What is an off-grid inverter?

[cajocars]

What is an off-grid inverter?

For regulatory reasons, I need an inverter that cannot export energy to the grid

I think I have a general idea of what an off-grid inverter does, but correct me if I’m wrong: it’s connected in series between the grid and all of my loads (it has an AC in and an AC out)

Suppose there are no batteries involved i this system: when my solar production is not sufficient to power all my loads, can the off-grid inverter draw only the difference from the grid or does it stop drawing from the solar panels and takes everything from the grid?

 

[JCSchwarb]

What is an off-grid inverter?

For regulatory reasons, I need an inverter that cannot export energy to the grid

I think I have a general idea of what an off-grid inverter does, but correct me if I’m wrong: it’s connected in series between the grid and all of my loads (it has an AC in and an AC out)

Suppose there are no batteries involved i this system: when my solar production is not sufficient to power all my loads, can the off-grid inverter draw only the difference from the grid or does it stop drawing from the solar panels and takes everything from the grid?

You are spot on. Think of it as adding an uninterruptible power supply ((UPS). The UPS is composed of batteries, and AC/DC inverter/rectifier, Solar PV input, and battery charger. The AC input is called upon when a set point is reached. Note there are many settings to configure for your specific use needs. The AC output from the inverter(s) can come directly from the AC input (bypass, pass through mode - like when batteries are below a usable range) or converted from battery and/or solar. My inverter priority is set to Solar, then battery, then utility (SBU).

In your case, without batteries, the UPS offgrid concept would be severely diminished. If your loads cannot be satisfied by solar, then the unit would switch to bypass mode. I am not aware of the inverter mixing a synchronized solar DC input and AC input at the same time. Bypass can allow Solar to charge batteries and AC input to power loads directly. This is my general understanding without seeing a theory of operations on a given inverter. You could certainly start with a system without batteries and add later if you are budget constrained. Consolidation of heavy loads when the sun is out is one way to make use of such an installation.

Hope this helps.

Best,
-Jay

 

[cajocars]

So it doesn’t make sense to have an off-grid inverter without batteries, as it would always be in bypass mode, right?

 

[JCSchwarb]

So it doesn’t make sense to have an off-grid inverter without batteries, as it would always be in bypass mode, right?

System would be in bypass mode, this time of year about 18 hours, depending on 1) where you live and other environmental conditions like trees/shade/shadows due to low sun angles, 2) PV positioning, 3) size of PV array, and 4) load demand.

So you could squeeze about 6 hours in FL or similar climate, using Solar PV, but I am not sure how the inverters will transition when you have demand spike’s exceeding the current PV supply. I would think the pulse shock would be very disruptive to your electronics. In a typical setup, when the battery hits 45.9V, bypass is synchronized and initiated so we see a quick flicker/pulse and then goes steady again as the bypass initiates. Same happens in reverse, my battery cut in is set to 48V and bypass transitions back to solar mode. Recommend reviewing the inverter manufacturer instructions on this mode of operation to see if supportable or feasible.

 

[OffGridInTheCity]

Agree with discussion above but.....

An off-grid inverter can also be just an inverter - no AC input, converting battery DC -> AC. The Battery is charged by solar or generator. The definition of an 'off-grid-inverter' in my mind is an inverter that does not connect to the grid / send power to the power company.

This is how my home solar system is setup. Under this approach, the inverter power is fed to ATSs so that when there is solar power to charge the batteries, the inverter turns on providing power to my home circuits. When the solar ends / battery runs down the inverter shuts off and the ATSs automatically switch back to grid. The use of ATSs requires UPSs for sensitive circuits.

These days, there are good options for 'off-grid' inverters that combine several functions (MPPT charge controllers, ATS/UPS, battery charging) but not all inverters have these functions.

 

[JCSchwarb]

Agree with discussion above but.....

An off-grid inverter can also be just an inverter - no AC input, converting battery DC -> AC. The Battery is charged by solar or generator. This is how my home solar system is setup. Under this approach, the inverter power is fed to ATSs so that when there is solar power to charge the batteries, the inverter turns on providing power to my home circuits. When the solar ends / battery runs down the inverter shuts off and the ATSs automatically switch back to grid. The use of ATSs requires UPSs for sensitive circuits.

These days, there are good options for 'off-grid' inverters that combine several functions (MPPT charge controllers, ATS/UPS, battery charging) but not all inverters have these functions.

The main question was offgrid system without batteries.

 

[OffGridInTheCity]

The main question was offgrid system without batteries.

The question was about the definition of an offgrid inverter. The key point is that an offgrid inverter is one that does not connect (tie) to the grid.

Do you know of a actual offgrid inverter (no grid tie) that doesn't use a battery? I'm trying to think of one. For example, there used to be a Sunny Boy? that could provide direct Panel -> 120v AC output up to 2000w without a battery but it was grid-tied.

To the OP's question - I believe a battery is always required for offgrid. If it's a really small battery, then that's OK - it just means that when the solar power is no longer enough, the inverter will shut off almost immediately.

 

[Steve_S]

The Samlex EVO Inverters are a good example of an Off-Grid Inverter.
They come with 2 AC Inputs, one for Grid In and one for GenSet AC Input but they cannot export AC power to Grid. These are fully programmable as to their priorities. Grid AC & Genset AC are used for Charging Battery systems when they reach Low Voltage threshold. The Samlex have built-in ATS Capabilities for both sides as well as an AGS (Auto-Gen-Start) relay to enable the use of generator start/stop modules. These inverters have built-in AC Bypass so that the Inverter can continue to power your loads even while charging from either Grid or GenSet, they can also power loads from Battery + AC input if required - all programmable.

Offgrid Inverters are typically Battery Backed (primary energy source) and will use either Grid or GenSet AC for "Charging batteries only" and have no ability to export AC to grid. The "Batteryless" type systems usually are MicroInverter based and only provide power when the sun is shining and have no fallback capabilities. Micro-Inverters convert the DC to AC. Quite a different beast with a few different possible configurations.

Hope it helps, Good Luck.

 

[cajocars]

I am not really sure I understand off grid inverters yet

I know that with grid tie inverters every shortfall in solar production is compensated by the battery (if not empty) or by the grid

If I have an off grid inverter and battery do I basically need to use the battery to compensate what the panels aren’t producing ?
And then when the battery is empty it starts to supply all my loads from the grid and only use solar to charge the battery?

 

[SparkyJJO]

I am not really sure I understand off grid inverters yet

I know that with grid tie inverters every shortfall in solar production is compensated by the battery (if not empty) or by the grid

If I have an off grid inverter and battery do I basically need to use the battery to compensate what the panels aren’t producing ?
And then when the battery is empty it starts to supply all my loads from the grid and only use solar to charge the battery?

The answer is it depends.

It depends on the capabilities and configuration of the inverter.

Decide what you want the inverter to do, then find one that has those functions.

 

[cajocars]

The answer is it depends.

It depends on the capabilities and configuration of the inverter.

Decide what you want the inverter to do, then find one that has those functions.

I want an inverter that is able to:

- if pv production is less than demand and battery IS empty: draw power from the grid, but keep using whatever the pv is able to produce (even if very little)

- if pv production is less than demand and battery is NOT empty: draw power from the battery, but keep using whatever the pv is able to produce (even if very little)

- if pv production is more than demand and battery is NOT full, use any excess to charge the battery

- if pv production is more than demand and battery IS full, modulate pv production to match demand

 

[OffGridInTheCity]

I want an inverter that is able to:

Let's take a typical all-in-one (AIO) off-grid (does not feed excess to grid) unit such as an MPP Solar..... https://usamppsolar.com/product/pip-3048lv-mk/ for the context to answer you're questions below.

This unit (and other AIOs) have AC input (optional) and PV input (optional) and Battery (required) input and provide AC output.

It's like this: AC Input / PV Input -> Battery -> Inverter -> AC Output. The Input will never exceed the Output once the battery is full.

- if pv production is less than demand and battery IS empty: draw power from the grid, but keep using whatever the pv is able to produce (even if very little)

Yes - PV + Grid inputs will charge the battery enough to keep the inverter running for all the loads. You can typically prioritize Grid input over PV input and vice versa.

Note: The battery can never by 100% empty or the unit will turn off.

- if pv production is less than demand and battery is NOT empty: draw power from the battery, but keep using whatever the pv is able to produce (even if very little)

Yes.

- if pv production is more than demand and battery is NOT full, use any excess to charge the battery

Yes.

- if pv production is more than demand and battery IS full, modulate pv production to match demand

Yes .

While I understand the thought experiment of bypassing (not needing) a battery....
This flow: AC Input / PV Input -> Inverter -> AC Output
instead of this: AC Input / PV Input -> Battery -> Inverter -> AC Output.

Its not practical for off-grid....
If you have AC then why bother going thru an inverter - just use the AC directly and avoid efficiency losses.
If you have PV then you need a battery as a buffer as the voltage is just up/down + here/there. A single cloud can momentarily wildly change PV power where an inverter needs steady / reliable DC voltage minute to minute.

BUT if you want to do grid-tie (instead of off-grid) then equipment such as SolArk that can be connected/synced with the grid and has many options - it's a different level of equipment and I don't have enough background to say what it can do or not do.

 

[Hedges]

I am not really sure I understand off grid inverters yet

I know that with grid tie inverters every shortfall in solar production is compensated by the battery (if not empty) or by the grid

"Grid Tie Inverter" often means one that only feeds power from PV into the grid, for net metering, only when the grid is up.
Sunny Boy as mentioned above offers "secure power" 120V 2000W with grid down. One contractor demonstrated it starting and running a 3 HP Milwaukee grinder.

Some are hybrids and have battery, can act as UPS when grid as down and also recharge from PV.
Some of them can do batteryless backup, power loads direct from PV when grid is down. Obviously, PV production must remain high enough.
Having a small battery could make them perform much better, riding through brief cloudy times and providing starting surge for motors.

If I have an off grid inverter and battery do I basically need to use the battery to compensate what the panels aren’t producing ?
And then when the battery is empty it starts to supply all my loads from the grid and only use solar to charge the battery?

Various models have different priorities, and abilities to blend multiple sources.
Some (grid interactive) can blend AC power from the grid with inverter generated AC from batteries and PV.
Some can charge battery from the grid but only when not passing grid to loads (!?)
Some have a battery charger to supplement with power from grid while also using PV.
May be able to charge/discharge at certain times due to time of use rates. Charge batteries in advance when weather report says storm coming, etc.

One good application of off-grid without batteries is water pumping, variable speed to match PV.

If you define the function and performance/power/capacity you need, then you can select a scheme and equipment to meet the requirement.
Separating some loads like always-on 12V security and communication gear, also power tools only operated in sunshine, can result in a more cost effective system than one which mimics grid power without user behavior adjustment.

 

[cajocars]

Let's take a typical all-in-one (AIO) off-grid (does not feed excess to grid) unit such as an MPP Solar..... https://usamppsolar.com/product/pip-3048lv-mk/ for the context to answer you're questions below.

This unit (and other AIOs) have AC input (optional) and PV input (optional) and Battery (required) input and provide AC output.

It's like this: AC Input / PV Input -> Battery -> Inverter -> AC Output. The Input will never exceed the Output once the battery is full.



Yes - PV + Grid inputs will charge the battery enough to keep the inverter running for all the loads. You can typically prioritize Grid input over PV input and vice versa.

Note: The battery can never by 100% empty or the unit will turn off.


Yes.


Yes.


Yes .

While I understand the thought experiment of bypassing (not needing) a battery....
This flow: AC Input / PV Input -> Inverter -> AC Output
instead of this: AC Input / PV Input -> Battery -> Inverter -> AC Output.

Its not practical for off-grid....
If you have AC then why bother going thru an inverter - just use the AC directly and avoid efficiency losses.
If you have PV then you need a battery as a buffer as the voltage is just up/down + here/there. A single cloud can momentarily wildly change PV power where an inverter needs steady / reliable DC voltage minute to minute.

BUT if you want to do grid-tie (instead of off-grid) then equipment such as SolArk that can be connected/synced with the grid and has many options - it's a different level of equipment and I don't have enough background to say what it can do or not do.

If I understand you correctly, then in case the pv production is less than the demand and the battery is (almost) EMPTY, then PV and grid will be charging the battery, which at the same time will be discharged into the loads; is this right?

 

[OffGridInTheCity]

If I understand you correctly, then in case the pv production is less than the demand and the battery is (almost) EMPTY, then PV and grid will be charging the battery, which at the same time will be discharged into the loads; is this right?

Yes.

Think of the battery like a swimming pool of water and the inverter like a pump. You can pump out water at a steady rate until the pool runs dry. You can add water by rain (PV) or a garden hose (Grid) at a fast pace or slow pace - the pump doesn't care and will continue to pull out at a steady pace.

Adding power to a battery by grid or PV and pulling it out by inverter happen at the same time independently of each other. The battery is the buffer that let's the inverter operate with a steady, minute by minute, stream of DC power.

 

[JCSchwarb]

The question was about the definition of an offgrid inverter. The key point is that an offgrid inverter is one that does not connect (tie) to the grid.

Do you know of a actual offgrid inverter (no grid tie) that doesn't use a battery? I'm trying to think of one. For example, there used to be a Sunny Boy? that could provide direct Panel -> 120v AC output up to 2000w without a battery but it was grid-tied.

To the OP's question - I believe a battery is always required for offgrid. If it's a really small battery, then that's OK - it just means that when the solar power is no longer enough, the inverter will shut off almost immediately.

I did some research on the MPP / EG4 AIO inverter types and a “Line mode” is defined to allow solar and AC input to operate synchronously without a battery connection. See attached screen shot.

 

[cajocars]

Yes.

Think of the battery like a swimming pool of water and the inverter like a pump. You can pump out water at a steady rate until the pool runs dry. You can add water by rain (PV) or a garden hose (Grid) at a fast pace or slow pace - the pump doesn't care and will continue to pull out at a steady pace.

Adding power to a battery by grid or PV and pulling it out by inverter happen at the same time independently of each other. The battery is the buffer that let's the inverter operate with a steady, minute by minute, stream of DC power.

A consequence of this is that if I want to be able to power a 6kW load, then the battery will also need to be charged and discharged at 6kW+?

 

[Hedges]

Yes.

Unless PV delivers a large fraction of the load.
My system has relatively small AGM battery, can deliver high current for a short period. Excessive PV, so A/C and other load are powered during the day even while batteries are recharging.

If you install excessive PV, make sure battery charge rate is limited to what battery should receive. Some equipment can do that, while still delivering much more to inverter.

 

[cajocars]

Also, considering this
AC Input / PV Input -> Battery -> Inverter -> AC Output.
and the pool example

If there is no PV production do I lose efficiency in converting AC (from the grid) to DC and then to AC again (loads)?

 

[Hedges]

Every conversion has an efficiency loss.
Some parts are as low as 1% loss, others could be as high as 15%.
Cheaper inverters and lead-acid batteries might be 85% efficient under some conditions.

Inefficiency starting from grid may be less significant since it is an unlimited source.
From PV and battery you may care more. Although, PV can be 1/10th the price of grid these days.

Your architecture can be selected in part depending on what path most power flows through.
Is it PV to battery, later battery to AC?
Or is PV used primarily to generate AC directly, either backfeeding grid or feeding loads during the day?

If most of your consumption is grid feeding loads, then something with a relay connecting them would be most efficient.
You also want a way to keep loads operating if electronics fail. Grid will be more durable and has a crew maintaining it.

I would like a failsafe transfer that returns loads to grid if inverter fails. Presently, grid only powers loads if my inverter is functioning because it qualifies grid. I imagine a snap-action knife switch pulled down by a weight, with inverter powering something to keep it in the up position.
For now I have manually interlocked breakers to bypass inverters.