Do I NEED a inverter?

Old Guy · January 2017

Situation: Off grid cabin
Items I need to run: LED lights, RV water pump (occasionally), small radio and one small transfer pump to fill my water tank once a month.

All these things run on 12v DC and I use them once a week and everything seems to be fine thanks to the help of some of the members here.

Would I have been better off using a inverter and 120v AC powered equipment?

If yes, please explain. Seems like an extra step I didn't need.

bill von novak · January 2017

You don't need an inverter if all your loads are 12V.

Important note - be sure to include an LVD on your 12V loads so you don't kill your batteries accidentally. Inverters have one built-in; for 12V loads you need a separate one.

Old Guy · January 2017

Thank you!

I don't have a LVD but I do have a main shut off and I disconnect the load when I leave. Nothing running but the controller charging the batteries when I am not there..

mike95490 · January 2017

What you should watch for, is the long 12V runs, you can loose voltage in resistance in the wire, forcing larger gauge wire ($$) to be used.

Old Guy · January 2017

I don't think I have any runs longer than one might find in a large RV.... But I did use heavy gauge wire. : )

706jim · January 2017

Even if you bought a 150 watt inverter (which will run from the typical 12 volt "cigarette lighter socket") you now have the flexibility of running wall wart chargers, an ordinary 120 volt TV (up to about 55" if you can believe that) and various other 120 volt appliances. I keep one of this size in my boat just for these loads.

Estragon · January 2017

The only reason to run an inverter is for loads requiring AC.

Reasons to avoid inverters include; less efficient - ~85% at best but often much lower if the inverter size and load size are mismatched, they have tare losses just to be on which can be substantial for larger inverters, and some loads don't work well with cheaper (square or MSW) inverters.

Old Guy · January 2017

Estragon said:

The only reason to run an inverter is for loads requiring AC.

Reasons to avoid inverters include; less efficient - ~85% at best but often much lower if the inverter size and load size are mismatched, they have tare losses just to be on which can be substantial for larger inverters, and some loads don't work well with cheaper (square or MSW) inverters.

I don't know anything about inverters but thats what I have always heard. It just made sense to me to use 12v devices in a 12v system.
At least in my application. I have not had any issues finding devices I need that run on 12v. My main interest was lighting and I use standard 120v fixtures with standard base 12v LED bulbs and its working out very well so far. The bulbs are 50 watt equivalent that only use 4.9 watts.

BB. · January 2017

One issue with any off grid battery system is that the battery bus voltage can range from 10.5 volts to ~16 volts (12 volt flooded cell lead acid battery bank). That is quite a wide range, and some DC devices just do not do well. (not easy to find AC inverters that will take ~16 volts on the input either--Can be a big issue for our friends up north--Cold batteries need higher charging voltages.).

Using a good AC inverter that can take that input range and then give "regulated" 120/230 VAC can work out pretty nice. Many of the Energy Star rated devices these days are almost as efficient as their 12 VDC counterparts.

-Bill

ramloui · January 2017

You don't need an inverter. But, like everything else, it is a matter of choosing the lesser of 2 evils for your situation. Let me explain.

Generating your own power (and using it...) means that you have to be heavily involved in every aspects, nothing comes without effort. For the same amount of power, your wires have to carry 10 times as much current on a 12V system compared to 120V (P = V x I) and there is a cost for larger wires. On the other hand, going to 120V means conversion losses.

Depending on your circumstances and location, 120V electrical equipment (wiring, switches, outlets, breakers, etc) may be more readily available at a lesser cost than 12V equipment.

And then there is the convenience. With 120V, you can use any regular plug in tool, appliance (within the limits of your system!).

For my situation, all 3 of the above were check marks in the "120V" column. When I bought my cabin, the previous owner had 12 V lights (truck backup lights...) with thin wiring. Basically worthless system. He also neglected to install fuses/breakers. It was not long before I ripped everything out and, after weighing everything I decided to go the inverter route. And I never regretted that decision.

In the end, I think it is a personnal choice that comes after careful consideration of what your expectations are. There are benefits and drawbacks to each option. Pick your poison.

Good luck!

Louis R.

706jim · January 2017

I wired my camp for both 12 volts and 120 volts; two completely separate circuits. I used #14 wire for everything. Well, a 500mah load on the 12 volt system dropped voltage by 0.5 and a larger load would have even been worse. When figuring out losses, line loss at 12 volts becomes horrendous at any sort of distance. And who could afford hundreds of feet of #10 wire not to mention the difficulty this stuff is to work with inside an electrical box. You're better off to swallow 15-30% inverter loss to benefit the much reduced line loss at 120 volts. My solution was the DR1524 inverter. The 12 volt wiring is not used for anything now.

Old Guy · January 2017

706jim said:

I wired my camp for both 12 volts and 120 volts; two completely separate circuits. I used #14 wire for everything. Well, a 500mah load on the 12 volt system dropped voltage by 0.5 and a larger load would have even been worse. When figuring out losses, line loss at 12 volts becomes horrendous at any sort of distance. And who could afford hundreds of feet of #10 wire not to mention the difficulty this stuff is to work with inside an electrical box. You're better off to swallow 15-30% inverter loss to benefit the much reduced line loss at 120 volts. My solution was the DR1524 inverter. The 12 volt wiring is not used for anything now.

What sort of distances are we talking? I don't think I have anything longer than 40'.

littleharbor2 · January 2017

With DC you have to figure the round trip distance so now you are talking 80'

BB. · January 2017

You can find lots of voltage drop calculators online.. Here is a very simple one I use here a lot:

http://www.calculator.net/voltage-drop-calculator.html

For example, if you want 40 feet (one way run), 3 amps, and 0.5 volt drop maximum, plug in the numbers and test different wire AWG, you get:

12 AWG:
Voltage drop: 0.38
Voltage drop percentage: 3.17%
Voltage at the end: 11.62

http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=5.211&voltage=12&phase=dc&noofconductor=1&distance=40&distanceunit=feet&amperes=3&x=48&y=16

-Bill

Old Guy · January 2017

Understood. And I understand some of you are running 120v devices with a much higher amp draw than anything I have and living full time with your system. I am only in the cabin on the weekend.

Looking at my usage I have:

7 - 4.9 watt LED bulbs wired with 14 gauge wire. They can operate on 6-15 volts.
**90% of my usage is 3 or 4 of these bulbs running.
1 - RV water pump wired with 10 gauge wire. Runs intermittently for hand washing, sink, toilet flushing and occasional shower.
1 - Small water transfer pump wired with 10 gauge wire. Runs for less than 10 minutes a month.
1 - Digital Class T chip amplifier wired with 14 gauge wire. It can operate on 10-14.4 volts >2A

I really don't need to add anything else as I have a propane heater and a wood stove that require no electricity and a propane stove that requires no electricity. I do have a 6 gallon electric water heater that I power with a generator when I need to. I could switch that to propane but I hardly use it.

I have not tested the voltage drop at each device but never felt the need because everything seems to be operating perfectly. In normal usage I have never seen the batteries drop below 12.5v running all day and night so I am very satisfied.

I understand there is loss but there is also loss when using a inverter. With my current load I think the loss was less going with 12v devices than it would have been had I used 120v devices with a inverter.

So, we will have loss either way. Other than a situation where the device could not work properly, what harm does the loss do?
How much loss do you feel is acceptable? If one of you math wizards could calculate my system for loss running 12v vs. 120v that would be pretty interesting.

Estragon · January 2017

With 12v wire losses, more current is needed for a given load wattage. This extra current could theoretically shorten the life of some loads.

Wire losses would have to be pretty horrendous to be more than the 10%+ (possibly much higher) loss through an inverter.

Sounds like what you have works fine for you. If it ain't broke...

Old Guy · January 2017

So if the loss could shorten the life of some devices would this have any negative effect on the batteries?

Estragon · January 2017

If you are only drawing them down to ~12.5v it isn't likely to be a problem for you. Inverters often have a low voltage cut-off which would prevent loads from taking the batteries down too low. You can set up something similar for DC loads too, but absent that, some of your loads may be able to drain batteries to potentially damagingly low levels.

Old Guy · January 2017

Thank you all again!

BB. · January 2017

I will add one other "question" about DC vs AC systems... That is arc faults (shorts, failing connections, etc.).

12 volt systems are just at the edge of being able to sustain arcs. 24 volt systems and above can sustain arcs very well (i.e., DC arc welders are better for most applications vs AC welders).

https://www.youtube.com/watch?v=Zez2r1RPpWY
With 120 VAC, you have much less current, and AC current that naturally is less able to sustain arcs.

AC offers isolation (transformers) and the ability to reduce surge current (again transformers/inductors) and shock hazards (transformer isolation and Ground Fault Interrupters at the ~5-10 milliamp level).

For small systems--This is not a big issue. For larger systems--These can be difficult systematic issues to resolve for DC systems.

-Bill

archsabino · February 2017

Hi Old Guy,

Thanks for the query. You don't need an inverter since all the circuits that you'll be running are 12 volts, but here's something you should be more worried about: voltage runs. Resistance will build up in the wire which will result to lose voltage. You'll then have to put up with larger gauge wires.

Thanks,
Archsabino

Do I NEED a inverter?

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