Is a 24V inverter safer than a 12V inverter at 2200W

karrak · February 2017

There are a number of 12V inverters made by reputable manufacturers that will output more than 2200W on a continuous basis so it puzzles me that some people seem to be of the view that it is unsafe running 12V inverters at this power level and insist you should use a 24V inverter.

The main safety concern seems to be that a 12V inverter running a 2200W load will be drawing around 200A from the battery compared to 100A for a 24V inverter.

Assuming that the correct size cables and connectors are used and that the connections are done properly I can't see any difference. On both the 12V and 24V inverter you still have the same amount of power passing through the cable and connections.

With any inverter if there is a fault, be it a bad connection or the cable is too small there will be too large a voltage drop at the site of the fault that may generate enough heat to start a fire. This voltage drop is limited by the inverter's low voltage shutdown voltage. For a 12V inverter this is usually 10.5V and for a 24V inverter it is 21V. The amount of power available to cause a fire by the fault will be the voltage drop * current. For a 12V battery this will be around (12.0-10.5)*200=300W and for a 24V battery it will be (24-21)*100=300W

Am I missing something?

mvas · February 2017

In theory you are correct, but in reality the higher the amps in the circuit = lower overall efficiency.
Balancing two or more 12 Volt batteries in parallel is not easy, as balancing batteries in series.
48 Volt systems are common.
Designed correctly, a 12 volt system can be as safe as a 24 volt system.
It is more difficult to extinguish high amperage DC arcing vs low amperage

KeithWHare · February 2017

Safe or unsafe is not really the issue.

The key phrase is "Assuming that the correct size cables and connectors are used". Cables are sized for the Amps. With 24 volts, the amps are half what they are for 12 volts, so you can use smaller cables with 24 volts than with 12 volts.

A 12 volt system will require more batteries in parallel than a 24 volt system. It is difficult to completely balance charge and discharge for parallel batteries.

Take a look at some battery specifications and configurations and calculate whether or not you can actually get 200 amps out of a 12 volt battery bank.

Keith

mcgivor · February 2017

Theoretically from a safety point of view, there is no difference assuming all conductors, connectors and most importantly overcurrent protection are utilized, the difference is generally not safety related but more an economic difference, lower voltages mean more current, so larger conductors, larger fuses/circuit breakers and larger battery capacity to cover the current demand, lower voltages may mean more parallel strings. Going to a higher voltage reduces the current so smaller "ballance of system" devices can be used, higher array wattage can be utilized and batteries connected in series are inherently easier to maintain. So generally there is a limit to the economic viability of a 12V system, perhaps in the 1000W range but as the demand gets higher it only makes sence to increase the voltage.

mcgivor · February 2017

Think we all wrote at the same time, so no connection to one another, but essentially the same reply.

706jim · February 2017

Key words are "properly sized and maintained". If connections become corroded, usually due to limited maintenance, the higher amperage at 12 volts is going to make more heat.

Another consideration is the amount of time the inverter is running at full steam. If only occasional high loads are experienced (Skill saw for eg) it would be less of a concern.

mike95490 · February 2017

Defects. A small defect has a much larger impact with a 12V system. A crimp internal to the inverter, or a bad spot on a PCB trace. It may pass a 5 min test, but the criticality at 12V 200A, is much more, than at 100A.

Photowhit · February 2017

Safety, I might give a small edge to 24 volt systems mostly for the reasons others have give.

...but Smart Design 24 volt has so many advantages;

Inverter and charge controllers will operate at lower temps for the same output (hence likely last longer).
Double the size array for each charge controller.
Fewer strings of batteries and panels
less wiring, breakers, fuses.

karrak · February 2017

Thanks for the replies.

Regarding issues of corrosion causing high resistance I would have thought that the extra surface area of the higher current connections would balance this problem up.

Regarding faults within the inverter, PCB trace widths, solder pads and plated through holes will be larger in higher current inverters which again I would have thought would balance the problem up.

Paralleling batteries up is another interesting topic. With lithium batteries if the wiring is done correctly as far as I can see it is actually beneficial to parallel batteries up. I have little experience with lead acid batteries in off-grid systems but wonder how many of the problems that we hear about are caused by poor battery interconnection design.

As a general rule I agree that the larger the power output the higher the voltage but don't think this should be the only criteria to base system voltage on. For use in RVs or marine based systems using the same voltage as the RV or boat would be a consideration. Another consideration might be that if your average power usage is low but you need high peak power output for running high power equipment like welders, pumps etc.

Simon

Estragon · February 2017

In terms of shock risk, higher voltage = higher risk so in that sense, I suppose 12v is marginally safer than 24v.

Photowhit · February 2017

Estragon said:

In terms of shock risk, higher voltage = higher risk so in that sense, I suppose 12v is marginally safer than 24v.

I considered that, but for passage through skin, I thought the threshold was around 30 volts. I'll check later, but certainly a consideration for 48 volt systems. I do believe the NEC has added restricted access for DC above 30 volts.

softdown · February 2017

I still like a rule of thumb that I read about years ago:
12 volt is OK for up to 1000 watts of panels.
24 volt is OK for up to 2000 watts of panels.

Overly simplified? Sure....but it usually works based on what posters have to say about their systems. I know that Photowit rues the day day that he settled on 24 volt because it makes expansion more expensive and difficult. It can be easier to do a 24 volt battery system though.

We have a lot of solar pros that can argue with anything. They know a lot more than me as a rule.

Photowhit · February 2017

softdown said:

... know that Photowit rues the day day that he settled on 24 volt because it makes expansion more expensive and difficult. ...

I don't know about "rues the day", my situation was that I already had a well sized 24 volt system for my cabin/home and had to move. I purchased a tin can (mobile home) and had more loads than the system was sized for...

Even then it was a close thing, as to whether it was better to replace the battery and inverter, $2500 + $1500, or live with what I had for 9-14 years until I would need to replace the battery and purchase a 24 v inverter large enough for the place and at least 1 extra charge controller $1500 + $500. I ended up living with the small cabin inverter and have now for 4 years.

Certainly not ideal, it will be a major expense shortly after I retire. I'm learning to live with-in the constraints of a 1800 watt inverter and an 1100 watt inverter that I turn on when doing outside work. I might yet put up my spare 1800 watt inverter and have a dedicated line for an air conditioner or water heater for sunny days. ...but that would require a separate line and a transfer switch and box.

It can be a pain running the hot water heater and forgetting and turning on the microwave. I likely pop off the inverter (over load and the inverter automatically shuts down) once a month during the hot months accidentally.

Softdown is right in my estimation for the rough break down of when it's advisable to move up to a larger voltage system. I had planned on a 3500-4000 watt inverter for this place. I will look toward this and a 48 volt system when I retire this battery. I have a 5000 watt array in place now, and will add a small south facing array and a dedicated hater heating array in the next little bit. I know I've been talking about this for a while, but have even cleaned off the porch to set up the drill press and band (or chop) saw to make the mounts, and have the lumber, wire, combiner box...

Is a 24V inverter safer than a 12V inverter at 2200W

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