Using MPP solar inverters to extend grid capacity

[invizko]

Hello friends. I live in Indonesia and have very limited utility input - only 3KW. Using multiple appliances simultaneously exceeds that limit and circuit breakers turns the electricity off. I have a battery rack and 2x Hybrid V2-5048 inverters that I bought for my expedition vehicle project, which are currently not being used. I'd like to use them to provide extra capacity to my house. So I have a few questions:

1) Can I configure inverters so that they output AC directly from AC input up to certain limit (i.e. 2.5KW) and everything on top of that would be fed from batteries?
2) Can i connect connect AC input directly with AC output of inverters?
3) If there's no good way to do any of the above, would my LiFePO4 battery pack degrade from being used constantly? I.e. If i charge it at night, and discharge during daytime (but not all the way)

 

[Roland W]

Those kind of hybrid inverters probaby cannot do what you describe on AC side. You cannot set a power limit for the grid supply as the primary source, as the grid is assumed to be the final limitless saviour in case battery is finished. You cannot connect AC In to AC Out as that would be just a bypass without control over the grid power limit you have.

As I see it, you only have option 3 available. Run your house virtually off-grid. All loads fully supplied by the inverters via the battery, and use internal grid AC charger which can be set to your required 2,5kW, and solar to assist and charge the battery. So the AC charger if you configure it by the voltage schedule in your UI, can relieve stress on your battery as the DC it creates will be directly taken and inverted back to AC.
LiFePo cells do have good cycle life. One cycle a day won't hurt them much if you stay away from the edges in you voltage curve..

 

[invizko]

Thank you Roland. That's a bummer! It seems that indeed I would have to run everything through the batteries. I wonder however if there're maybe other devices that exists that can do this "merging" of two ac sources into one, that could prioritize usage of grid.

I could use current sensor and some MCU to switch inverter modes if the load exceeds certain current. But I feel like that would be a bit fragile/complicated for my non-essential need. I am not sure if it could act quick enough to avoid circuit breakers doing their thing.

 

[italianuser]

Hi there, you should also consider the wiring you already have, because it could not be able to manage more than 3-3.5KW.

I could imagine you have 13AWG wires (in Italy we call them 2.5mm) for heavy loads and maybe 15AWG wiring (1.5mm) for light loads/lights.

What happens with italian energy meters (the ones from the energy company) is that they do not disconnect immediately when a current overload happens; but they wait 20/30 seconds before disconnecting, so you have 5KW running around the wires... and they really get hot.

Last week I had a 13AWG melted wire for a 5KW overload which lasted I think 20 seconds. And MCBs don't trip unless they reach a higher temperature; my solution for this will be to add a circuit to trip after a few seconds overload...

 

[Roland W]

Thank you Roland. That's a bummer! It seems that indeed I would have to run evribingerything through the batteries. I wonder however if there're maybe other devices that exists that can do this "merging" of two ac sources into one, that could prioritize usage of grid.

I could use current sensor and some MCU to switch inverter modes if the load exceeds certain current. But I feel like that would be a bit fragile/complicated for my non-essential need. I am not sure if it could act quick enough to avoid circuit breakers doing their thing.

What you are describing, would be a grid-tied inverter, which needs to kick-in once you reach your grid supply limit. There might be hybrid inverters having such features, but if you would try to DIY something, you would really need a ultra fast controlling of a separate grid-tied inverter connecting when needed. It would just be a pain. I don't see much of a chance other then to run all through the MPP as stated above.