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I have a well pump. I want to build out a battery backup (and, eventually, charge the batteries with solar and, more eventually, move to solar full time).

It's 1/2 horsepower on a 240v circuit. It's a Webtrol WS1005L with a Franklin Electric motor. Per the manufacturer's documentation, it draws a max of 6 amps on the two wires with a maximum load of 960 watts and only needs a 15 amp breaker.

manufacturer specifications

The manufacturer also says it can be run with a 2000 watt generator.

generator specifications

It has a beefy wire with a four-prong plug.

four prong plug

I'm trying to figure out what combination of inverter and batteries I need to power it.

For the inverter... I can't find an inverter that has an outlet like that. Is there one? Do those funky looking universal outlets work for that? Can I use an adapter to get to a more "normal" plug?

For the batteries... I assume I need some number of 100Ah sealed AGM batteries, and if my calculations are correct, one battery will get me about... 8 seconds of run time. How many should I have? Should I wire them together as 24V or leave them as 12V?

Any help would be appreciated!

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    If you're going to go full solar eventually, why not look at a "whole house" battery, like a Tesla Powerwall? That should tick all your boxes, though you didn't make any mention of budget. – PhilippNagel Aug 13 at 20:56
  • that looks bigger than a 15 amp, there should be some numbers like L15-20 or L14-20 possibly 30 but it looks bigger than a 15a. I would guess it is a 30 amp plug , the correct plug should be used is there an inverter? I have seen the wrong twist lock used the numbers will tell. As far as batteries it depends on what solar voltage you want to set up. But to give them an idea 10 series batteries would provide 120v you need 240 plus losses and you really can not discharge the batteries beyond 50% so you can see you are rapidly running out of power with 10 batteries 4 hrs ballpark fully charged – Ed Beal Aug 13 at 21:16
  • How many wires from the pump to the plug I also mentioned the plug type in the comments if you have 3 wire control with a ground going to the pump you may have start run caps and are using all the wires that’s why I mentioned the 3 phase plug if this method is used that is the proper plug not the L14- not in both cases the 30 a would not meet code. – Ed Beal Aug 13 at 22:03
  • How important is it to be able to extend this system to other loads in the future? Do you want to be able to support it with a generator as well as solar panels? – ThreePhaseEel Aug 13 at 22:57
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    @ThreePhaseEel yes – Andrew Kvochick Aug 18 at 3:23
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Motor startup surge is going to be the biggest issue. You will probably need a 2000W inverter for this.

The motor will surely be 240V. Pump motors are run at the highest practicable voltage to reduce voltage drop. A motor is typically at the bottom of a potentially deep well, and so the run from house to wellhead, then down the well itself can be 600 feet in some cases. Running that on 120V would be totally unworkable.

As far as draw, 6A x 240V is 1440 VA. VA is the "whole waveform", "watts" is the part of the waveform the pump actually uses. Fortunately the inverter only has to make the part. So you can figure 960W draw from batteries. That will be

  • 80A at 12V
  • 40A at 24V
  • 27A at 36V
  • 20A at 48V

Not worth going higher because DC power starts to get dangerous above 48V, and Code requires you handle it much more carefully.

Which voltage is really decided by the cost and availability of the appliances being attached to the battery bank. Also keep in mind that voltage drop in wiring is much worse with lower voltages. Beefy aluminum "ought" wire is your friend!

To run the pump for an hour would take 960 watt-hours. To run it for a minute would take 16 watt-hours.

Battery capacity

Your 100AH battery can supply 100A for 1 hour -- but only a few times. A deep discharge like this damages and reduces the life of a lead-acid battery. If you need to regularly deep discharge a battery, use a nickel or lithium type. A better rule of thumb for a quality deep cycle lead-acid is to figure on using 30% of its capacity on a daily basis - so count on 30AH @ 12V for a single deep cycle. That is also 360 watt-hours.

So if you have a 2 battery system, that is 30AH @ 24V, or 720 watt-hours.

Draw vs capacity

As said, the pump draws 16 watt-hours per minute of run. A 2-battery system gives you 720 watt-hours (reasonably, daily)... so that'll run the pump for 45 minutes.

To recharge that, you need to put - throw a fudge-factor in there for charge efficiency - 1000 watt-hours from solar panel. Assuming 5 hours of respectable sun, that's 200 watts of solar panel.

Mind you, a leak would really screw you up. Because it would make the pump start periodically as water pressure leaks down. I would not simply have the pump energized 24x7. I would have a pushbutton at each water spigot that enables the pump for 15 minutes.

Don't run the inverter unless the pump's pressure switch kicks on.

Having an inverter "spun up" 24x7 waiting for an intermittent load like a pump motor or a fridge, is wasteful of electricity, because the inverter has a "standby load". The standby load isn't terrible on paper, but the load is active 24x7, so it really adds up. It's similar to the "vampire loads" from cable TV boxes and wall warts. As such, you should rig up the pump system so the pump's actuation switch actually switches on the inverter. The inverter should then shut off when pump current draw has gone to zero for a few minutes.

If able, don't store electricity - store water up high

If you have the terrain to pull it off, focus on your ultimate goal: pressure and flow of water when you turn a tap. You can do that with a big tank, up high! This is a passive system that will work as long as the water lasts. It is much better to store water in a tank than electricity in batteries.

The tank only needs to be refilled from time to time, and your solar system can do that with "dump" power -- leftover excess power it would throw away because all loads are satisfied and batteries are full. In short when the system sees 100% battery and tank not full, run the pump for 5 minutes. If this isn't keeping up with water demand, then add panels to the system.

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  • This is good stuff, and it makes me question my strategy. Now I’m wondering if I should just make a mini water tower and build out two single-battery solar generators that I can combine in an emergency. Then I would just use them to top off the water tower. What are your thoughts on physically connecting this bad boy to an inverter? Are there 2000W inverters with a plug that will get me up and running? With an adapter? – Andrew Kvochick Aug 14 at 11:57
  • @AndrewKvochick I'm proposing hardwiring because horking it up with extension cords as a permanent installation violates NEC 400.10 and 110.12. If you want to do it that way, then the first stop is determining YTH this pump has a NEMA L14-30 plug when it only needs a 6-15. Then it's just a matter of identifying a "solar generator" (read: battery + inverter + wheels/handle + 300% markup) with a 6-15 socket on it. You don't need to parallel them, you can just use one to some prudent discharge level, then move the plug to the other. – Harper - Reinstate Monica Aug 14 at 16:13
  • it sounds like I would be within reason to call the electrician back out to redo it with a 6-15 plug/receptacle. No? – Andrew Kvochick Aug 14 at 16:24
  • @Andrew Certainly worth a conversation with the electrician as to why that particular connector. Honestly you are putting yourself at a serious handicap trying to do something so specialized with electrical power, but without the skills/chops/confidence to DIY even a simple thing like a plug swap. It's less about the $100 service call... and more about the time needed to schedule and wait for it. That's why I change my own oil lol... it means not having to give up the car. – Harper - Reinstate Monica Aug 14 at 16:29
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If you have not yet bought the pump, you may want to reconsider. If you buy a 3 phase pump, you can use a VFD to power it. Use a VFD that will accept a DC input directly, then you don't need the inverter (it IS an inverter). That will avoid the starting current surge of the motor, because VFD controls that. The down sides: Your DC voltage for a 230V pump will need to be around 330VDC, so that's 28 12 volt batteries in series (I'd go 30 to give you some head room as the batteries get old). It's also going to be expensive.

Alternative #2; get a solar powered well pump, basically a Permanent Magnet DC pump motor, no inverter needed at all. Also expensive, but MUCH simpler.

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  • You still have starting currents the vfd can take longer to start With reduced voltage but the load on the system will last longer. But if the vfd is capable of a direct dc feed that would eliminate some losses , there are also single phase VFD’s I have used them to vary single phase pumping speeds not as efficient as 3 phase but they are out there. – Ed Beal Aug 13 at 21:55
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There are a lot of questions in your question, I'll answer the one relating to the plug. That looks like a L14-30 type plug (L14-30P is the plug, L14-30R is the receptacle). They are a locking type plug (locked by twisting).

These are pretty common on generators. If you can't find an inverter with this type of receptacle, you can easily source either a premade adapter or make your own - there are cord ends available at most home improvement stores or online.

But as Ed Beal pointed out in his comment, this does not seem to be the correct plug for the amperage of your pump anyway. You should switch it out for one that matches the current draw of your pump and the breaker that protects this circuit. The details for the rules on this can be found in the answers to this question.

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  • Thanks! For the plug, I guess my question was more basic. I have seen adapters that will convert it to the three-prong style and a normal household 15-amp style plug. Is that OK as long as the wire is thick enough? I don't want to plug positive into neutral or something like that. – Andrew Kvochick Aug 13 at 21:04
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    No, I saw that one too and that is not acceptable to use. You need a NEMA 14-30P to L14-30R adapter, such as this one (just happens to be the first one I found, not a specific recommendation): homedepot.com/p/… – PhilippNagel Aug 13 at 21:14
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    Actually a 30 amp on a 6 amp pump is a code violation. – Ed Beal Aug 13 at 22:04
  • Even though that is what the pump has from the factory? Or maybe we shouldn't assume this plug was factory installed. – PhilippNagel Aug 14 at 1:18
  • It did not come from the factory. I asked an electrician to put a plug on it, and that’s what I got. What is the correct plug @EdBeal ? – Andrew Kvochick Aug 14 at 11:42

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