Advice, building an off grid irrigation system, running a 1.5 HP pump from 12v or 24v?

I’m planning an off grid, stand alone lawn sprinkler system. It should only operate two mornings a week for about 1 to 1.5 hrs BEFORE the sun rises.
Based on my sprinkler calculations I need a 1.5-hp pump operating on 115vac to satisfy water needs. A 1.5-hp pump uses about 19amps running. 19a * 115v = 2185 running watts, X 3 = 6555 watts starting surge. So my inverter needs to be sized to handle that.
Running watts should be: 2185w * 1.5 hrs = 3278 total watt hours each cycle it runs. (1.5 hrs twice a week)
Battery sizing using 280ah LiFePo4 cells:
12vdc at 280 ah is: 3584 watt hrs.
*Barely enough power
12vdc at 560ah is: 7168
24vdc at 280 ah is: 7168 watt hrs.
*More than double needed power

Plans to use 4- 100 watt panels and a 40 amp MPPT SCC to charge batteries. Then add an 12/24 vdc inverter to handle pump and sprinkler controller.

My question is; Does/Should the battery watt hours, be greater than the startup surge of the pump/system?
Could 12vdc handle that load even if I used a 12vdc 560ah diy battery?
Please check my math, I feel maybe I’m missing something? Any experience with off grid irrigation is helpful.

I would plan ob 5x running watts...
Unless you get a soft start pump

Shawn Hayes said:

I’m planning an off grid, stand alone lawn sprinkler system. It should only operate two mornings a week for about 1 to 1.5 hrs BEFORE the sun rises.
Based on my sprinkler calculations I need a 1.5-hp pump operating on 115vac to satisfy water needs. A 1.5-hp pump uses about 19amps running. 19a * 115v = 2185 running watts, X 3 = 6555 watts starting surge. So my inverter needs to be sized to handle that.

Click to expand...

I see motor surges closer to 4-5X run. Did you measure this with an INRUSH meter, or is it calculated.

I had a 3/4hp pump that runs at 850W send a WZRELB 24V/2000W/4000W max inverter to overload-ville.

A soft starter may be needed.

Shawn Hayes said:

Running watts should be: 2185w * 1.5 hrs = 3278 total watt hours each cycle it runs. (1.5 hrs twice a week)
Battery sizing using 280ah LiFePo4 cells:
12vdc at 280 ah is: 3584 watt hrs.
*Barely enough power

Click to expand...

Only if you get absolutely nothing from your PV that day... it's pretty rare.

Shawn Hayes said:

12vdc at 560ah is: 7168
24vdc at 280 ah is: 7168 watt hrs.
*More than double needed power

Click to expand...

See below.

Have you considered the idle power of the inverter? This is the one use case where the "ECO" mode might be workable if you manually run the pump.

Shawn Hayes said:

Plans to use 4- 100 watt panels and a 40 amp MPPT SCC to charge batteries. Then add an 12/24 vdc inverter to handle pump and sprinkler controller.

Click to expand...

~2kWh/day.

Shawn Hayes said:

My question is; Does/Should the battery watt hours, be greater than the startup surge of the pump/system?

Click to expand...

No, but the battery cells and BMS need to handle the run and surge currents.

Shawn Hayes said:

Could 12vdc handle that load even if I used a 12vdc 560ah diy battery?

Click to expand...

Depends on BMS. Rather than ONE 12.8V 560Ah battery, I would build 2X 12.8V 280Ah battery each with its own BMS. If you have trouble with one, you still have the other to fall back on and two BMS give you more margin on start.

Shawn Hayes said:

Please check my math, I feel maybe I’m missing something? Any experience with off grid irrigation is helpful.

Click to expand...

Helped my neighbor work out the inverter system needed for his 3hp 700' deep well pump, so I'm not a total dunce.

DC pumps are available. Sometimes there is no need to invert to get the job done.
https://www.thesolarbiz.com/our-products/water-pumping/submersible-pumps.html

A friend of mine dug his own well in a sweet spot in the dry country Eastern Montana, he was able to bench down and get to water with his excavator.
He put in one of these pumps and controllers.

skyking1 said:

DC pumps are available. Sometimes there is no need to invert to get the job done.
https://www.thesolarbiz.com/our-products/water-pumping/submersible-pumps.html

A friend of mine dug his own well in a sweet spot in the dry country Eastern Montana, he was able to bench down and get to water with his excavator.
He put in one of these pumps and controllers.

Click to expand...

Not an option. I’ve got two (3500 and 2500 gallon) rain collectors to gravity feed from.

sunshine_eggo said:

I see motor surges closer to 4-5X run. Did you measure this with an INRUSH meter, or is it calculated.

Click to expand...

Calculated, as I do not have the pump yet but researching before buying.

sunshine_eggo said:

I had a 3/4hp pump that runs at 850W send a WZRELB 24V/2000W/4000W max inverter to overload-ville.

A soft starter may be needed.

Click to expand...

Soft start devices was something I’ve considered but perhaps it’s a must have now.
Any recommendations?

sunshine_eggo said:

Only if you get absolutely nothing from your PV that day... it's pretty rare.



See below.

Have you considered the idle power of the inverter? This is the one use case where the "ECO" mode might be workable if you manually run the pump.

Click to expand...

I figure I will oversize the batteries to compensate. I’ve not settled on an inverter until I got feedback from the forum. However I will check idle draw specs.

sunshine_eggo said:

~2kWh/day.
No, but the battery cells and BMS need to handle the run and surge currents.

Click to expand...

Agreed, that’s why I figured a diy LiFePo4 280ah battery with a JK BMS would be a safe bet.

sunshine_eggo said:

Depends on BMS. Rather than ONE 12.8V 560Ah battery, I would build 2X 12.8V 280Ah battery each with its own BMS. If you have trouble with one, you still have the other to fall back on and two BMS give you more margin on start.

Click to expand...

Agreed. 2 X 12V 280ah each with JK BMS.
I was unsure if 12v would handle that, but X2 is better all around.

sunshine_eggo said:

Helped my neighbor work out the inverter system needed for his 3hp 700' deep well pump, so I'm not a total dunce.

Click to expand...

Thanks, glad I posted!!! I hesitated to ask for opinions on this. It seem pretty straightforward but felt I may be overlooking something.

Shawn Hayes said:

Not an option. I’ve got two (3500 and 2500 gallon) rain collectors to gravity feed from.

Click to expand...

This pump would work perfectly with a tank situated above it.

https://www.thesolarbiz.com/scb-24v-40p-booster-pump-scb8.html

They are ideal pumps for home pressure systems, sprinkler systems or in-line booster pumps. Like the SCS series, SunPumps SCB series pressure pumps are high-quality, maintenance-free, DC pumps specifically designed for water delivery in remote locations.
This 24 volt DC model is designed for battery direct operation

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Pump curve
http://3415913.shop.netsuite.com/co...6yitCtZmTHjFZneIbeetoEmOtZKgxxkdRwAe&_xt=.pdf

Check out those motor watts on the second page of the pump curve PDF. Pretty tasty!

That saves making a bunch of AC amps with an inverter.

Shawn Hayes said:

Calculated, as I do not have the pump yet but researching before buying.

Click to expand...

I would count on 5X. Soft starts tend can reduce this to about 30-40% of the 5X peak.

Shawn Hayes said:

Soft start devices was something I’ve considered but perhaps it’s a must have now.
Any recommendations?

Click to expand...

I would start with Micro-Air - the folks that make them primarily for RV A/C units, but they have units for up to 6 ton A/C units.

Shawn Hayes said:

I figure I will oversize the batteries to compensate. I’ve not settled on an inverter until I got feedback from the forum. However I will check idle draw specs.

Click to expand...

Shawn Hayes said:

Agreed, that’s why I figured a diy LiFePo4 280ah battery with a JK BMS would be a safe bet.

Click to expand...

Shawn Hayes said:

Agreed. 2 X 12V 280ah each with JK BMS.
I was unsure if 12v would handle that, but X2 is better all around.

Click to expand...

Shawn Hayes said:

Thanks, glad I posted!!! I hesitated to ask for opinions on this. It seem pretty straightforward but felt I may be overlooking something.

Click to expand...

The process is very straightforward... the details are what kills you.

If you're not trying to pull from a well, I gotta second the 24v DC pump and just skip the inverter and conversions all together.

That pump I listed is a 2/3 HP, and only ~500 watts vs 2100 watts. Plus conversion losses and complexity. I think it makes up for the cost difference quickly.
@Rednecktek I would plumb in tees and ball valves, try the 1 pump and see. If not enough, put in the second pump and check valve off those tees and ball valves.

I will definitely looking into that. However, based on initial conversations with a company that does irrigation layouts and equipment. It seems the lift (elevation) needed, number of sprinkler heads and length of a couple zones, seemed to put most dc pumps out of consideration.
My intention was to use a normal rain bird controller to run the system. As well as a small pressure tank to shut off the pump automatically when system stops. Basically like a normal well tank/pump system.