I could use some advice about a 12VDC to 120 VAC inverter project:

Aug 22, 2009
Pittsburgh,PA U.S.A.
Hi Everyone, I am going to be buying a 12VDC to 120 VAC clean sine wave inverter soon, but could really use some advice about what brands are the best.

Someone in our house is on an oxygen generating machine that make oxygen from air and runs on 120 AC, and the name plate on it says 310 Watts maximum input. This machine must run 24/7 non stop. We have a backup bottle of oxygen that should last about 4 hours if needed, and I have a 2200 Watt Generac inverter generator and 10 gallons of fresh gasoline treated with Sta-Bil that in case we have a long power outage and I am home.

But I am not always home, and when I am not some other adults are taking turns being here. If there were a long power failure I would be nice to have a 12 Volt DC to 120 Volt AC inverter as an additional means of powering the oxygen generating machine that they could connect to there car battery (not the 12 Volt power connection inside of a vehicle as that is limited to 15 amps or less, and run a long 14 or 12 gauge extension cord from the inverter.

I figure the minimum inverter size to get would be 500 Watts but will probably get a 1000 Watt one for some extra heat sink ability if used on a very hot day, and also the oxygen generator has an air pump that is powered by an electric motor, so there is the start up inrush current to keep in mind.

To play it safe I will be getting something with a very clean sine wave output. And also it is likely that more expensive clean sine wave inverters would be of higher quality and more reliable.

So, what brands of inverters are have a clean sine wave output and are the most reliable?

Thanks in advance.
I'd look into using an uninterruptible power supply to start with, to give them time to crank up plan B into action.

It may help to know what kind of power supply is in the o2 generator, as switching power supplies are actually quite tolerant of dirty power.

Put the thing on a kill-a-watt meter to see what it actually draws, to appropriately size your response.
I'd look into using an uninterruptible power supply to start with, to give them time to crank up plan B into action.

It may help to know what kind of power supply is in the o2 generator, as switching power supplies are actually quite tolerant of dirty power.
Mods please delete this ^. I tried to reply as below but when I tried my computer would not type because something had got onto it.
Last edited:
Consumer grade Bestec on Amazon is what I use in the car. Not sure if it would be up to your task though. They have high output models in addition to small cigarette lighter types.


Problem is, most batteries have a reserve time around 90 minutes at 25 amps drawn. Say 360 watts, that's 30 amps. So than 90 min run time.

With low battery protection, you may get a few minutes before it shuts down to avoid killing the batt. You could leave it running at significant load to the alternator. A 100 amp alt outputting 30 amps plus it's own demand would be around half of rated capacity at idle. Quick way to burn up an alternator if you run it for an extended length of time.
You'll do more than fine with a normal 16 ga extension cord and your 3 amp (max) draw. It'll probably be harder keeping people from borrowing the cord for "other" uses, and you don't want to overcomplicate things by demanding they only use this particular cord.

I get that the inverter is a backup to a backup, so it'll be ok to idle a car until the generator can be fixed.
We have an oxygen bottle with a liters per minute adjustable output regulator that should be able to supply oxygen for about 4 hours while we set up some kind of alternative power for the 120 VAC powered oxygen generator.

I plan to always run the engine while using the inverter, and will tell others to also do that if they end up using their vehicle when I am not here. I figure that the alternator has to have extra overhead designed into its capacity to handle all the vehicles outside lights and the AC clutch and the vehicles factory stereo all on at the same time and all of that should add up to more than 30 Amps and all of this will be off when the inverter is being used with the engine of the vehicle running. Plus the alternator has to be able to recharge the battery from when it supplies power for the electric power steering if the battery has to make up the difference when the alternator is not spinning fast enough, and the electric power steering also will not be demanding power because no one will be turning the wheel. So about 30 Amps continuous should not be demanding too much from the alternator. Though I do not know if vehicles will have to be ran above idle to put out enough.

If I have to, I will get a 220 Amp alternator installed, but I hope that in general everyone who also takes watch would be able to use there vehicle as is. There is a company called "Power Bastards" that makes a 220 Amp direct replacement that puts out 110 Amps at idle and 220 amps at 1200 engine RPMs for my 2016 CR-V, for $475.00 including shipping. But it looks like if we limit the load on the inverter to only the oxygen machine then standard vehicle alternators should be able to handle the load as long as everything else is kept off in the vehicle while the engine is running.

The local alternator rebuild shop (Ed's Generator service) are friendly and helpful guys, and have an adjustable load set-up for testing, and I plan to call them next week to see if it can go as low as 30 Amps and to sometime go there to do some testing. If I have to I will rig something up myself to do the testing but right not I would rather not spend the extra time rigging something if I do not have to.

What would really be helpful is if someone know what brands of inverters are the reliable ones.

BTW, we probably will be using 100 feet of extension so even at about 3 amps I will probably go with at least 14 gauge.
Last edited:
are 2 pure sine inverters on my wishlist, in that order.

A parked idling alternator at 50%+ of output, gets much hotter, much faster, than when vehicle is moving with more.engine rpm spinning alternator fans faster with underhood airflow.

Also voltage regulator itself gets hot, fast, when the rpm is low and load is high.

How hot depends on specific vehicle.and the actual load.

I thermo epoxied k type thermocouples to my alt and vr, as guessing blows.

My external adjustable vr's got additional heatsinking and 60mm fans as hitting 175f+, in 3 minutes, when maxed out, idling parked, scared me.

I dont trust hot engine, hot alternator, idle output claims.

I intend to be able to run a 15 amp table saw via battery + alternator - psw inverter one day.

Perhaps shopvac at same time.

Wish i bought the 2/0 cable last year.
I'd look into using an uninterruptible power supply to start with, to give them time to crank up plan B into action.

It may help to know what kind of power supply is in the o2 generator, as switching power supplies are actually quite tolerant of dirty power.

Put the thing on a kill-a-watt meter to see what it actually draws, to appropriately size your response.
Bigger units, >1000va can use external batteries connectivity.
I'd consider hooking pv panel on that, with skilled electrician.
i've been using an EDECOA 3500W 12V Pure Sine Wave Power Inverter (about $350) to power my 3/4hp sump pump that draws some 30A @120VAC at startup for over 4yrs and it has performed flawlessly. It is always on 24/7 in case power goes out when I'm not home, which is significantly more rigorous than your use case. these things are not really designed to be used in continuous operation for years on end, but its doing it.

just get a grp 29 marine battery from walmart, or the grp 31 FVP marine battery from Menards during their 11% rebate. Keep it on a maintenance charger in your garage or basement and use that rather than running it off your car. that will power your O2 machine for 3hrs without running your car. I'd also recommend using real inverter cables/connections (not jumpers) to connect the battery to the inverter. I've had bad luck trying to use jumpers for this kind of stuff. the inverter comes with real cables.

When hot, my engine, in drive, stopped, foot on the brake, idles at 525 rpm and the one 50/120 alternator can make its 50 amp rating.
The other alternator with same 50/120 rating, struggles to make 40. Both K type thermocouples on each alternator are climbing rapidly the whole time I am sitting at a traffic light with alternators maxxed out. 220f is the danger zone but I start dialing my alternators down, via my adjustable voltage regulators when they hit 185f

Take it out of gear and it idles at ~600 rpm, the first alternator can now make 62 amps, the second makes 55 max
rev it to 800 rpm and the first alternator can make 68 amps and the second can make 80 amps. The alternator which struggles at 525rpm vastly outperforms the one which can, from 800 to about 2000 rpm. At that point the second maxes out at ~ 116 amps and at 2300+ rpm the first alternator tops out at 124 amps

Both get hot way too quickly( 185f+ and rising quickly when maxed out iding, parked.
Both will not exceed ~145f at 65mph, making 110+ amps each.
Sub 25mph is not much better than parked, temperature wise, in my engine bay.

Their K type thermocouples are adhered to stator casing, and insulated from airflow, not the rectifier, so not perfect data.
the first alternator has one pulley mounted puller exhaust fan, the other alternator has two smaller internal fans. which pull air in from the ends and expels it radially.

This is data collected from one engine bay and 2 separately regulated alternators on same crank pulley, same belt, with fat cabling to batteries, and loads which can max out the alternators.

I'd expect all vehicles to be different, as to how much alternator heating occurs at what load, and would caution against assumptions without actual data to back them up, in the actual vehicle.
I was told by someone highly qualified, that the dual fan alternator I was contemplating, would outperform the single fan alternator of same idle and max rating, at hot idle, but when bought and installed, actual data proved otherwise.

The Killer was 525 rpm idle in gear foot on the brake. If 800 rpm was the minimum then he would have been right.
Of course he did not know my idle speed or pulley ratios, and made some incorrect assumptions from previous experience, but these were/are huge factors and the difference between being right, and wrong.

Beware, the higher rated well marketed Alpha alternator, which also claims high output at idle, might only make that when cold, and when a smaller pulley (not included) and 100+ dollars, is used.

The power curve graph below, all the alternators are the same exact size and case
leece neville power curve copy.jpg

The 185 amp alternator at 1400 rpm can make 100 amps.
The 325 amp alternator at 1400 rpm makes 40.


The cables I've seen provided with many noname budget 1500W+ inverters are often doubled 4 gauge cables, with pisspoor ringterminals crimped poorly with hammer crimps. They could even be copper clad aluminum cables with aluminum ring terminals, hammer crimped. If run anywhere near their rating I'd recommend checking for excessive heating at the connections. Thick copper cabling should be able to pull heat from connections and act as a heatsink, as opposed to weak aluminum undersized cables and pisspoorcrimps adding to the heatload at the connections and heating inverter internal components more.

One ideally wants short Fat copper cables with thick copper proper ring terminals with hexagonal crimps, between inverter and battery, and preferably no alligator clamps.
A big fat Anderson connector can help eliminate alligator clips, and yield a safer easier more foolproof connection.

The more wattage needed, the more important low resistance cables and connections become.

Also consider the fact that it takes no less than 3.5 hours, to get a healthy Lead acid battery from 80% charged to 100% charged. Those 3.5 hours assume the battery is being held in the mid 14 volt range the entire time, something few vehicles will actually do, even if they are driven for 3.5 hours.

The deeply cycled battery is much happier, and perform better and live longer, when it can be returned to 100%, as opposed to 98%.
Older batteries can take significantly longer than 3.5 hours to get from 80% to 100% state of charge, even with ideal voltages held that entire time, and these older batteries worked hard with a deeper cycle, really want to be returned to a true 100% state of charge, or they start looking for the nearest cliff to jump off of. Plugging in and havnig the time required for plug in charging surce, to hold battery at the correct absorption voltages for enough time, to actually reach a true full state of charge, can prevent premature battery demise.

Alternators are only about 55 to 60% efficient. Each 25 amps your alternator makes eats up about 1 engine HP.

Charging a lead acid battery to about 80% charged is fairly efficient, but 80% to 100% gets much less efficient the closer it gets to 100%.
Running an engine to fully charge an 85% charged Lead acid battery to 95%+ charged, is a waste of time and fuel.

The largest marine battery which can be stuffed into the engine bay, will likely handle occassional inverter/deep cycle duty far better, but marine battery warranties are usually far less robust, and marine batteries of today might not be any different than their starting cousins, but for their inclusion of both posts and threaded studs, with that shorter warranty and lower CCA rating and slightly higher weight( hopefully).

Does the Kill-a watt meter even capture surge/start up amps? I know some of these devices can, but not all of them do.
@wrcsixeight the problem I had with "true" deep cycle batteries (GC2s) in the past is:
1) they required periodic watering, and despite my obsessive nature I still managed to neglect them, leading to early death. the average user will simply ignore them altogether and they'll die in 18 months.
2) by design they don't provide huge surge current without large voltage drop -- that is not their use case, but it is what I need for a large sump pump. so my pair of GC2s would drop to <10.5V when the pump kicked on, and that would trigger the inverter to shut itself down even though the batteries were still at 80% SOC. obviously that is not a good thing. I experimented with large capacitors across the batteries to alleviate this with limited success. ultimately this is why I switched to marine, and have had no shutdowns in 8yrs, and the batteries are still going strong.

so yeah, marine batteries are really not much different than regular starting batteries, except they come in bigger sizes for less $$. Not sure you can get a grp 29 SLI battery for <$100, or a grp 31 for <$110. if you can let me know, I'll buy those next time.
I have the 300w and 600w version of this unit.


ive run the 300 at about 50% power for hundreds of hours while teleworking out of our travel trailer. It’s been great.

the 600w lives in the truck. It rarely gets taxed, but had no problem running a crock pot while transporting it from home to work. They seem to handle their rated load, but I’d suspect for longevity they’d best be kept beneath it. Note, the cables on the 600w are known to be crummy, don’t use them.