Small Grid-tie inverters ? (1 Viewer)

[e9999]

I just had a sudden bout of (dangerous?) inspiration...

I just realized I do have a bunch of small solar panels lying around from various projects and tests and mostly doing nothing. It dawned on me -belatedly- that I could put them to good use -possibly- with a small grid-tie inverter. I somehow previously had this notion that grid-tie inverters were these big affairs for large solar systems with complex disconnects but I was wrong. A quick look on Amazon revealed that there are some in the 1000+ W range that are quite inexpensive ($100-200). So, in principle, I could just quickly hook up a couple of 100W panels I have in storage and probably take care of all the parasitic losses in the house, which would make my Sustainable self happy. A back of the envelope calc suggests a couple of years for payback, which is pretty good.

However, oddly, on Amazon at least, most/all of that type of small units have very few / not encouraging reviews. Looks like this is a fairly new type of product with little/dubious history. Not great. All generic chinese stuff from little-known brands, it seems. Victron does not currently sell a small self-contained unit like that that I could find. So, do you know of any good small grid-tie inverter you'd recommend? (It would have to have an auto-disconnect per the NEC, I believe.)

(I should also look and figure out what would happen to my meter if I were to generate more than I use at any time. That could well be a problem.)

Also, all the ones I saw were MPPT types. Do they have any that you can use with just batteries as input, not solar panels? Cuz then I could use my existing MPPT chargers, a battery bank, and be golden, even at night.

 

[peesalot]

Sunny boy, a few years ago the code changed and inverters must have a disconnect at the inveter, the older sunny boy & others are pretty much gone but you can find them at a great price as they are not eligible for permitted systems. Once you have the inverter , (6,000W) you can add battery bank and be self contained. Throw a small , quiet gen to supplement.

 

[e9999]

thanks. Had a quick look. Seems like the Sunny Boys I see online are quite a bit bigger (around 3-8kW?) than what I am looking for, but will look a bit deeper. Not looking at a big system here, just a few odd spare panels, maybe 500W total.

This whole bizness about auto-disconnect is interesting. Sure, you want to protect the linemen, but if you can't run the thing at all without the grid, it does defeat the alternative independent power idea in case of a blackout. Is there a way to bypass that auto-disconnect if I were to switch off the grid connection completely first? Like running a generator into the house and then having the grid-tie inverter use that as its "grid", synchronize, and then wake up and contribute?

 

[Mrtwooooo]

Things like this have crossed my mind also. I would love to just have maybe 1500 watts worth of panels and a small system tied back into my house to basically just cover what my HVAC system is using.. or mostly cover it. That would drastically cut down on my power bill.

 

[e9999]

been thinking about this some more. I'm guessing that it should be OK to connect one of those micro grid-tie MPPT inverters directly to a battery set. The issue there, though, may be that -unlike with solar panels- it would automatically draw its full rated power (and hopefully not more than that) since the batteries should easily be able to produce that much. IOW it would be going full blast all the time. That may not be so good for longevity. May want to set a fan on it. From the few reviews I've seen on YT, it seems like the little fans on these type of units are a weak point. Better, I should try to figure out a way to limit the draw if I were to ever try that, but that may not be easy or efficient.

 

[db3]

I just had a sudden bout of (dangerous?) inspiration...

I just realized I do have a bunch of small solar panels lying around from various projects and tests and mostly doing nothing. It dawned on me -belatedly- that I could put them to good use -possibly- with a small grid-tie inverter. I somehow previously had this notion that grid-tie inverters were these big affairs for large solar systems with complex disconnects but I was wrong. A quick look on Amazon revealed that there are some in the 1000+ W range that are quite inexpensive ($100-200). So, in principle, I could just quickly hook up a couple of 100W panels I have in storage and probably take care of all the parasitic losses in the house, which would make my Sustainable self happy. A back of the envelope calc suggests a couple of years for payback, which is pretty good.

However, oddly, on Amazon at least, most/all of that type of small units have very few / not encouraging reviews. Looks like this is a fairly new type of product with little/dubious history. Not great. All generic chinese stuff from little-known brands, it seems. Victron does not currently sell a small self-contained unit like that that I could find. So, do you know of any good small grid-tie inverter you'd recommend? (It would have to have an auto-disconnect per the NEC, I believe.)

(I should also look and figure out what would happen to my meter if I were to generate more than I use at any time. That could well be a problem.)

Also, all the ones I saw were MPPT types. Do they have any that you can use with just batteries as input, not solar panels? Cuz then I could use my existing MPPT chargers, a battery bank, and be golden, even at night.

You have a couple of issues to deal with besides the hardware. First, most utilities require a gird tie agreement for you to back feed their system, plus a special meter set. This usually requires a building permit with a licensed contractor installation. They need knowledge of your system to protect their linemen who might be working in your area during an outage. Obviously the same goes for a standby generator system. There is a lot of information about off grid and grid tied systems on this website: NAZ Solar Electric | Off Grid & Grid-Tied Solar Power Systems - https://www.solar-electric.com/ I started with a couple of panels with a Victron charge controller, a couple of golf cart batteries, with a 2000 watt inverter.

 

[e9999]

I would, of course, check with the Utility before connecting anything, but I want to figure out first if it's feasible hardware-wise, and learn something in the process.

FWIW, I've been doing some general reading about using a battery as input on a mppt device and that looks controversial. With some Victron MPPT chargers (not inverters or DC-DC) at least, if appears (reportedly) that when the mppt charger detects something odd, it may short the input, odd as it may sound. That is probably safe if the panels have a short circuit current spec within the limit specs of the charger. However, with a battery there is no such current limitation, so if the charger were to short the input it could melt everything. So, not good for that kind of chargers at least. Now, with a micro MPPT inverter, that may be different and I have indeed seen some vids of somebody using a battery as input to a cheap chinese micro grid-tie mppt inverter with no ill effect, but in the big picture it seems unlikely that I would get good info on such an inverter, and that would be a crap shoot and maybe risky in case of a short. So for now at least I think I'll forget the notion of using the micro grid-tie inverter with a battery input. But I'm still curious about the OP premise of using it with panels. And, of course, if anybody knows of an affordable small grid-tie inverter that is designed to be used with a battery input, kindly let me know.

^DB, that 2000W inverter of yours, what kind is it? Grid-tie? Solar MPPT?

 

[db3]

I would, of course, check with the Utility before connecting anything, but I want to figure out first if it's feasible hardware-wise, and learn something in the process.

FWIW, I've been doing some general reading about using a battery as input on a mppt device and that looks controversial. With some Victron MPPT chargers (not inverters or DC-DC) at least, if appears (reportedly) that when the mppt charger detects something odd, it may short the input, odd as it may sound. That is probably safe if the panels have a short circuit current spec within the limit specs of the charger. However, with a battery there is no such current limitation, so if the charger were to short the input it could melt everything. So, not good for that kind of chargers at least. Now, with a micro MPPT inverter, that may be different and I have indeed seen some vids of somebody using a battery as input to a cheap chinese micro grid-tie mppt inverter with no ill effect, but in the big picture it seems unlikely that I would get good info on such an inverter, and that would be a crap shoot and maybe risky in case of a short. So for now at least I think I'll forget the notion of using the micro grid-tie inverter with a battery input. But I'm still curious about the OP premise of using it with panels. And, of course, if anybody knows of an affordable small grid-tie inverter that is designed to be used with a battery input, kindly let me know.

^DB, that 2000W inverter of yours, what kind is it? Grid-tie? Solar MPPT?

My inverter is an old school Xantrex PROwatt SW true RMS unit. It's over 9 years old and has had extensive use in the few years. My only complaint is while the rated output voltage is 120 VAC, it only outputs 115 VAC under load, as measured with a Fluke 177 true RMS meter. Didn't look too hard, but doesn't appear to be any way to adjust the output voltage. Really a minor quibble for such a very reliable unit. Although I have two larger grid tied inverters, I have no experience with the smaller micro units. However, from what I understand, the Germany made Sunny Boy units were very good.

 

[e9999]

^ are you saying that you are using a regular inverter -not a grid-tie one- and just connect it to your house system?

I don't think that 115V is a problem TBH.

 

[db3]

^ are you saying that you are using a regular inverter -not a grid-tie one- and just connect it to your house system?

I don't think that 115V is a problem TBH.

Yes, a regular RV type inverter that I used for isolated loads, and emergency lighting. Tried using it with my generator isolation panel, but since I only had 400 watts of solar and a 200 Ah battery, it really won't power much for very long. So for many years I used it for a water distiller, a small dorm size refrigerator in my shop, and keeping my RV battery charged. The 115 VAC output was really not an issue except for the long run to the generator transfer panel, where I lost more voltage if I tried to run even a moderate load, so I just used it for the isolated shop loads.

 

[LandLocked93]

If I understand OP correctly, what is it that Ford Lightening owners are doing to tie their trucks into house wiring?
Isn't that essentially what you want to do?
Any other time I've seen the term 'grid-tie' it referred specifically to large solar arrays capable of 'selling pwer back to the grid' in lumps of thousands of watts.
Curious if the power company would be bothered to accommodate a piddly couple hundred infrequent watts.

 

[e9999]

^ well, the main feature of modern micro grid-tie inverters is that -unlike your typical standalone inverters that you'd use camping or in a pinch when there is a blackout- they first analyze the grid they are connected to so they are able to synchronize their phase to that of the grid, set themselves up to produce a slightly higher voltage to ensure the current flows in the desired direction, and -importantly- will automatically shut down if the grid they are connected to stops delivering power. The latter is to prevent current from flowing back into the grid utility lines if there is a blackout, which could potentially hurt the linemen out there fixing the blackout. My understanding is that the latter feature is mandated by code now.

I would not have expected that a truck's built-in inverter could do all that, but that could be true I guess, I don't know. OTOH, if you were to disconnect your house from the grid with the main panel switch first, then I guess none of those phase etc "checks" matter, technically speaking.

 

[e9999]

well, I'm pleasantly surprised. I did some better calculations of potential return if I were to grid-tie with some spare panels I have. I used a solar calculator to estimate my actual local irradiation based on historical data, and with 300W of panels in top condition, and the usual assumptions for losses etc, it predicts I could make about $150 / yr at my ungodly rate of about 30c / kWh. That means payback on an inexpensive grid-tie meters would be of the order of 1 year (and possibly less depending on inverter cost and panel arrangement). That's pretty good. That's for my location with my energy cost, and already having the panels paid for, of course.

On the not so great side, a quick look at my Utility guidelines on this sort of things suggests it would be a serious pain to do the paperwork etc on this, as expected.

 

[e9999]

Update: I did some more reading about this whole concept. This is quite complicated. But if there is anybody interested at all in the topic, I'll just tell you briefly where I am:

I'll spare you the details but from the technical perspective the general idea of having a small solar system that covers a minor fraction, maybe the base level, of your house AC consumption through a grid-tie inverter seems easy to implement, although there some important safety considerations to keep in mind. That was interesting, I learned a lot.

However, from what I can gather so far, the short story is also that my Utility appears to be making this very difficult logistics/paperwork-wise and / or likely difficult if not impossible to come out ahead $$wise, if that small system would be such that you need to coordinate with the Utility.

Unfortunately, I have to be realistic, and basically at this point I don't think it's worth the trouble to do this. I guess that was predictable, in retrospect.

I may revisit this in the future when I have more time to dig in deeper.