Inverter line side tap to main panel, how to zero the grid usage?

[JoeX]

I am trying to do my first DIY solar system. 12 Kw panel, a string hybrid Inverter (Sol-Ark12k) and battery (30 kWh). Simple.

My requirement is normal: Single family house, Gird-tie system. 200A main panel. During the day time, use the electricity from solar panel, if there is more electricity than I need, it will be used to charge battery, if the power from solar panel is not enough, then pull more Amp from gird. In the night, use the electricity from battery. If the battery is low or not enough power, then pull something from gird. This will zero the gird feeding back and save me the most at electricity bill.

I only have one main 200A panel. I don’t plan to install a new critical subpanel, so I plan to do a Line Side Tap, which will connect the inverter’s AC output directly to the main service line after meter, but before the main panel’s breaker.



If I understand everything correctly and config my inverter correctly, in the day time, the solar will feed my main 200A panel as much as it can if needed, and use the rest of electricity from solar to charge the battery. If my main panel is using more than the solar can provide (12Kw), it will pull more from gird.


Now, my question is: Since I have two sources to feet the main panel: gird and output of Inverter and they are connected together (by Line Side Tap), how does my main panel know which electricity source to use first? In another word, how does my panel know to use the Amp from inventor first, but not using the Amp from gird? And if the Amp from inventor is not enough, then pull more Amp from gird? It's the same 110V/220V voltage both on gird or inverter AC output. The Amp might be different, but that seems not the reason for main panel to choose.

I know this might be related to that CT sensor, since I read a word in the Sol Ark manual that:
"CT sensors installed between the grid meter and the main service panel to measure the amount of current the Sol-Ark system is pulling or pushing to or from the grid. The Sol-Ark inverters use the reading from the CT sensors to calculate how much power to push to the main service panel in order to zero the grid usage".

It sounds like inverter is trying to push more Amp to the main panel to force these loads to use all power from inverter, but gird.
I never doubt they will be able to do this. But I just trying to understand the logic behand this. Since if the voltage is the same, just simply push more Amp to the main panel will not result the panel to use more Amp from inverter.
Do I understand this correctly? Or do I miss any other important part in this system?

 

[timselectric]

The CT's measure how much power is coming from, or going out to the grid.
The inverter tries to push just enough power to keep the measurement at zero.

 

[DIYrich]

If you connect the main panel to the load connection on the sol-ark, there is an internal CT that measures the consumption on the load line. No external CT required.

Voltage is not the same. If the inverter voltage is slightly higher than the grid, the inverter pushes power onto the load line.

 

[ULR]

Example numbers for 18K-PV:

In grid assist mode:
- The bypass relay (from the AIO's line input to its output) is closed. Line power (up to 200A) can go from the AC INPUT (connected to the POCO's supply line through the main disconnect) to the AC OUTPUT (and thus to the {all loads or priority loads} panel), or from the AC OUTPUT to the AC INPUT.
- The (main) inverters (one for each side of the 240V split-phase) feed up to 12 kW continuous (more for short peaks) from the DC bus (batteries and solar input via the MPPT controllers) to the AC OUTPUT terminals. (The inverter frequency and phase are synchronized with the voltage at the AC INPUT, so the inverters and line can combine to feed up to 250A total {continuous} to the panel attached to the AC OUTPUT.)
- There are current transformers on the lines from AC INPUT to the POCO line disconnect. These sense how much power is coming from or going to the POCO. The AIO uses this signal to turn down the output current from the (main) inverters, so no power, or no more than the desired amount of power, is fed to the POCO. (The panel doesn't "know" squat about where the power comes from. All that is controlled by this mechanism in the AIO.)
- So (presuming you're set to "no export") the inverters provide as much power as your loads are taking, up to the limit of the inverter outputs, and if that's too little the POCO provides the rest. If you're set to some amount of export, the inverter output is turned up further until it's powering your loads plus the export - again up to the limit of the inverter outputs. If they fall short, first the amount of export drops, and the POCO again provides the rest of the needed power if the loads are taking more that all the inverters can produce.
- If the line power fails (detected by the frequency drifting too far off the line power's official frequency or some other problem with power quality), the AIO opens the bypass relay and takes over control of the frequency, effectively going into stand-alone mode. The inverters power the AC OUTPUT terminals and nothing from the inverter powers the AC INPUT terminals but the POCO. If the POCO power comes back, after it's stabilized (and has been for several minutes), the AIO adjusts the frequency and phase of the inverter outputs to match the line, then closes the bypass relay and goes back into grid assist mode.

- On the 18K-PV and other external-current-transformer designs, you can hang a non-critical-loads panel on the lines between the POCO disconnect and the AC INPUT terminals. This panel will lose power if the grid dies (or the POCO feed disconnect is opened). If you put it on the AIO side of the current transformers, the inverters will (try to) power it when in grid-assist mode. If you put it on the POCO side of the current transformers the inverters will NOT try to power it, so its load will end up on your power bill. (Unfortunately, as DIYrich poined out, the Sol-Arc has the current transformers inside. So loads on a non-critical panel on the AC INPUT side will appear on your power bill, or come out of your "sell" power credits, rather than being powered by your solar setup.)

 

[JoeX]

Thanks all you guy's explanation.
I think I kinda understand that.

But I just noticed I missed an important thing:
If I understand everything correctly, if I line side tap the inverter AC output to my main panel, I wouldn't be able to use any electricity if the gird is down. Is that right?
Since there is not mechanism to automatically disconnect the main service wire from gird to main panel. If the inverter keeps sending Amp to the AC output when gird is down, the electricity will end up go to the girs and this is illegal.

Is my only choice now to install a sub panel?

 

[Bkord]

Thanks all you guy's explanation.
I think I kinda understand that.

But I just noticed I missed an important thing:
If I understand everything correctly, if I line side tap the inverter AC output to my main panel, I wouldn't be able to use any electricity if the gird is down. Is that right?
Since there is not mechanism to automatically disconnect the main service wire from gird to main panel. If the inverter keeps sending Amp to the AC output when gird is down, the electricity will end up go to the girs and this is illegal.

Is my only choice now to install a sub panel?

The inverter load can not be tied to the grid. You need a sub panel or run the main panel from the inverter load with no grid connection to the main. The inverter can back feed the main panel from the input but not from the output. Sol ark has wiring diagrams for different set ups. I have a 12k sol ark and my electric bill is $0.

 

[Koyaanisqatsi]

If you must use a line-side tap, you should also install a breaker between the meter and the tap. I always recommend to NOT use a line-side tap unless there are no other good alternatives, for two big reasons. 1. they are extremely dangerous to install unless you have your utility cut power to your house at the pole or wherever your feed comes from while you install it and the related connections. And 2. they increase the danger of overloading your utility feed because the tap usually comes before the main panel's breaker. Look at it from a worst-case scenario, and one can see how the combined loads of the main panel and the inverter in a failure situation could draw enough to blow the meter or melt main feed wires.

In your case, it might be justified. You will need to install a physical lock-out device between the input breakers on the main panel feed and the input breakers of the the load output from your inverter. Or put a transfer switch that chooses between grid (bypass) and inverter output going into your main panel feed breakers. It's likely your utility or local authority will require something specific here.

But my suggestion is to install a small-but-mighty distribution panel between the meter and main panel. This would give you the option of grid-only outlets, for example. But critically, it would give you the breakers necessary to feed the inverter's grid input and also the "bypass" line from this new panel to the main panel. And retain a SINGLE main breaker between the grid and all loads. You still would need the lock-out or transfer switch I mentioned above. This just gives you a lot more flexibility for utilizing grid power when needed. Think of things like a welder or large power tools, or something like a pool or hot tub, which you would not want your inverter to have to support.

Here's a chunk of my diagram, which I am still refining. But I'm going after a similar setup. In my case, I'm calling the distribution panel my main panel, and my original main panel is now called the load center. I don't know whether Calif. allows for lock-outs like this, or if I'll have to use a transfer switch instead.

 

[JoeX]

If you must use a line-side tap, you should also install a breaker between the meter and the tap. I always recommend to NOT use a line-side tap unless there are no other good alternatives, for two big reasons. 1. they are extremely dangerous to install unless you have your utility cut power to your house at the pole or wherever your feed comes from while you install it and the related connections. And 2. they increase the danger of overloading your utility feed because the tap usually comes before the main panel's breaker. Look at it from a worst-case scenario, and one can see how the combined loads of the main panel and the inverter in a failure situation could draw enough to blow the meter or melt main feed wires.

In your case, it might be justified. You will need to install a physical lock-out device between the input breakers on the main panel feed and the input breakers of the the load output from your inverter. Or put a transfer switch that chooses between grid (bypass) and inverter output going into your main panel feed breakers. It's likely your utility or local authority will require something specific here.

But my suggestion is to install a small-but-mighty distribution panel between the meter and main panel. This would give you the option of grid-only outlets, for example. But critically, it would give you the breakers necessary to feed the inverter's grid input and also the "bypass" line from this new panel to the main panel. And retain a SINGLE main breaker between the grid and all loads. You still would need the lock-out or transfer switch I mentioned above. This just gives you a lot more flexibility for utilizing grid power when needed. Think of things like a welder or large power tools, or something like a pool or hot tub, which you would not want your inverter to have to support.

Here's a chunk of my diagram, which I am still refining. But I'm going after a similar setup. In my case, I'm calling the distribution panel my main panel, and my original main panel is now called the load center. I don't know whether Calif. allows for lock-outs like this, or if I'll have to use a transfer switch instead.

View attachment 250305

I agree with you that line side tap is not a DIY friendly way to connection due to the fact. Many people still choose line side tap is because the the back feed using breaker to the main panel is usually caped at 40A breaker in a 200A panel with 200A Main breaker, unless I replace that 200A main breaker.

It looks like it's inevitable for me to install a critical load subpanel and by doing that it make the whole thing much easier. I will just go ahead to do that way.

 

[wheisenburg]

You can install a new "Grid Panel". Power that from the grid. Then use the power from the "Grid Panel" to power your inverter. The inverters will power the your existing "Main Panel". It is much easier to move a few large loads from your "Main Panel" back to the "Grid Panel" than to move a whole bunch of stuff from your existing "Main Panel" unto a new "Critical Loads" panel. No the 4 watt LED light in the 3rd bedroom closet probably isn't that critical, but powering it isn't a problem either. An electric water heater, a wall oven, and a swimming pool pump are the kind of loads you might what to move to the "Grid Panel".

You just move the breaker to the "Grid Panel", Run your hot wires from there back to the "Main Panel". Wire nut them to the wires you removed from the breaker. If you have a GFI type breaker, the neural will also need to be routed to the the "Grid Panel".

A good move here would be to get an electrician used to working with the power company to install a 200 amp breaker between the meter and the "Main Panel" as your first step. Once this is done you can turn the breaker off and work on anything down stream with the power off.