and genset for back-up

That is what we do offgrid or we run ACV out to the pump from the house system or genset for back-up. You already have that right?

As for over engineering, I would make sure you do not destroy those batteries by charge or discharge currents. Seen too much of that lately with LFP!  Good Luck!

james9001

Thanks Dave.  Yes, I thought I had it all designed out and then discovered the complications associated with wells pumps after the fact.  I don't currently have any generator backup, I figured I would just make it all solar if I was going that route.  Too late to return the batteries now, but if I can't make a reliable solution for my well pump, I guess I will just need to change over to a different project for solar.  It is just surprising to me that there aren't more people out there who have been able to successfully go from grid to off-grid on an existing well pump.

mike95490

STOP BUYING STUFF !!

Your pump has a pressure switch control box 
  it might have a 3-wire control/starter cap box.   Make sure you know the difference

a 3/4 pump has a huge starting surge on the AC side, which means you inverter has to supply that, and the batteries feed the inverter
3/4hp surge 9,200watts starting
inverter will consume 11,040 watts starting it
 battery @ 24V will supply 460 amps for about 1 sec to start
    a 48V battery will supply 230 amps for starting.

The inverter has to be robust enough to handle this surge several times a day, and not give out.
 Good inverters have a complete data sheet displaying their limits, so the customer can make a wise decision

Dave Angelini

james9001 said:

Thanks Dave.  Yes, I thought I had it all designed out and then discovered the complications associated with wells pumps after the fact.  I don't currently have any generator backup, I figured I would just make it all solar if I was going that route.  Too late to return the batteries now, but if I can't make a reliable solution for my well pump, I guess I will just need to change over to a different project for solar.  It is just surprising to me that there aren't more people out there who have been able to successfully go from grid to off-grid on an existing well pump.

People have successfully run an existing well pump offgrid! We do this all of the time here! They just need to design for it. As Mike said it takes the right power system equipment to be reliable. We want this to last at least 10 years or more right ?

james9001

Thanks for the reply Mike.  Here is a photo of the control box in the pumphouse.  I can confirm that it goes to the well pump, not the booster. By pressure switch, are you referring to the booster pump, or does the well pump have a pressure switch also? Sorry for my ignorance, I'm learning as I go here, lol.   I have made no other purchases at this point other than 2 batteries. 

mike95490

Nothing on the label gives me a clue if it's a 3 wire or a 2 wire pump.
 So you can research it yourself, or open the junction box and see how many wires go to the pump (not counting the green wire)

So, ways to get out of the hole. 
  can those batteries supply 230A and can they be wired 4x for a 48V bank ?    Otherwise, back to basics.

And maybe start with a 6kw conventional generator.  If you take something off grid, you need backup generator for it, or do without.

james9001

Thanks again Mike for the solid data points, I will look into what I have.