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I am new to this community so I hope this is the right place.

I am looking into buying a new inverter for my off grid cabin running from solar. I would like to purchase THIS 3000w inverter. My question is... in the unlikely event that I run this at its full 3000w capacity, what is the draw from my 12v battery bank in either amp hours or watt hrs. Thanks in advance.

  • 3000 / 12 is 250A ignoring losses so a bit more in reality. So 1 hour is 250 amp hours – Solar Mike Aug 18 at 15:53
  • Is there a reason you're running the battery bank at 12V instead of at a higher voltage? Also, inverters aren't something I'd chintz out on if I were you...and where are you on this planet, for that matter? – ThreePhaseEel Aug 18 at 15:54
  • @ThreePhaseEel I'm running to battery bank on 12v because it's a 12-24v solar system. Also, if I reveal my super secret location, it kinda defeats the purpose of my off grid cabin. Lol. Lastly, is there a reason you think this inverter isn't good enough? It sounded like you thought it was a cheap option. – BigCountry Aug 18 at 16:18
  • @SolarMike thank you. I thought it would be something like that. My dad and I were talking about it and I had said that(correct me if I'm wrong) 3000w worth if work on the ac side would require 3000w the DC side. Example 120v 10a pump is 1200 watts so we would need 12v 100a from the dc side. They both equal 1200w. If this is true then could you run that pump for 30 min on a 12v 50ah battery? Only considering numbers not reality and efficiency. – BigCountry Aug 18 at 16:23
  • @BigCountry -- don't need or want GPS coordinates, but a country and perhaps some info about what rules your AHJ follows would be nice :) – ThreePhaseEel Aug 18 at 16:31
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I suspect there's a sizing issue here because I'd be surprised if a 3000W inverter could start a 3000W motor.

If you run the inverter at its nameplate capacity, your immediate 12 VDC current draw will be 300 amps or more. Aside from inverter losses, you will have considerable voltage drop in whatever wires you are using to connect to the batteries, and indeed, through the batteries themselves. The inverter will compensate for voltage sag by pulling more current.

So that means you'll need 300AH per hour. 5 AH per minute.

Invert as soon as possible ... For the big loads

You'll want the inverter as close to the batteries as safety permits, with battery-battery and battery-inverter interconnects of heavy 2/0 or even 4/0 wire. I've seen a solar project fail because they used (seemingly generous) #10 wire to carry 20A@12V eighty feet.

The goal is to cover distance at the highest voltage possible, because raising voltage both lowers current and lowers the importance of voltage drop. The benefit is (voltage/voltage) squared - e.g (120/12)^2 is 100x better to transmit as 120V than 12V, and (240/12)^2 400x better for 240V.

With 120V you must also watch voltage drop, but it becomes a factor over much longer distances in the 100-200' range (and don't forget the distance down the well, unless the motor is at the top; most are at the bottom).

Use a voltage drop calculator with your distance, voltage and continuous current draw (not startup surge). Override the absurd 3% and allow 40% or so, merely to see how bad it will be at the minimum allowed wire size. Anything more than about 6-8% needs remediation.

  • You can recompute with a 6% value and see what it says for wire size.
  • If it's recommending hundreds of dollars of wire, recompute for 240V. If that number is much more favorable, then it's time to go 240V on the well pump (but do not increase horsepower!)

You don't need a 240V inverter. You can make 240V (at half the current) with 120V and a simple transformer. The 120V side of the transformer looks like any other 120V load. A motor of same horsepower will have the same current draw (from the 120V side). Transformers have a trivial power drop of 1-3%. They handle startup surge without worry (unlike inverters) so size for the motor's continuous draw.

Deep cycling is a problem, though

As discussed, you'll need 300A+ from a 12V battery. That's 5AH per minute or 300AH per hour.

Beware lead-acid, as they take damage from discharging, so their practical, everyday capacity is nowhere near their nameplate capacity. Seriously. If you draw only 20-30% of a lead-acid's capacity and then recharge it, the battery will work for years. If you start dipping it to 50%, you shorten its life to a few hundred such dips, and if you discharge 80% of its nameplate capacity, you'll greatly shorten its life to dozens of such cycles.

So a 300AH lead-acid battery would very quickly fail. Best practice is a 4x oversize, so now you are talking about a 1200AH battery.

Lithium batteries do much better, and nickel batteries are almost impervious. Apple and Tesla get phenomenal performance out of lithium batteries (this 7yo iPad is run down daily and still has nearly full range) by including 20-30% more battery and not letting you use it. Once Tesla pushed a software patch to hurricane customers that let them use the full range of their battery. This gave them 100+ more miles!

Make hay while the sun shines

If you have a location at a higher altitude than your house, you can put a storage tank there. Water in the storage tank pressurizes your system and delivers water to the house. The pump tops up this tank when the sun is shining and the panel is at peak delivery.

Faucet pressure is based on altitude difference, 1 PSI per 2 feet of altitude.

So for instance if the solar panels are producing 2000W of power, your 3000W load is only drawing 1000W from the battery, which has a big impact on sizing.

If the solar array is 3000W then the battery sizing issue pretty much goes away (least as far as the pump is concerned), you're only borrowing from the battery for startup surge.

As a bonus to this, you can spin down the inverter when you're not using the pump, so you don't have the parasitic "standby losses" of the inverter, which really beat you up because they're 24x7.

Also, this lets you do irrigation at sensible times rather than at the height of the day when the sun is shining. The tank size is the limit; essentially you're storing "pumping energy" hydraulically rather than electrically. Tanks are cheaper than batteries.

  • I appreciate all this extra information! We actually currently have a tank up the hill for the exact reason you mention. So our system already covers that. The problem is that where the well is, it is very deep and in order to get it from the ground and up the hill is a tremendous job for the pump so it burned out. We started using a smaller compression tank and that seems to work well but we still need a stronger pump. For what we need, we have to go to a pump on 120v. The pump in particular I believe is about 800w. I never planned on getting a 3000w pump. – BigCountry Aug 23 at 19:15
  • we wanted to get an inverter at least 3x the motor wattage due to how induction motors work with that heavy startup. If you're still wanting to know more, we are considering moving to a 24v system or possibly a 48v as suggested above. We are also trying to price batteries to create a robust enough bank that we should never get down below 80%. Hopefully this sounds good to you. I'm trying to piece everyone's info together and so far, that's where its leading us. – BigCountry Aug 23 at 19:20
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After seeing many comments, I believe the best answer to my question is 250amp Hrs. when only considering the numbers provided.

3000w/12v = 250A

In order to maintain the 3000w output, you would need more on the input due to conversion loss. If the inverter is 90% efficient, then you would need 3000W/90% = 3333W on the input side.

Thank you Solar Mike for the original answer. Thank you jsotola for the extra info that helped with a better final answer. And thanks to the rest of you for helping and making me think of more questions! :)

  • 250A is insane, you would need wiring thicker than your arm. you have no need of such capacity, mentioning only about 0.5kw of load with your pump and lights and movies all running... – dandavis Aug 19 at 16:22
  • @dandavis I agree it is a lot. We don't plan on using that much either. My dad and I were just looking at a project we're working on and were writing out some hypotheticals. In order to ask the original question I just decided to use the numbers from a potential piece of equipment involved. – BigCountry Aug 20 at 3:55
  • @dandavis a 4/0-4/0-4/0-2/0 SE cable is not exactly "thicker than one's arm" :P – ThreePhaseEel Aug 22 at 23:28
  • @ThreePhaseEel: heh, some exaggeration applies. I still think feeder-level current capacity is a mismatch for OP's cabin. – dandavis Aug 23 at 16:53
  • 4/0-4/0-4/0-2/0 Al would make acceptable DC feeder because you'd parallel blackgreen- redwhite+, presuming your current or distance is sensible. If both current and distance are insensible, rethink. If they must be insensible, then you're buying structural Al bar, and wrapping with tape. Al not least to make the pieces handleable. Copper is twice the weight! – Harper Aug 23 at 19:55

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