Emergency battery back-up for 220V water pump....

I hope this is the right forum for this question.

I'm looking at this inverter as a general investment in battery power to various parts of my home down the road: http://www.wholesalesolar.com/2954842/magnum-energy/inverters/magnum-energy-ms4448pae-inverter

It's a 220V pure sine wave AC from 24V DC unit, heavy duty and not cheap, that would for immediate purposes be installed to run our 3/4 hp, 2 wire well pump. Our reason for wanting this alone on battery back-up is that we have animals that need daily water and it's about that time when you most need it that the grid power goes out. Since we are still grid-intertied, this unit would serve as an inverter-charger...maintaining the batteries when grid power is on and immediately transfer-switching to the batteries when grid power is down. The real life need would be to pump about 50 gallons from the well **very intermittently** over a span of about 3 hr. It would not be "needed" again for another 24 hr.

The question: What amp-hour rating of battery do I need to drive this train? As the Magnum inverter requires 24V DC input, I was thinking of series-connecting two 12V batteries, but have not seen a good, easy way to calculate battery bank size for this configuration and this kind of load option. I might be partial to AGM batteries for low maintenance and low use. Any insight you could provide on this would be appreciated. And if possible, please point me to your favorite online "sizing calculator" for such a need. I realize this is "financial overkill", but I hope to integrate the inverter unit into other aspects of our home power down the road. Thanks.....

Hi John,

Haven't used a Magnum inverter myself, but they're on the rather short list of brands I'd consider... Good rep.

You've linked a 48V inverter, but from your text I assume you mean the 24V version, the MS2024PAE, right?


Have you considered a water reservoir? I get that this inverter will be able to serve other functions, but if cost is at all a factor this is surely worth a look. A thousand gallons of water on hand would suffice for 20 days @ 50 gallons a day... allow for careful household usage and you're still looking at a couple weeks of water... Could add a generator (propane, so the fuel doesn't go stale) for half the cost of the inverter, and run that to refill if you have no power for weeks on end... A thousand gallons is only 4x IBC totes, which are often available(1-trip used, food grade) for less than a hundred bucks each in my area. Granted, you won't have pump pressure to put this thousand gallons where you want it, so you'd need to locate your reservoir carefully...

For that matter, well pumps do occasionally fail. A week or two of water to make this a less time sensitive problem sounds pretty damned good to me!


I don't have exact numbers for you as far as battery bank goes. Sizing to handle surge doesn't appear to be an easy thing to google for, or at least that's my story since I'm striking out...

Sizing to fit the power requirements, well... how much of those 3 hours is the pump running? If it was all 3 hours, at around 560W you'd run through about 1.7KW per day; round up for inverter and resistive losses and call it 2KW, and that would mean at least a 3KW AGM bank; that would give you 65% DoD when the bank was new, and with 80% as an absolute max there's a bit of margin there...

3KW is only 125Ah @ 24V, so two ~125Ah 12V in series...

Of course the pump is running 'very intermittently', so you'll have to quantify that and fix the numbers and then decide how many days power you want...


Closest comparison I've got is a 95Ah 12V deep cycle in my van as a temporary house bank; it will run a 750W load on my 3KW inverter, but it sags badly, by about 1.25V. I don't expect it would run it for long, and have only done it for testing purposes. That's a resistive load from a spaceheater, no startup surge to worry about; I'm pretty sure the low voltage cutoff on the inverter would trip with a startup surge.

Given that after startup, you'd be drawing less than half the amperage, plus the AGM benefits... I'd guess you'd be fine. But that's definitely a guess!

Maybe the way to proceed is to start with that 125Ah(or whatever you get when you redo the math...) and check with the manufacturer for specific batteries available to you to verify they will have acceptable amounts of sag and adequate capacity at your ~26A discharge rate?

Hi Dillon, Thanks for your response here.

"You've linked a 48V inverter, but from your text I assume you mean the 24V version, the MS2024PAE, right?"

Yep....thanks....my bad on that one. I am interested in the 24V model.

RE: Water reservoir or generator. The generator is already an option, but is not set up as a turnkey standby option. What I mean is that all things animal feeding/watering are my wife's hobbies and all things mechanical are in my domain. If I am away from the property when the power fails, no way my wife (mid-60s) is going to mess with trying to get a generator running....she has flat out stated that as a concern. So the generator works fine with a transfer switch at the power pole....but only when I'm present. The water reservoir is less of an option, since we are not interested in trying to keep the water in a liquid state at the point of delivery, which is an unheated quonset building. We have on average 90 days at below 30F, many of these days around 0F which will freeze up a water supply pronto. Although one option would be to sink the tank into the ground, the frost depth is routinely 5 - 6 ft, which is why all water lines are sunk 7 - 8 ft deep. Also, as you noted, one can truck in water....we recently bought a 180 gal. tank for just that purpose and filled up at the local fire hall. This happened because the pump pressure switch died suddenly, but otherwise the weather was fine. This latter part is key: Most power outages here are due to ice storms....no one is driving to pick up water during an ice storm and few will meet you at the fire hall to do so.

You are right that pumps occasionally do fail, although in my experience, they tend to give you some advanced warning that something is not right (poor pressure, etc). During desperate times when the ice on the river is not so thick we will pump from there to water animals and the garden.

"....how much of those 3 hours is the pump running?"

Actually probably no more than 30 - 45 min. I went to Trojan battery's website and found a calculator that seemed a be it more user friendly than most......came up with a configuration that would involve a couple of 12V 200Ah AGMs wired in series to give the 24V. This would be an absolute minimum and is based on an 80% DoD.


I get what you are saying about trying some 125Ah batteries, which are much more available and less expensive, but I think I read somewhere that for a 2-wire pump, I should over-estimate a bit on the inverter and batteries for this system. And I could do 4 of these batteries instead of 2 of the larger size, but available space begins to be a consideration. I'd rather not find out the hard way that I undersized the batteries for this project, and if they last me a while will be money well spent. The batteries would be located in a basement where it stays relatively cool. In the end, I look forward to using some of the components down the road for other home loads.

Thanks again for helpful respone....this helps me feel I'm at least on the right track.

I hadn't yet figured out where the heck 'RRV' was when I answered... the cold and ice storms would pose a definite challenge to many of the alternatives. We very rarely get ice storms here, but I have vivid childhood memories of listening to trees explode during a severe one.

I get what you are saying about trying some 125Ah batteries, which are much more available and less expensive, but I think I read somewhere that for a 2-wire pump, I should over-estimate a bit on the inverter and batteries for this system. And I could do 4 of these batteries instead of 2 of the larger size, but available space begins to be a consideration. I'd rather not find out the hard way that I undersized the batteries for this project, and if they last me a while will be money well spent. The batteries would be located in a basement where it stays relatively cool. In the end, I look forward to using some of the components down the road for other home loads.

Sounds good to me; not a fan of undersizing, and my math was obviously fairly rough. I figure people who are selling batteries will err on the side of more than really needed, but I'm generally fine with that.

Plus, 2x12 in series is just a nice config. I'd always prefer a series bank vs a series-parallel... and I'd rather use 12V cells than 6V for flexibility. Should one cell bite the dust well before the other, you can repurpose a 12V battery for many uses, but what would you do with 3x 6V cells?

Should give you capacity for a few days, and good battery longevity.

Its only a 3/4 hp pump. And could do 5-6gpm.

Is it a deep well submersed pump? What kind? How deep and what diameter well?

200ah is the makers minimum recommended battery, that would be for most inverters in its power range. A 3/4 hp pump will not destroy that battery inverter combo. If you are only pumping 50 or so gallons a day =15-20 minutes of run time a day. Or a rough 750 watts x a rough .5 hour per day for household and livestock watering.

A 200 ah battery would have 3,800 watt/hours ups and 2,400W/hr off grid useable energy. The reason for the minimum rating may be fudgable if you can keep the inverter from demanding max current. Your short duration load is about a fifth of the inverter max continuous output. Measure the surge and running watts and compare with battery manufacturer specs for a particular battery you wish to not destroy. Outback makes a beauty of a 2500w 24v inverter or 3600 watt if you need more.

Have you considered one of the "trickle pumps" that direct-tie to a panel?
(If the float is down...and you have sun, it trickles into the tank. If the float is up...no connection.)

Saw one of these work in Texas. Amazingly good.
Do not know the model or maker, but what they were getting out of one 250 watt panel, all stand alone without a battery...was impressive.

(I could have almost showered with full-sun flow rate.)