Tulex
Solar Wizard
LOL, I keep clicking on this bold text, nothing happens.
Confusion on meaning of Off Grid vs Grid-tie vs Zero Export
- Thread starter jdufford
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hwy17
Anti-Solar Enthusiast
I don't think a lot of the marketing terms stand up to linguistic scrutiny. Hybrid seems ok, but it's ambiguous: Hybrid of what?
I would make two categories:
Grid tied or Grid Following - Can only operate with an external AC supply to support. Includes micros, regular string inverters, most 1500v solar and battery systems.
Grid forming or Grid Interactive - Capable of forming it's own independent AC output.
Almost everything we discuss on this forum firmly falls into the latter category. It's really the grid tied inverters that make a grey area when they do things like SMA sunny boy's emergency solar only output or enphase's solar only function.
"Off grid" inverters barely exist. Inverters called that are almost all grid interactive. Maybe Outback made some stationary install off grid inverters without grid interaction capability?
I would make two categories:
Grid tied or Grid Following - Can only operate with an external AC supply to support. Includes micros, regular string inverters, most 1500v solar and battery systems.
Grid forming or Grid Interactive - Capable of forming it's own independent AC output.
Almost everything we discuss on this forum firmly falls into the latter category. It's really the grid tied inverters that make a grey area when they do things like SMA sunny boy's emergency solar only output or enphase's solar only function.
"Off grid" inverters barely exist. Inverters called that are almost all grid interactive. Maybe Outback made some stationary install off grid inverters without grid interaction capability?
timselectric
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I try to stay away from the terms "grid forming" and "grid following". Because they already exist and are actually the opposite of what you would think. (Too confusing)
timselectric
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There are 3 categories.
"Grid-tied" , sends all production to its grid connection and relies on the grid to function.
"Off grid" , sends all production to battery, to be used emidiatly or in the future.
"Hybrid" , a combination of both above.
"Grid-tied" , sends all production to its grid connection and relies on the grid to function.
"Off grid" , sends all production to battery, to be used emidiatly or in the future.
"Hybrid" , a combination of both above.
hwy17
Anti-Solar Enthusiast
I have heard what you are saying but I'm not sure I'm convinced. Grid Forming in utility has been used to describe inverters that provide voltage support but cannot actually run standalone, which confuses things. But conceptually I think the terms are still weighted towards their linguistic meaning.
Grid Forming vs Grid Following ?
Grid forming and grid following inverters are both important technologies for integrating renewable energy sources into the power system.
energycentral.com
timselectric
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That's only one source for the definitions.I have heard what you are saying but I'm not sure I'm convinced. Grid Forming in utility has been used to describe inverters that provide voltage support but cannot actually run standalone, which confuses things. But conceptually I think the terms are still weighted towards their linguistic meaning.
Grid Forming vs Grid Following ?
Grid forming and grid following inverters are both important technologies for integrating renewable energy sources into the power system.
And I could almost go along with their interpretation.
But, it's not the majority.
So, I just avoid the terms.
Ampster
Renewable Energy Hobbyist
Those links do not show up on small screens of phones and perhaps some tablets.
MajicDiver
Solar Enthusiast
I don’t disagree with your statement, but the issue is that the various combinations and variations that fit the definition make the term useless in describing anything specific. Telling someone an inverter is a hybrid tells them nothing about the inverter.
timselectric
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Yup, just checked my phone. It doesn't.
I'm always on tablets, it does show on mine. Not sure about others.
MajicDiver
Solar Enthusiast
That didn’t occur to me. I don’t recall how I found his build thread last year, but it probably was on one of the rare times I was on a computer……
timselectric
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It tells them that it's both. A grid-tied inverter with battery backup.
wattmatters
Solar Wizard
That's understandable. Even the industry mixes terminology up.
I think there are two paths for you:
i. keep researching, think long and hard about what it is you are actually wanting to achieve, and then design and implement a solution to deliver on those goals, or
ii. just get started with something, anything, and learn by doing.
They are equally valid approaches.
i. makes sense if the big picture matters most, and is probably lower cost over the long haul. But it can also mean a much larger financial commitment up front and that is daunting, especially if you have doubts or perhaps unsure if the time frame for it to make sense is longer than you are comfortable with.
ii. is good in that you can just make something happen, gain some benefits and start with something lower cost. However starting small/low cost and expanding later inevitably means abandoning what you have built for something more suitable for the end goal / big picture.
ii. is also partly about enjoying the process of learning new stuff. That may or may not appeal.
As has been mentioned, one of the key factors to understand are the local regulations concerning connecting any generation source to the grid. Utilities will typically have pretty strict requirements for that and the costs of complying will vary enormously depending on where you are.
There are four basic options (but there are nuances and frankly it's almost a spectrum of options):
1. Grid-tied PV
The inverter is connected to the grid. Its AC power output is synchronised with and operates in parallel with the grid. This type of inverter will (typically) draw every last watt it can from the solar PV and that production is added to the grid's power supply.
How your home is deemed to consume energy from your PV system and the grid is then a function of the utility's metering, and that's a whole 'nother level of operational variance across regions/the world. With modern meters and an approved installation, the normal metering set up is your home first consumes your own production, and the grid manages the balance, be that importing any deficit or exporting any excess. Because the energy sources operate in parallel and one of them is the grid, you have little concern with being able to cover load demand.
But there are variations from this typical set up. One of those are where export limitations are applied by the utility. In these cases the inverter may be required to restrict or prevent export to the grid and the inverter must be capable of complying with these restrictions. Usually the inverter will have a means of monitoring the power flow to/from the grid and adjust output accordingly so as to remain within the regulatory limit.
Grid-tied inverters will need to be approved for use in your region/country and have all the necessary safety systems to ensure it can never energise the grid should the grid's supply go down. Most standard grid-tied PV inverters require a grid signal to operate, and without it they simply shut down. No grid, no PV and this is how they comply.
But understand they provide zero backup power in a grid outage. There are some exceptions to this but they are not common, examples are Enphase's promised battery-less backup and Fronius PV backup point (which is not available in the US) - but these operate with very limited capacity/capability.
2. Grid-tied Hybrid PV + battery
As per #1 above excepting these are hybrid systems operating with batteries. Such systems usually (but not always) have the capability to continue to operate when grid power is lost, so they need an approved method of completely isolating from the grid should there be a grid outage. Under no circumstances can they be allowed to energise grid power lines if the grid power is out.
There are multiple solutions like this. It is the most flexible option for a home since under normal operation they operate in parallel with the grid so no concern with supply capacity to meet load demands while they can continue to operate should the grid power supply be cut (but keep in mind the power supply is then limited to what the inverter/battery/PV can supply). Downside is they can be very costly and of course the local regulatory environment can make life hard/expensive.
The can work with or without solar PV, the idea being to charge the battery when power is cheap (be it from solar PV or during off-peak grid tariff periods) and discharge it to power the home when grid power is expensive. How financially sensible this is will depend on the tariffs in your region and the costs of installing the system.
Here in Australia grid-tied PV is a no brainer because install costs are low (and is why 1/3rd of all homes already have such a system) and they will easily cover their cost within a handful of years. But a hybrid system with battery (or even an AC coupled battery add on like a Tesla Powerwall) is generally a dud deal because batteries approved for grid connection are so damned expensive here. There is no DIY option for grid-tied anything here. The equations for each however vary greatly depending on where you are located.
3. Off-grid with grid support:
This is an off-grid inverter which can accept power input from the grid but it does not synchronise with nor operate in parallel with the grid. It has no capacity to export to the grid.
Most will require a battery to operate, and even the ones which can operate without one, really should still have some battery.
Most of these inverter types will offer the feature to pass grid power straight through to loads and some can blend the grid and PV so as to reduce the amount of energy required from the grid. If grid power is lost they will auto cut over to run from the battery and PV (if available). Like a big UPS as mentioned already.
They can also be set up to run primarily off-grid from PV and battery and treat the grid like a backup, e.g. when battery state of charge gets low and/or there is insufficient PV output to power the home and charge the battery.
The primary limitation is the loads you can support at any one time are limited to the output capacity of the inverter. In some cases only selected loads are connected to the inverter's output, while heavy loads may be left to be supplied by the grid only.
Another limitation is if the inverter fails, then you have no power for loads. In this case it is recommended to have a means by which the off-grid inverter can be bypassed and the grid (or an alternative power source) is able to power loads directly.
They are flexible in how they can be used and with their various functional capabilities are a pretty neat option. As a rule of thumb the regulatory requirements are less onerous since you are not connecting a generation source to the grid but of course all the normal electrical regulations/standards apply.
4. Off-grid with no grid support:
Pretty much same as #3 but without any grid power supply at all. PV and battery connected to an inverter supplying loads. Both the loads and the inverter are entirely independent of the grid and can only support loads up to the rated capacity of the inverter and battery. It is often desirable to have an alternative source of power should it be needed (e.g. a generator), typically used to keep batteries charged if solar PV cannot.
Where I am there is zero chance of grid-tied DIY unless I am working alongside a licensed electrician who also has their solar installation certification. It's a closed shop. So while that kinda sucks, at least professionally installed grid-tied PV installation here are cheap. However grid-tied battery systems are not cheap, they stupid expensive.
5. A Combination
Naturally you can have a combination of more than one of the above. Pick and choose which is suitable for your needs, capabilities, budget and local conditions.
I have both #1, a grid-tied PV system as well as #3, an off grid PV + battery system with grid support. The latter runs most of the general power supply to our two homes and shed, while heavy loads are supplied by the grid only. On weekends my utility provides a 2-hour free electricity period (noon to 2PM), so I will be ramping up the supplemental grid charging of my off-grid system battery, as well as charging up the EV. I was going to also switch the water heater onto full power but it has already finished heating from PV.
Note: Some small edits made for clarity.
Last edited:
timselectric
If I can do it, you can do it.
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Or possibly happened upon it in "show and tell".
MajicDiver
Solar Enthusiast
So, Is the backup output separate from grid tied output? Does it pass the grid thru it to loads or does it sync in parrell with the grid? It goes on and on and on….
Ampster
Renewable Energy Hobbyist
I do not think the term hybrid is intended to describe anything specific. All it tells us is that, as @timselectric said, it is a combination of off-grid and grid tie. One needs to read the specs to to understand the functionality and probably the manual to understand to programming options. I went from and Outback Skybox to a SolArk based on what I read here and what I learned from downloading and reading the manual. The biggest differences among inverters is their functionality. In addition the functionality continues to evolve. My first Radian did not even have AC coupling functionality.
timselectric
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Usually the answer is always yes.
Because it does it all.
Ampster
Renewable Energy Hobbyist
It depends on which output you are talking about. The output of a hybrid is typically to a critical loads panel. When it is exporting in grid tied mode, that output is to the main service panel. A grid tied inverter typically sends its output to the main service panel but could be wired to a sub panel.
To me that is the same thing. It has to sync to the grid because you cannot have two different AC frequencies in a home. Depending on whether it is importing then the grid passes through and mixes with the inverter output. When exporting the loads are covered first and then the excess is exported.
MajicDiver
Solar Enthusiast
Nope, it very different. One difference, if it passes thru, the inverter must be capable of passing the entire house load. If it fails, house loses power.
If it’s what I’m calling parallel, house remains powered regardless of what happens to inverter.
Of course there are virtually endless variations on this little piece of functionality.
timselectric
If I can do it, you can do it.
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Not necessarily. It just has to be capable of passing the loads connected.
If it's large enough, this could be the entire house. But that's just a difference of size, not functionality.
But it doesn't change what category it falls into.
In the simplest form.
Grid-tied can export.
Off grid can stand alone.
Hybrid can do both.
MajicDiver
Solar Enthusiast
Of course
