Brand new and need assistance in figuring out what inverter I need

Bud in Fl

New Member
We put an RPS solar pump in for our garden irrigation a few years ago. That system has 400 watts of panels feeding the controller which supplies 24vdc to the in-ground pump and also maintains the 24vdc battery bank which is currently two each 100ah 12V AGM batteries.

SWMBO (my wife) wants to be able to have water running to the house in case of a hurricane or similar power outage. The line is already piped so all I have to do is open a valve to connect the solar system to the existing system.

I tried a 24v Shurflo pump that worked OK for a few months but now won't maintain enough pressure to run a sprinkler. It was recommended to go to a cast iron 1/2 hp 115vac pump. I have this one on order - https://www.lowes.com/pd/Acquaer-1-2-HP-Dual-Voltage-Cast-Iron-Shallow-Well-Jet-Pump/5001226199
This pump pulls 7.2 amps running on 115vac. It has a built in start capacitor. My in-ground pump fills a 1600 gallon storage tank set about 5 feet from where the transfer pump will be. I'll have less than 3 feet of vertical lift if the tank were near empty but when it's full, the pump is actually has 4+ feet of static pressure on the supply line.

What size & brand inverter should I look for. This will be an off grid installation and I'll probably need to run an auxiliary fan to help cool it. It'll have to be in a screen cage, too since it'll be under a shed and fair game for wasps here in central Fla.

Also, how many more 100ah AGM 12v batteries should I add to my bank?

Thank you for any info! I was a millwright/nuke pipe welder/machinist and I'm trying to learn the electrical side at 70!
 

MichaelK

Photon Sorcerer
Here is a pump table that documents power requirements. I don't see a pump that matches your specifications exactly, but what you should pay attention to is the starting amps vs running amps. Your running amps is the 7.2A value. Your starting amperage is likely to be 3-4X higher. For my pump it's 3.8X times higher. This means the inverter you need has to be capable of powering 7.2A X 115V X 4fold starting = 3312W. So, 2000W high-frequency inverters are out. Look for something in the 4000W+ range.

I would NOT advice you to get AGM batteries, especially 12V. You'll get better performance buying buying lower voltage batteries and wiring them in series to get your desired voltage. Look at 24V sine-wave inverters. I have this one that I am very happy with it. A low-cost 6V battery would be Costco's 6V golf-cart battery. Was 99$ the last time I looked. 7.2A X 115V = 828W.

From a 12V battery, you'd have to pull 828W/12V = 69A to power the load.
From a 24V battery, you'd have to pull 828W/24V = 34.5A to power the load.
 

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Bud in Fl

New Member
Thank you!!! Your explanation makes a lot more sense that what I was coming up with.

I need to clarify one thing - I'd suspected that the difference in 115v and 230 v would be a trade off and basically a wash but if I'm following correctly, I'd have better results sticking with 115v?

Thanks again!!!
 
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MichaelK

Photon Sorcerer
I need to clarify one thing - I'd suspected that the difference in 115v and 230 v would be a trade off and basically a wash but if I'm following correctly, I'd have better results sticking with 115v?
Well, it depends. For the smallest pumps, sticking with 110/120V makes sense because you only need one voltage.

But, as pump size increases, it becomes physically impossible to get that much power through a lower voltage pump simply because the amperage will be too high. Remember, that most domestic wiring is standardized for 15A, which as 1800W at 120V. So, the most power hungry equipment like stoves, whole-house air-conditioners, and water heaters runs at 240V to keep the amperage down.

My own cabin is wired with a standard split-phase electrical box, so 120V goes to the wall sockets, and 240V goes to the well-pump. My split-phase inverter performs this task very well, and I've had zero problems.
 

Bud in Fl

New Member
Thank you! Hopefully the 115v pump I ordered will all I ever need on the inverter. It's supposed to be delivered tomorrow. I can temp pipe it and use an amprobe to see what my actual start & run amps are with a 9kw genset. The pump coming is supposed to had a capacitor on it to help the start up but I won't know for sure till I see what it does.

The main reason for this project it to have a back up water supply in case of an outage here in the hurricane area. I'm from near the SC Lowcounty, I lived on a beach in NC (hurricane magnet!), went through ice storms in SC & western NC & a "derecho" in WV. Life without water is the worst part of it. If I can get that covered, with this system I may add some panels on my shop roof to pick up a few lights in the house but that's about it.

Thank you again for making some sense out of the numbers for me!
 

MichaelK

Photon Sorcerer
The pump coming is supposed to had a capacitor on it to help the start up but I won't know for sure till I see what it does.
My three-wire 240V pump has a Franklin starter box. Inside are capacitors about the size of a clenched fist. Even with those, the starting amperage of my pump is 3.8X higher than the running amperage. Just having a start capacitor doesn't automatically mean it will be easy. The best why to know is to put a inrush clamp meter on the hot wire when you start up the pump. Most likely the startup surge will be even higher with a 50-100' collumn of water over the pump.
 

Guido1

New Member
Thank you! Hopefully the 115v pump I ordered will all I ever need on the inverter. It's supposed to be delivered tomorrow. I can temp pipe it and use an amprobe to see what my actual start & run amps are with a 9kw genset. The pump coming is supposed to had a capacitor on it to help the start up but I won't know for sure till I see what it does.

The main reason for this project it to have a back up water supply in case of an outage here in the hurricane area. I'm from near the SC Lowcounty, I lived on a beach in NC (hurricane magnet!), went through ice storms in SC & western NC & a "derecho" in WV. Life without water is the worst part of it. If I can get that covered, with this system I may add some panels on my shop roof to pick up a few lights in the house but that's about it.

Thank you again for making some sense out of the numbers for me!
"Life without water" soon becomes death. The common mantra is: 3 minutes without O2, 3 days without HO2, 3 weeks without nourishment.
None of those seem like a good way to go.
What's a "derecho"?

-Guido in Houston (hurricanes, no derecho's)
 

Bud in Fl

New Member
"Life without water" soon becomes death. The common mantra is: 3 minutes without O2, 3 days without HO2, 3 weeks without nourishment.
None of those seem like a good way to go.
What's a "derecho"?

-Guido in Houston (hurricanes, no derecho's)
I'd never heard of one before I was in one, either! It's a straight line storm with hurricane force winds. We were living near a mountain top not far from Beckley, WV, the sun was shining and a breeze started. With in 10 minutes we were looking for cover and the sun was still shinning. When the actual storm got to us, it was dark with trees taking out power lines. It was over 2 weeks before we got power back.

As much as I hate being on a cell phone "leash", it was kinda nice to be able to call relatives in Ga & Colorado an find out what had happened.


 
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