The most beginner of beginner questions.

[JakAHearts]

How exactly do hybrid solar power inverters function in the context of being connected to the grid, and what role do they play in terms of grid integration? I hope I'm phrasing this property so bear with me. Say I am making this system but perhaps on a smaller scale - https://www.mobile-solarpower.com/48v-complete-system-blueprint.html. If I connect grid power to the inverters how does this affect the sub panel power? Do the built in MPPT have logic that uses the solar to charges batteries/run load when solar is available but then switches to grid if and when needed? Ive read a good bit and watched even more YouTube videos but oddly have never seen anyone address the exact specifics of how this works.

For reference, Im planning on building a setup similar to above and gradually adding panels/moving loads from the main panel to the solar connected sub-panel over time. Would what I am proposing work and the only extra grid load would be to run the inverters when the batteries are depleted and no solar is available? Or am I completely off base and out of my mind? (wife says yes. That part is true)

 

[zanydroid]

There are multiple models of operation. Also it’s in large part a function of the inverter stage, rather than the MPPT. In short not all inverters are created equal

In the cheapest / simplest one, the load is carried by either the inverter, or the grid, but not both at the same time. And then after that is grid supporting inverters that can use a combination of grid and inverter power

On a related note most cheap inverters cannot AC charge from utility at the same time that they are inverting due to the same hardware being used for both purposes, and not designed in a rapidly reversible way. There are some power topologies that are intrinsically reversible. So again you need to understand the details of the specific unit you want to use.

Generally speaking this kind of detail is hard to convey in an edutainment or review video, you’d need like a lecture video

A grid supporting inverter is almost certainly going to require an interconnection agreement to be legal in the US.

As for priority of charging vs loads. This is configurable and the capabilities/terminology will vary by hybrid. Look up SBU, SUB, etc mode. Those letters stand for Solar Battery Utility.

 

[JakAHearts]

Thanks, Zany! That all makes a lot of sense. So just to clear up something, an expensive inverter with appropriate features and that that DOESNT back feed the grid but is fed from the grid is what Im looking for, right? I know I saw someone else in another thread talking about inverters that were specifically made to never feed the grid. Or is it better advice to just appropriately size your panels and batteries to just be off grid?

 

[JakAHearts]

Also, amazingly, the super cheap and simple interconnection agreement in my state expires tomorrow. Ha!

 

[zanydroid]

Thanks, Zany! That all makes a lot of sense. So just to clear up something, an expensive inverter with appropriate features and that that DOESNT back feed the grid but is fed from the grid is what Im looking for, right? I know I saw someone else in another thread talking about inverters that were specifically made to never feed the grid. Or is it better advice to just appropriately size your panels and batteries to just be off grid?

Not clear on your goals. What level of code compliance / permitting are you going for.

Backfeed is pretty imprecise. People say that to mean anything from not exporting to grid but still having the grid available to serve loads, to being isolated from the grid.

If you have some electrical background, a key concept is parallel operation - AC inverter power output and grid power both connected to the same set of circuits. That is how grid tie inverters with no off-grid capability operate 100% of the time.

Off-grid AIOs with SBU typically will be isolated from the grid when running on solar or battery. This is the safest in terms of being legal (in the sense of what the power company cares about) with cheaper equipment. It may not be legal in terms of what the building code enforcement cares about. Usually isn't because code compliant things with batteries (aka UL9540 certified inverter/battery pair) are expensive.

The 6500EX linked in the plans set is completely off grid. It's also only code-legal in states that have not yet adopted the UL9540 requirement.

Hybrid (in the ideal definition, not all things labeled as hybrids match this) as a baseline when operating on grid will have the possibility of sending power into the grid. If you happen to have loads at your house (either critical loads subpanel or on the "AC-In") of the inverter, then those will draw this power being sent into the grid. Somewhat related to this - just because something is labeled AC-in on a hybrid, doesn't mean it's input-only. Quite often it's a bidirectional connection wrt the direction power can flow. And AC-out is also sometimes bidirectional.

And just because a hybrid says SBU or has a feature called SBU doesn't mean it's always this power topology. There's no legal mandate for any of this terminology to be consistent except when it comes to which certifications a particular piece of equipment has.

In terms of expensive inverter... if you buy an expensive inverter and its matching UL9540 certified battery, then you will have no code nor grid legality issues provided you file the interconnection application.

 

[JakAHearts]

Thats great info. Thank you. Im in South Carolina so typically our codes/laws aren't as overbearing as other states but I'll have to check. Appreciate the long and thorough responses. Back to more research.