Stupid question about solar planning for house

I've been doing solar for a while now, so I should know this, but I find that I am stumped over what should be a trivial question.

Under normal operation, I want the solar panels to grid tie into the house wiring, send excess to the batteries, but be incapable of exporting. This way, solar is used for all loads including the dryer and water heater. When the grid is down, I want the solar and battery system to power a critical load subpanel, which would include the A/C system (17A run/ 40A start @ 230V [3910W/9200W surge]).

I have been looking at some of the mpp inverters - like the LVX6048 and the LV6548. These are all in one inverters that have MPPT and battery charging built in. I'm not sure if they can do zero export grid tie or not.

Any suggestions to get me back on track?

Do what I did and call the manufacturer and ask for tech support.
sure fire way to get an answer, and if it's not the answer you want,
maybe they can suggest somebody's unit that will do what you need done.

Unless I am mistaken the units you selected are not Grid Tie models.

Thanks, I did contact the Skybox people and they said the Skybox wouldn't do what I want. I may contact the MPP people and see if there is a solution there. The MPP inverters seem to do a lot for the price, but do they deliver the watts they claim and last more than one season of continuous use?

I was thinking about using a separate grid tie inverter and a separate backup system like the LVX6048. But then how exactly do I share solar panels? When the grid is up, the 6048 can simply charge off the AC (with huge losses). If the grid is down, the grid tie inverter shuts off, but the backup inverter still needs solar to keep things charged and running.

Basically, I want a system that runs off mains AC most of the time and augmented by solar, so that the batteries aren't cycled too much.

Then to complicate things, there are some grid tie inverters that are fake split phase. In the US, we use split phase which has two 120V legs and a neutral. Any grid tie inverter that relies on the transformer on the pole to balance the load would be unacceptable. These bad inverters are using single phase inverters instead of the more costly dual inverters. I'm pretty sure that this would cause self consumed current to flow out and back in from the meter which is unacceptable.

Northernchateau said:

Unless I am mistaken the units you selected are not Grid Tie models.

Click to expand...

The LVX6048 is supposed to be grid interactive, support 450V strings and is a single unit split phase, but the LV6548 doesn't seem to be grid tied and requires a second box for split phase. Very little data for either of them unfortunately.

T

n4mwd said:

Thanks, I did contact the Skybox people and they said the Skybox wouldn't do what I want. I may contact the MPP people and see if there is a solution there. The MPP inverters seem to do a lot for the price, but do they deliver the watts they claim and last more than one season of continuous use?

I was thinking about using a separate grid tie inverter and a separate backup system like the LVX6048. But then how exactly do I share solar panels? When the grid is up, the 6048 can simply charge off the AC (with huge losses). If the grid is down, the grid tie inverter shuts off, but the backup inverter still needs solar to keep things charged and running.

Basically, I want a system that runs off mains AC most of the time and augmented by solar, so that the batteries aren't cycled too much.

Then to complicate things, there are some grid tie inverters that are fake split phase. In the US, we use split phase which has two 120V legs and a neutral. Any grid tie inverter that relies on the transformer on the pole to balance the load would be unacceptable. These bad inverters are using single phase inverters instead of the more costly dual inverters. I'm pretty sure that this would cause self consumed current to flow out and back in from the meter which is unacceptable.

Click to expand...

The solution is two Circuit breaker boxes. One of fed from the Grid the other is fed from your All in One box.

its not a simple question because very very often the "rules" about grid tie are mostly concerned with NEVER allowing power to flow if the grid goes down. This is a major safety concern for power line repair teams..and rightly so.

If you want to allow a grid tie system to run without the grid, well..a few things to consider:
1)make sure you have grid tie inverters that will operate WITHOUT the grid (some inverters if they do not sense the 60hz grid they will not work)
2)check all your local regulations as you will definitely want a properly designed system of interlocks that guarantee you will not re-energize the grid if it goes down
3) this is a potentially lethal situation not for you as much as some line repair person, have a professional handle this...

Northernchateau said:

T

The solution is two Circuit breaker boxes. One of fed from the Grid the other is fed from your All in One box.

Click to expand...

I assume you mean a sub panel for critical loads.

Anyhting grid tied would have to support zero export mode. I presume this means two sensors at the panel to detect current flow. If only the sub panel is grid tied, then the inverter itself could potentially monitor for any backwards current flow. The only problem there is that it can't feed the non-critical loads without sensors at the main panel.

Also, with the LVX6048 or LV6548, it isn't officially known what their surge rating is. I think there are rumors that they will surge to 2x the rating, but nothing official last I checked.

n4mwd said:

I assume you mean a sub panel for critical loads.

Anyhting grid tied would have to support zero export mode. I presume this means two sensors at the panel to detect current flow. If only the sub panel is grid tied, then the inverter itself could potentially monitor for any backwards current flow. The only problem there is that it can't feed the non-critical loads without sensors at the main panel.

Also, with the LVX6048 or LV6548, it isn't officially known what their surge rating is. I think there are rumors that they will surge to 2x the rating, but nothing official last I checked.

Click to expand...

Your over thinking things.
Your all in one box is connected to the grid for input power ONLY, never exporting to the grid.
Critical loads are connected to your All in one box ONLY. It will draw from the grid if the panels are not producing enough thru your all in one or the batteries of the grid goes down.

Diysolar123 said:

its not a simple question because very very often the "rules" about grid tie are mostly concerned with NEVER allowing power to flow if the grid goes down. This is a major safety concern for power line repair teams..and rightly so.

If you want to allow a grid tie system to run without the grid, well..a few things to consider:
1)make sure you have grid tie inverters that will operate WITHOUT the grid (some inverters if they do not sense the 60hz grid they will not work)
2)check all your local regulations as you will definitely want a properly designed system of interlocks that guarantee you will not re-energize the grid if it goes down
3) this is a potentially lethal situation not for you as much as some line repair person, have a professional handle this...

Click to expand...

No, if the grid goes down, only the sub panel gets energized from batteries and solar. The rest of the house goes without.

The goal is to have a grid tied system most of the time that will never export. Then if the grid goes down,automatically switch the solar panels to an all in one inverter that will energize only the critical loads panel.

Y&H 1000W Grid Tie Inverter Power Limiter DC25-60V Solar Input AC110V/240V Auto Switch​

https://www.amazon.com/Inverter-Limiter-DC25-60V-AC110V-Switch/dp/B07GC53QBD/ref=sr_1_1_sspa

Something like this?

Northernchateau said:

Your over thinking things.
Your all in one box is connected to the grid for input power ONLY, never exporting to the grid.
Critical loads are connected to your All in one box ONLY. It will draw from the grid if the panels are not producing enough thru your all in one or the batteries of the grid goes down.

Click to expand...

The LVX6048 specs claim that it can export. https://www.mppsolar.com/v3/lvx6048/

It says that "Grid Injection may be enabled". But the specs are inadequate at saying whether it can be regulated so that it doesn't export out of the main panel. For that matter, its not even clear if its injecting only down stream or both directions.

I'm thinking that it might be possible to have some sort of supervisor circuit that could monitor the current flow at the main panel and then disconnect the solar panels if it tries to export.

n4mwd said:

The goal is to have a grid tied system most of the time that will never export.

Click to expand...

So your statement above is not the same as in your first post:
" I want the solar panels to grid tie into the house wiring"

If you have your inverter tied to the same wiring as your normal house wiring that is tied to the grid, well, you are MOST DEFINITELY going to export power hehe.

if you are powering a seperate subpanel only that is NOT tied to the regular house wiring, well...
ahhh, thats not a grid tie system then hehe
You are just building a standard solar panel/inverter/charger system that accepts an input ac voltage and DC solar panel voltage into a completely seperate circuit...get any all-in-one and you are good to go, its automatic

OnTheRoadAgain said:

Y&H 1000W Grid Tie Inverter Power Limiter DC25-60V Solar Input AC110V/240V Auto Switch​

https://www.amazon.com/Inverter-Limiter-DC25-60V-AC110V-Switch/dp/B07GC53QBD/ref=sr_1_1_sspa

Something like this?

Click to expand...

Yes, but bigger. That is a grid tied inverter with built in zero export capability. Only 60V strings and only 1000W. Right idea, but wrong scale. I think some of the SMA inverters will work the same.

Diysolar123 said:

So your statement above is not the same as in your first post:
" I want the solar panels to grid tie into the house wiring"

If you have your inverter tied to the same wiring as your normal house wiring that is tied to the grid, well, you are MOST DEFINITELY going to export power hehe.

if you are powering a seperate subpanel only that is NOT tied to the regular house wiring, well...
ahhh, thats not a grid tie system then hehe
You are just building a standard solar panel/inverter/charger system that accepts an input ac voltage and DC solar panel voltage into a completely seperate circuit...get any all-in-one and you are good to go, its automatic

Click to expand...

Yes and no.
Its still the same requirement, just stated differently.

Under normal operation when the grid is up and supplying power to my house, I want to be grid tied (or maybe I should say "AC coupled") to the entire house including the sub panel. However, the inverter must be capable of shutting down AC injection if it determines that it is about to export to the external mains grid.

Some inverters have this built in and call it a variety of things like grid-zero, net-zero, meter-zero, or simply Non-export mode. The problem is that some simply shut off the inverter if the load gets too low. What is best is one that can inject power on a continuously variable scale depending on load. I'm not sure which that is.

The idea is to augment power required by my loads with solar power. So if my loads are 3000W, and my solar is producing 1000W, then draw only 2000W from the external grid. BUT, if my loads are only 500W and my solar is making 1000W, then I want the inverter to only inject 500W to still run my loads without back feeding the grid.

Then when the grid goes down, stop AC injecting and power just the backup sub panel from the same solar panels that previously worked the grid tied inverter.

It would be nice to do this with just one inverter, but its really looking like two inverters will be required. One for AC coupling and one for critical load sub panel backup.

Your setup sounds very similar to what I'm looking for. Did you ever get any further with your plans / research?

moose09876 said:

Your setup sounds very similar to what I'm looking for. Did you ever get any further with your plans / research?

Click to expand...

Its been a while and I pretty much gave up. The inverters I needed really couldn't do what I wanted. Or were unreasonably expensive. Basically, the way to do it properly is to have a subpanel that is purely powered by solar AC which is physically impossible to backfeed the grid. Then physically switch to grid AC when the sun is down.

Another problem is that in Florida where I live, the summer tends to be rainy with little sunshine. That is actually when the need for power for the A/C is the greatest. In the winter, when we don't need the A/C very much, we have plenty of sunshine.

This guy has it set up pretty much exactly how I'd want.


Like you said, the inverter is extremely expensive for what it's able to do.