# question

asked

## Solar Invertor AC coupling of Mulitplus on AC input?

Hi all,

The context of my question is for a grid coupled solar system with local battery.

I've been reading documentation on the Victron (MultiplusIIGX) and some posts here on AC Coupling (https://www.victronenergy.com/upload/documents/MultiPlus-II_GX/MultiPlus-II_GX-nl.pdf)

What I've understood so far (https://www.victronenergy.com/live/ac_coupling:start):
- AC Coupling is done on the output of the MP/MP2
- Rule: MP2 Power rating >= Solar invertor power rating
- Battery capacity (Lithium): 1,5 kWp installed AC PV power requires 4.8 kWh of battery storage

The questions I have:
a) Is there a hard requirement to couple the solar invertor on AC Output of the MP2?
b) Can some of these rules not be avoided by coupling on AC Input (see reasoning below)?
c) How to control the MP2 to only charge the battery when there is solar power available?

I found this post (https://community.victronenergy.com/questions/62049/ess-with-solar-grid-inverter-on-input-to-multiplus.html) but very little other info.

The reasoning for this question:

Suppose the MP2 is coupled to the grid/solar invertor on input: i.e. the Solar invertor generates power if there is sun, and injects into the grid when there is not enough demand. The MP2 takes care of the loads coupled on the AC output, i.e. when there is demand and Solar power available in the AC input the loads can be fed from the AC input, when there is no solar the loads can be fed from the the battery and if the battery is depleted the loads are finally fed from the AC input (i.e. grid).

In this way the power rating of the MP2 can be selected in line with the power required by the load behind the MP2 and the battery can be sized accordingly.

For a household, a big part of an electricity bill is caused by low power devices that are powered for a large period of time, i.e. (led) lights, routers, clocks, computers, ... -> i.e. a large part of the bill could be avoided by a relatively small MP2/battery system.

The only thing I've not found is to control the MP2 when to charge the battery, thus only charge the battery when there is solar power or when the grid is cheap.

What am I missing?

Thanks for helping out.

Best regards,

Ruben

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1 Answer
wkirby answered ·

A) No.
B) Yes.
C) By using Carlo Gavazzi energy meters.

Use an energy meter to measure the production of the PV inverter at AC input. Configure the meter as a PV inverter in the GX device. Fronius PV inverter, for example, would not need a meter here because its production can be read by GX device directly over ethernet.
Use another energy meter at the entrance point of the network, before any loads or PV inverter connection. Configure this meter as Grid meter in the GX device.
The GX device now tries to make grid meter read zero (or your chosen set point). Multiplus will start charging proportionally if there is excess energy trying to escape into the network. At night the MultiPlus feeds back to try and prevent energy coming in from the network, powering loads and trying to make the grid meter read zero.
Indeed, the system can work without anything connected to AC output of the MultiPlus, but if the network fails then nothing works at all.

Did you see the ESS documentation for details of the various configurations?
https://www.victronenergy.com/live/ess:quick-installation-guide

1 comment

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·
Hi Wkirby,

thank you for the quick answer.

If I understand you correctly then my idea would work, but with the drawback that as the Solar invertor is coupled on AC In, it will not get any sinewave to slave on when the grid goes down. So the solar invertor will stop and no solar power will be available, only the battery can be used until depleted via AC out?

I will look through the ESS documentation, I missed that in my search. Thanks for the pointer.

best regards,

Ruben

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