how do I run a long curcuit off a cheap inverter?

heynow999heynow999 ✭✭✭✭Solar Expert Posts: 80 ✭✭✭✭
I have a cheap harbour freight inverter like this one

I want to put it in my shed with a 12v battery system and use it for backup power of my house. I am experimenting with a small windmill that will charge the batteries. The idea is I will run a conduit from the shed to the house and add one new receptacle that will run off the inverter. I realize that a 2000 watt inverter will not give me a lot of power, the idea is to be able to keep the fan going on the woodstove, a few lights, maybe the tv and be able to microwave some food , but not all at the same time. If the windmill produces good power, we can use the plug as a "green" plug that we can use of vacuuming or something like that.

So the problem is that the output on the inverter is a plug, not a terminal that I can hard wire a circuit to. I was thinking I could open it up and just wire off of where the plug is wired to, but I don't want to open it up. I could just put a plug on the end of the wire that goes from the shed to the house, but that seems wrong as well. Is there a proper way to deal with this? I want to use this inverter as it is what I have laying around.




  • icarusicarus ✭✭✭✭ Solar Expert Posts: 5,304 ✭✭✭✭
    Re: how do I run a long curcuit off a cheap inverter?

    The issue of the "cheap inverter" not supplying " a lot of power" is not that the inverter can't supply the power,,, but more that the batteries can't supply the power. As we say all too often here,, do your math. A 1000 watt load from your inverter will draw ~83 amps from a 12 volt battery. Unless you have BIG battery,, if you put that kind of load on a simple deepcycle battery the inverter will go to low voltage disconnect.

    Just as a simple math exercise,,, let's say your tv burns 125 watts,, if you have a 100ah battery, and you don't want to discharge it more than 50% (the maximum safe discharge level) that leave you ~50 ah,, or 600 watt hours to use off the battery. If you figure in line loses and inverter loses,, you might have some where in the neighbourhood of 3-400 wh to use. That leave you between 2-3 hours of tv. Micro wave at 1500 watts would about 20 minutes.

    Charging the battery becomes another matter.

    The basic wiring rules for 120vac will apply as per wire size vs wire run and amp load on the circuit. Find the link on this size for a wiring loss calculator, to find the proper wire size, breaker size etc.

    Good luck,, and welcome to the site. There is mountains of good information here. Take advantage of it to avoid the "ready, fire, aim" syndrome. Read the threads on small scale wind,,, and you may find that you would rather put your eggs in the solar basket.

  • hillbillyhillbilly ✭✭ Solar Expert Posts: 334 ✭✭
    Re: how do I run a long curcuit off a cheap inverter?

    There is a good chance that inverter is NOT intended for the kind of use you're talking about. Just having a receptacle, and no access to the terminals is a good indication that it isn't meant to be hooked up to a circuit as you're describing. As I recall one possible reason is that the negative may be bonded to the chassis inside the inverter, potentially causing some issues with grounding when a circuit is connected to it (such as with a cut power cord or some other creative solution). I would be cautious about proceeding with this without consulting the manual, or contacting the manufacturer specifically. It's possible that there is a simple work around, but I'd want to know from the "horse's mouth" so to speak.
  • icarusicarus ✭✭✭✭ Solar Expert Posts: 5,304 ✭✭✭✭
    Re: how do I run a long curcuit off a cheap inverter?

    Hillbilly makes a great point that I had forgotten to mention.

    Most small inverters cannot be set up with a grounded neutral, as the inverter neutral is floating. It may be an esoteric concept to some,, but the net effect is that if you ground the neutral of a cheap inverter it will go up in smoke,,, right now. I know,, I burned up several a number of years ago before I know any better,,, all the while telling my self,," There's no reason it shouldn't work"

    I can also assure you that the 'manual' from a cheap inverter will not give you much useful information. Much of the stuff from Harbour Freight is pretty questionable.


    PS. For the woodstove,, consider one of these,,, comes in two sizes,, moves tons of air,, and makes no noise:
  • loreleclorelec ✭✭✭✭✭ Solar Expert Posts: 200 ✭✭✭✭✭
    Re: how do I run a long curcuit off a cheap inverter?

    With a lot of the portable inverters, the output stage is transformerless. Long story short, when you ground the neutral wire, you are causing a direct short across one (or two) of the output FETs. So these FETs blow immediately and short the DC-DC converter, which usually takes out a couple of FETs there, too. Aside from using an isolation transformer on the output of the inverter, there isn't a way around this.

    Actually, second thought -- on some of these inverters, if you leave the DC side of things ungrounded, you might be able to avoid this problem. It depends on the particular design (whether the internal circuit ground is connected to the chassis ground or not). This would probably present some code-related issues, but I believe if your system is under 50 volts, you're not required to ground it.

  • dwhdwh ✭✭✭ Solar Expert Posts: 1,341 ✭✭✭
    Re: how do I run a long curcuit off a cheap inverter?

    Don't open the inverter up. You don't need to hardwire anything inside of it.

    Go ahead and plug into the receptacle on the front of the inverter - I would, since with your rig a "quick disconnect" might be for when that HF inverter goes bad and you have to take it back.

    Make sure that your "green plug" is completely separate and not connected to anything in your house wiring system.

    Make sure you have enough batteries to run what you think you are going to run...the stuff you've described will suck a normal battery dry in minutes.

    It's unlikely you'll get enough power from a small wind turbine to keep the batteries topped off if you use that "green plug" on a daily basis - so you'll probably also need to figure out some other way to keep the batteries charged.
  • heynow999heynow999 ✭✭✭✭ Solar Expert Posts: 80 ✭✭✭✭
    Re: how do I run a long curcuit off a cheap inverter?

    Some great replies, thanks.

    I didn't want to make a long initial post, but I have thought it out. The power goes out in our area maybe once or twice a year, not much at all, and only for a few hours. We are not way out in the country, more like suburbia. The whole idea behind the plan is that I want to build a windmill for fun, so I thought it would be a good idea to make it into back-up power as well. I plan on building a F&P windmill like these

    It might put out 300 watts, but again, it is more for the fun of doing it than the actual energy I will get out of it. I have access to 510 amp/hr ( at 5 hr rate) telecom batteries that I plan on using. I think they will last a few hours!

    I agree that the Harbour Freight inverter is junk, I bought it to put in my RV but never got around to doing it so I want to use it. The plug does appear to have a ground on it, does the ground do anything?

    So now that everyone has a better idea of what I want to do, is there a reasonably priced inverter that I could use to do the job? Maybe two Morningstar suresines? run two circuits? Probably not.
  • BB.BB. admin Super Moderators, Administrators Posts: 29,634 admin
    Re: how do I run a long curcuit off a cheap inverter?

    The safety ground should "ground" to the metal case of the inverter and the metal case of any power tool/appliance (that has a three prong plug). The two line outputs (120 VAC) should be isolated with respect to ground (or you will let the magic smoke out of the inverter).

    The Morning Star 300 Watt TSW inverter is probably the best one of its size out there at this time. Won't run a full size fridge (with auto defrost)--but would run any of your smaller gear out there very nicely.

    The manual (PDF Link) shows them safety grounding the white connection == but no mention (that I saw) about grounding of the battery connection (or not).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • vcallawayvcallaway ✭✭✭✭✭ Solar Expert Posts: 157 ✭✭✭✭✭
    Re: how do I run a long curcuit off a cheap inverter?

    I've been lurking here for a while but this question is pretty much what I've done in my home.

    I have a 2000 watt Vector inverter (modified sine wave) that is connected to an Aims 3-way switch.. It is inexpensive and does the job perfectly.

    It is wired just like a manual transfer switch for a generator. The bonus is that it has input for a generator and will switch to that power when it is running.

    The inverter has a standard 3 prong plug and is earth grounded. I just put a plug on the end of some 12/2 wire and wired it to the switch.

    My inverter is connected to a single Trojan J185G battery. It has run my needs for many outages without having to dig out the generator.

    I am sooo ready to go to the next level which is why I'm soaking up the knowledge here. :)

    BTW, The inverter was purchased at the black and decker outlet store for $100. The switch was $200 (online at several places) and the battery was picked up off craigslist for $75. Add another $100 for cables and wood for mounting. All in all fairly cheap for backup power.
Sign In or Register to comment.