DIY Grid Tied Inverter (doesn't Feed the Grid) UPS Alternative

by Kieren-J

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Intro: DIY Grid Tied Inverter (doesn't Feed the Grid) UPS Alternative

This is a follow up post from my other Instructable about making a grid tie inverter that doesn't feed back into the grid, since it is now always possible to do this in certain areas as a DIY project and some places do not allow feeding into there grid ( and its pretty obvious why the grid doesn't want just anyone feeding into the grid as i will explain a little later on)

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The Concept

Many people want to have solar panels to reduce their impact on the environment or reduce the cost of their electricity and there two ways about doing this, going fully off grid that requires a large bank off batteries and a decent inverter or subsidising your electricity with both grid and and renewable energy using grid tied inverters that feed your power back into the grid.
The problem is that going off grid is not always possible, designing a system that would power everything you want without issue would be very difficult and unreliable. and with grid tied inverters you would need a qualified electrician to install the inverter so it conforms to regulations when feeding back into the grid which is not very cost effective for everyone or ideal for your application. so my solution is a small scale solar system with a "grid tied non feedback inverter" using basic components that are easily available. this allows you to produce and use your own electricity without feeding into the grid but still able to use the grid power when you run out.

STEP 1: The Basic Principle (an Overview Explaining the Changes)

So a few things have changed since the last time upload about grid tied inverter, one being that i am no longer using ups (uninterruptible power supply), This is for a couple of reasons, the main few being that i was not able to pull the amount of electricity I needed without overloading the "UPS" at which point it safety features would cut in and shut it down, not great when you need to unlock several items to turn on one ups! another issue i was running into was the dc current was exceeding the rating of the relays meaning that i had to double them up to try and reduce the current but was still to warm.

I have also moved away from the complex switching system that was using relays and ssr, simply because when you run into problems fault finding can be difficult and tricking a "UPS" to switch over isn't the most flawless way for electricity, and that when I was pulling allot of current you would notice things like light flicker and some devices didn't like it, mostly computers!

so as a result i have done away of UPS and Relay circuits and simplified it with common items that are just as accessible to most people, and is currently my preferred way of controlling my system.

STEP 2: Parts Used and Brief Description

So Finally We can get into Everything i have used on this project, and this time i will be more in depth!

But first, a little safety disclaimer, This project involves AC (alternating current) and DC (Direct Current) electricity both of which are extremely dangerous and can cause harm or even kill if not installed correctly. If You DO Not Know What you are doingor understand it fully then do not attempt this or other electrical projects of a similar nature. That being said anyone can learn this stuff, just seek help from people who do understand it and stay safe!


Parts (linked to places to buy) :

Quick Description:

  • Victron Energy Solar Controller MPPT
    • Controls the charging of the batteries from the 275w pv array it turns the 30v output down to 13v to charge up the batteries and stops charging them when there full.
  • Sun Tech 275W PV Panel
    • They convert sunlight into 30v dc which then goes to a charge controller to charge batteries, I Brought damaged ones cheap then repaired them with a clear resin.
  • 20A PWM (Pulse Width Modulation) Charge Controller
    • Does the same as the first charge controller, just for a lower current, this is used to take power from my 20w solar array and is wired in parallel with the first charge controller.
  • 20W Solar panels
    • Takes power from the sun and converts it to dc power
  • 100AH 12V PowerLine Leisure battery
    • This is where all the electricity produced is stored
  • APC 16A ATS (Automatic Transfer Switch)
    • This device is the brains and switches between an inverter and the grid (more in depth explanation further on)
  • Victron Energy Battery Protect 65A
    • Is a low voltage cut off to stop you from running batteries completely flat
  • 12v 500w Pure Sine Wave Inverter
    • Converts 12V DC and turns it into 230v Ac Sine wave (like grid power)
  • Sonoff Wifi Controller
    • not overly necessary, but allows you to wireless control circuits attached to it via WiFi
  • 2 way consumer unit with RCD Main switch
    • Protect you and your circuits from faults that may occur either from devices you plug in or fault that an inverter might cause (will go into more depth why you should have this)

STEP 3: Simple Diagram to Show How This Works and Explanation

So, to cut a long storey short, instead of using an "ups" im now using an automatic transfer switch and inverter. The automatic transfer switch allows for two power inputs and switches to the other when one fails, you are also able to choose which one is default.

These switches are mainly used for large servers to switch from one ups to another seamlessly and allow up to 16 amp switching. Which when you take into consideration a normal radial circuit wired in 2.5mm cable is attached to a 16 amp MCB in the UK will be more than enough for my switching needs, and since this is all contained in one device makes it a lot safer and simpler .

so the way i have wired this system, is that the inverter is connected to the battery protector, this turns off the dc voltage to the inverter when there isn't enough power in the batteries. The inverter is then wired into the automatic transfer switch along with the grid and i have set the "ATS" to use supply one as default (this is the inverter) now when the battery protect turns off the inverter the "ATS" will seamlessly transfer to grid, and back to the inverter once the batteries have recharged.

***Added Feature***

The sonof wifi switch runs off the 12v battery bank, and is wired into the remote control of the battery protector, this means i can control whether the inverter is on or off via Alexa or my phone, i have set up some timers on it as well since i am not home during most the day the inverter wont actually be turned on till about 2:00pm this means that for most the morning my batteries are charging up and i can stay on battery bank power late into the evening making the most of the energy ive generated. and that i can automatically control inverter without having direct access to it.

STEP 4: Types of Inverter

Why i chose to use a pure sine wave instead of a cheaper modified sine wave.

Well the truth is, I didn't get a choice. I originally did this set up with a 2000W modified sine wave and ran into issues because the automatic transfer switch wasn't able to switch seamlessly it had to cut power completely then restart every time it switched over, not to mention modified sine wave makes an irritating buzzing sound on everything you plug into it. so i had to switch out to a pure Sine wave and the "ATS" worked perfectly.

After looking into this a bit more, i found out that the reason the modified sine wave inverter didn't work with the "ATS and grid power because of what is called "phase mashing" which is when the "ATS" tries to push a modified sine wave onto a load that is already accepting a perfect sine wave, and when you look at a picture of a modified sine wave and pure sine wave you can see why devices wont like switching instantly. pure sine inverters work because its the same as the grids wave.

And its because of phase mashing that the grid doesn't just want anyone feeding into it and that you need permission so they can see your system and make sure it works correctly and would shut down safely and not feed in to there power if there was a power cut. its all down to keeping people safe.

STEP 5: Using an Consumer Unit

The consumer pictured above is the one i installed after the automatic transfer switch this is because unlike a ups the transfer switch can not detect faults on a system, so if the breaker feeding the grid side of your Ats trips because of a fault on the load side of this system it could potentially switch to inverter and your fault would still be live and dangerous, the point of an rcd is to protect against imbalances in current so offers the most protection to people, were as the mcb protects the circuit from damage.

its always good to keep in mind that, most protective devices are there to protect the circuit not you, this is another reason why its good to have this consumer unit on the load side of an "ATS" becuase it will protect the inverter from short circuits and overloads, as well as devices that might become faulty.

As an (Apprentice) electrician are goal is for there to be discrimination between circuits ,having a consumer unit protects your solar loads from affecting normal grid appliances, since this is only a small scale system and its more likely to trip this first before your house breaker. always better to be over kill and safe!

if i were to improve this system i would switch the mcb for rcbos since they offer the maximum protection for you and protection your equipment

STEP 6: Whats Next (The Future)

as always this is a working progress project and the next things i am looking to do are;

larger bank

More solar panels

More Ats

Larger inverter

STEP 7: Video Overview Coming Soon

video will be up in the next day or so

46 Comments

FadiM8 5 years ago

Great article. best ive seen on the topic yet. was wondering why you went for a rack mounted ATS rather than a 'normal' mechanical one since the price is vastly different?

andytechdude 5 years ago

Great project Kieren!

I have had a keen interest in the renewables subject for 20 odd years now so I take great interest in projects like yours. I actually have a property that is entirely off grid, so the fundamental economics differ from yours but the principles are the same (just without the grid). Ignoring the modified sine wave inverters (only good for running a power drill), I notice that the inverters used in many of these projects are of the high frequency radio shack variety. High frequency inverters form the output AC waveform in the electronics and pass the waveform out to the load. This is fine when it only needs to supply power to limited devices for a short period eg UPS. A far better (and robust) system is low frequency where the electronics form the waveform and present it to the primary of a large transformer, the load is then powered from the secondary (rough and ready description, google is your friend). In low frequency inverters the electronics are protected from inductive spikes etc behind the transformer, this makes for a far more robust inverter. The surge capability of the low frequency design tends to be an order of magnitude above the high frequency equivalent too. Now before anyone shouts out "but at what cost?" there are low frequency inverters from China that are almost comparable in cost to the high frequency types, the inverters to which I refer also double as potent battery chargers when they are fed from a generator (or grid). I use one of these for a stand alone system in a shed and it runs everything from computers to motors without issue, and handles the large inrush current of my saw bench etc. These inverters not only charge the batteries when an external AC source is available but they also have internal contactors to switch the incoming AC through to the output. An example is

https://www.ebay.com.au/itm/3000W-9000W-PEAK-24V-P...

We have installed one of these in a neighbors house and it runs the whole place, 3 years on it is still putting in good service. Economics wise. My neighbors system cost roughly $1000 in electronics, inverter, solar regulator, battery isolators, solar circuit breakers etc. A further $6500 was spent on an 1100Ah 24V battery bank and $2000 on solar panels. All in all it is a break even on grid power costs in Australia over the 10-15 year battery lifespan. There is more to this than economics of course. For those purely interested in economics I always say, start by turning off the lights when you leave a room.

Great stuff Kieren, all the best for you future as a sparky!

grendell 5 years ago

Hi there,

Not sure I agree with your comments about high frequency inverter only for supplying power to limited devices for a short periods. What about double conversion UPSs where the load is fed 100% of the time from the HF "inverter" and only the input source transfers from grid to battery?

G

Korishan 5 years ago

Good points. I agree. Also the AIMS power line is a good inverter as well. The Sigineer Power inverter is also good. It's basically the AIMS wrapped in another package. There are a few others, but I can't remember who they are.

andytechdude, check out HBPowerwall. He's near Brisbane, if I remember correctly. SecondLifeStorage.com is his forum and there's lots of us there working on lithium based battery storage. Smaller, lighter, more energy dense, and cheaper to maintain in the long run.

grendell 5 years ago

Good resource showing what items are available these days. One point on the ATS. I understand it isolates line and neural (hot and cold). On the inverter, is the neutral tied to the earth (ground)? I have done a bit of work with UPSs and had problems when switching back to grid power as the UPS did not have a neutral ground link. The problem is that a lot of kit has input filters which, in addition to inductors, have capacitors L-E and N-E. If the UPS is not referenced to ground both legs can float 120-0-120 (on a 240v system). No problem there at all. However, if you now switch back to the grid you have a load of N-E input capacitors which can be charged up which will discharge via the utility N-E link. The result is that standard RCDs (GFIs) can trip. If anyone has this issue, try using a time delay RCD to overcome this.

G

GeoR1 5 years ago

Hi , Great article on how to do a solar installation. Valuable.

Question. You have shown all your batteries wired in parallel in your circuit. Eventually one or more batteries will degrade before the others depending on age and condition. If you have all the batteries in parallel are you depending on that battery going high impedance when it goes low voltage and cant hold a charge ?.

How does one bad battery affect the rest of the good batteries , when it goes bad ?

Again, Thanks for your work. This is great.

Kieren-J 5 years ago

Hi

Since the batteries are wired in parallel they all get the same treatment of a load and all get the same charge, in my case the batteries have all had the same treatment and are the same age, so the likelihood is that they would all die around the same time (in theory), tho this is a long time off since I don't fully empty my bank. I only wait for it to get to 50% then the inverter will shut off.
When one or more do fail the normal sign that gives it away to me is that the banks overall voltage drops and if I isolate all the batteries the failed ones olwont be holding a charge, you'd also notice the failed one to have higher temperature that's my give away.

GeoR1 5 years ago

Kieren

Thanks for the extensive reply. So using temperature then, if you used a weather station remote monitoring wireless device

you could watch each of the temperature of the individual cells. For example, for $40 US you could remotely monitor 3 batteries. Just keep adding units to monitor more batteries. It wouldn't be too hard to pull the thermocouple out of the remote unit , wire an extension coax (for emi shielding) and stick or glue them to each of the batteries

https://www.amazon.com/Ambient-Weather-WS-21-8-Cha...

Thanks , I've learnt something.

tictag 5 years ago

Great project but, humbly, you might want to consider changing the title - this is not a grid tied system. In fact, this is specifically not grid-tied because you disconnect the grid using your ATS. A grid-tied inverter is where the voltage and frequency of the grid is used to control the voltage and frequency of the inverter output - your project does not do this.

For example, the Victron Energy MultiPlus inverter allows you to not only feed back into the grid but also combine grid supply with your inverted supply so perhaps your inverter is 1,000W, if a load is presented at 1,500W it takes 1,000W from your batteries and 500W from the grid to satisfy the demand. This is a grid-tied system.

Kieren-J 5 years ago

I disagree any system that is attached to a grid supply, is grid tied the difference is i am not exporting my power back to the grid (the whole Point of this) the victron multi plus is used to tie grid power into solar power and any excess it exports (which i don't want to do) it also charges battery bank with grid power when it is available (again i don't want to do this) As for the ATS it synchronises the two supplies for seamless cross over, There is no down time,an exactly similar product would be the switchover unit that victron makes.

tictag 5 years ago

The meaning of grid-tie is where the grid supply is synchronised (phase, voltage and frequency) to another source. Please check online references e.g. wikipedia "In order to inject electrical power efficiently and safely into the grid, grid-tie inverters must accurately match the voltage and phase of the grid sine wave AC waveform.". Your APC SU043 does not do this, please check online references e.g. your user manual on page 13 it states that "Both UPSs must be the same sine-wave model." Your project is a grid-connected UPS, I agree, but not grid-tied - these are two different things.

n4mwd 5 years ago

I was hoping to find a grid tied inverter that would automatically switch off instead of back feeding the grid when solar output was greater than power consumed. Around here, they don't allow grid tied inverters to back feed the grid without a special permit. The meters are now smart enough to detect it too. I was thinking about wiring something to the AC compressor (4KW) and just allow the inverter to work when the compressor is running. If the grid tied inverter didn't have such an input, maybe just a relay in series with the solar panels.

JohnC430 5 years ago

Unless you have a battery backed system connecting the compressor to the SP system may blow up your compressor motor. the inrush current is very high

Korishan 5 years ago

The ATS is that relay. When there is grid power, the ATS is on grid, when the grid goes down, the ATS switches over to solar. You can also make this more dependent on loads, too. So if you have 1kWh of solar, and your loads are light, then you could run on solar. When the load goes up and over loads the solar feed, it'd switch over to grid to handle the higher load until the load drops back down. But that takes using some smart programming/switching and such.

This system doesn't feed power pack into the grid. This isn't grid-tied. This is basically a large battery backup UPS. The ATS is required to make sure the inverter doesn't feed back to the grid and it disconnects all 3 wires (Hot, Hot, Neutral/Ground) from the grid.

VincentT57 5 years ago

I still didn't realize, what's the difference between your scheme and billions of others "energy savers"? Same principle: accumulate solar energy and use when no main power. Or use partly, whatever. What is principal invention here?

And sorry for my pessimistic view, but I think COST of all solar toys is MUCH more expensive than you "harvest" from mutable sun. Even two suns. :) Did you make any calcs WHEN your system will bring profit?

In SA (sunny enough country) we have solar panels for ~$200/355W. Obviously you need few of 'em (ideally covering a whole roof :) ) and it's just beginning! Batteries - 4-5 units is OK to not worry about powerless nite. In a simplest scheme - Controller + inverter. And if controller is not so complex device (good DIY idea), good inverter (of course sine wave! and powerful enough) will cost you so much that you will wait years until you start making "free" energy! It's not saying batteries will die in 4 years (even with proper usage) and solar panels also degrade after years. Hell, are you still sure you make "free energy"?? :)

JohnC430 5 years ago

The return on investment is approx 10 years.

Korishan 5 years ago

$200/355W panels is a pretty good price. That's $0.56/Watt, and about average pricing. A good inverter can be bought for about $1000, depending on power output needed. AIMS is a good inverter. There are lots of inverters that "look" like an AIMS with different color/badging. Internally it is the same and will run the same, just different distributer. A lot of inverters are done this way.

The Batteries should be deep cycle marine grade type batteries. They can last 5+ years if treated properly and only use 50% DOD. If using lithium batteries, then about 70% DOD can be achieved and life can practically be doubled and they don't cost that much more (even cheaper if you recycle cells; look at the 'diy powerwall' community for an example).

There will always be maintenance. Replacing the batteries at some point will be that. But in the end, the maintenance will always be cheaper than the grid.

Depending on where you live will depend on return on investment. I live in Florida, and I have one of the cheapest power in the state at $.13/kWh. I pay out about $120/month. If I build a system that costs $3000, then it'll take about 25 months to get my return; which is actually a really good return. Even if I had to do $5000 system, that's 42 months. Still not bad as long as I am producing enough power.

The energy is free, the work to collect it and transform it into something usable is not ;)

Korishan

secondlifestorage.com

ron342 5 years ago

I had wondered if, for a small solar or wind system, you could have your grid tied inverter feed just a single or maybe group of 2 house circuits which had relatively continuous loads like a fridge, or ac so those loads would soak up most of your generated power and then use simple blocking diodes in the grid feeds into those individual circuits so the inverter couldn't export??
The diodes wouldn't have to be huge since they would be passing maybe 20 amps each?

PhilTilson 5 years ago

Blocking diodes? In an AC circuit? Erm...

ron342 5 years ago

Oops, you're right, forgot about the negative swing!
Must be a simple way to limit an inexpensive grid tied inverter from back feeding the mains without shutting it down though?
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