[Request] Support for limiting the output via GPIOs

[janhieber]

Is your feature request related to a problem? Please describe.

In Germany there is a new law since February, the "Solarspitzengesetzt 2025".
It requires installations >7 kWp to support remote control of the output power.

Specifically:

• there is a control box ("Steuerbox") coupled to the electricity meter which the grid provider controls
• it has 4 floating contacts
• the contacts, when closed, shall force limits of 0%, 30%, 60%, 100% of the output power

Describe the solution you'd like

There could be a UI element in OpenDTU where the GPIOs and the limits can be configured, similar to this example from a Fronius inverter:

Describe alternatives you've considered

As long as this is not available/supported the system shall be limited to 60% of its kWp.

So either this or one has to use the original DTU once it supports this, which we don't want of course...

Additional context

No response

[gitisgreat2023]

The output power is not limited, but the grid feed-in, ain't that right? So self-consumption is allowed. If you consume all the energy yourself, it would never be limited.
That's an improvement over the past, where the inverter output power itself was limited and worst case one had to buy electricity from the grid when the renewable energy production was very high and the energy provider put a 0% limit out.

[janhieber]

Yes that may be, and I don't know if the percentage is related to the kWp or the inverter max kW.
Maybe both options, hard-limit and grid feed-in limit would be nice to have (for those people not having the measurement equipment).

[OutdoorRob]

Hi @janhieber

I wonder if something like that would require some kind of certification / is accepted by your grid operator!?

I'm in a similar situation with my installation. But since the whole plant is just below 7 kWp, I don't need the control box and can hard limit it to 60% which is fine for me by now.
Since the inverters can just be controlled by the Hoymiles proprietary radio network (and not a more reliable cable connection), I would never expect it to be allowed in Germany.

Keep us informed if you have any news if your grid operator accepts such a solution.

[stefan123t]

You can always buy the original Hoymiles DTU Pro (S). The Hoymiles Pro models have a connector for the Australian DRM port. This can be used to throttle the inverters to 0, 50, 75 and 100%.

4.2 DRM Port (for Australia and New Zealand only)

DRM port is provided to support several demand response modes as shown below by connecting external
control device with a standard RJ-45 connector. DTU-Pro-S supports DRM0/5/6/7/8 if used with Hoymiles
microinverters.

Mode	Requirement
DRM0	Operate the disconnection device
DRM1	Do not consume power
DRM2	Do not consume at more than 50% of rated power
DRM3	Do not consume at more than 75% of rated power AND Source reactive power if capable
DRM4	Increase power consumption (subject to constraints from other active DRMs)
DRM5	Do not generate power
DRM6	Do not generate at more than 50% of rated power
DRM7	Do not generate at more than 75% of rated power AND Sink reactive power if capable
DRM8	Increase power generation (subject to constraints from other active DRMs)

https://www.hoymiles.com/uploadfile/1/202506/5ea8b286bb.pdf

[janhieber]

Good to know, so the chinese cloud might be the last option as backup.
I think for the OpenDTU solution you need a "good" electrician who can approve it ;)

My grid op for now just told me to limit to 60%
And the vendor of the inverters is willing to write an Kind oft certificate for me :)

[stefan123t]

In California / US there seems to be moves to make OpenADR 3.0 a standard for limiting or automating demand by responsible use of energy using so called Home Energy Management Systems (HEMS).

See the whitepaper called "Development of a Low-Cost Automated Demand Response Controller for Home Energy Management" at https://www.mdpi.com/2076-3417/14/23/11434 or here in PDF form: applsci-14-11434-with-cover.pdf

Besides the communication between the Demand Response Controller (DRC) setting the Inverter / Power Plants Demand Response Mode (DRM) it also talks about the communication between the two which seems to be using the Open Automated Demand Response protocol (OpenADR) in v2.0a/b.
The two participants are called Virtual Top Node (VTN) and Virtual End Nodes (VEN), where a VTN and VEN can be the same device. For example a DR aggregation server can act both as a VEN for a utility DR signal, and as a VTN for end devices.

• OpenADR what is Demand Response https://www.openadr.org/what-is-demand-response-
• OpenADR Frequently Asked Questions https://www.openadr.org/faq
• OpenADR Seminar Presentation https://www.openadr.org/assets/openadr%20plma%20seminar_final.pdf
• Everything You Should Know About OpenADR – But Were Afraid to Ask https://www.cortexo.com/about-openadr/
• Fujitsu Overview of OpenADR compatible system https://www.fujitsu.com/global/about/resources/news/press-releases/2013/0819-01.html

You can download the OpenADR 2.0a Profile Specification and the Schema here:
OpenADR 2 0a Profile Specification_v1.0

You can download the OpenADR 2.0b Profile Specification and the Schema here:
OpenADR 2.0b Profile Specification v1.1

You can download the OpenADR 3.0 Specification and the yaml file here:
OpenADR 3.0 Specification v3.0.1 revision August 23, 2024 - To get access to the GitHub repo email OpenADR.

[stefan123t]

@janhieber regarding your suggested solution with a DIP switch / RJ45 port connection we could consider adding an I²C Port Expander to the OpenDTU Fusion Board for setting and querying those DRM flags as configurable in the Fronius example mockup.

@markusdd would we be able to create another addon board for OpenDTU Fusion which we could stack on top or is this something for the CAN Bus Controller Board v2 ? 😉

[markusdd]

First of all the actual breadth of the use case needs to make sense. For an official add-on board to make any sense we would essentially need to be sure to sell hundreds or a few thousands. Not sure if that applies to such a fancy setup. I mean in general there are i2c Pins available on the fusion and generic IO expanders can be had on AliExpress or wherever for probably 1 or 2€.
Very hard to justify to enter such a market

[stener2009]

Additional enhancement proposal for GPIO-based output limiting

I would like to extend the scope of this feature request with a more advanced use case that combines GPIO control with a global power management function in OpenDTU.

Background:
In my setup I run up to 10 Hoymiles inverters (mixed HM and HMS models). For compliance with VDE-AR-N 4105 (Germany) and the upcoming “Solarspitzengesetz”, as well as for stability in island/off-grid systems (e.g. Victron Multiplus-II), it is essential to:

• Trigger predefined global limits via local GPIO inputs (dry contact from Smart Meter Gateway, ripple control, or manual switch).
• Also be able to set/change the global limit dynamically via MQTT/HTTP API.
• Apply coordinated limiting across all connected inverters rather than per-inverter manual settings.

Proposed control logic:

• Global max power value (Watt or % of installed capacity) is set either:

  • by MQTT (opendtu/set/max_power), or
  • by GPIO (preconfigured fixed value for each pin or trigger state).

• OpenDTU sums the current production of all connected inverters (data already available via telemetry).

  • If total > max power:
    • Throttle inverters starting with the one producing the lowest current output, then the next lowest, until the target is met.
    • Leave the highest-producing inverters running at full power as long as possible.

• Optional: hysteresis to avoid frequent toggling.P

• Publish current global limit and active/inactive state via MQTT/state topics (for external systems).

Benefits:

• VDE-AR-N 4105 compliance for grid-connected PV (local control via Smart Meter Gateway or ripple control).
• Victron MP2 / off-grid stability by preventing inverter overloading and allowing prioritised battery charging.
• Reduced RF traffic (only throttle necessary inverters).
• Works with mixed HM and HMS inverters.

Willing to help:
I can provide testing on a real up to 10-inverter setup (HM + HMS) and validate behaviour in both grid-connected and Victron MP2 island modes.

[spcqike]

@stener2009

if you want to, you may test the fork "openDTU-onBattery", as this should be able to do, what you want. at least via mqtt.

it can dynamically limit your inverters to a total maximum output.
in my case, i have 2 inverters set to 800W, and 0 export.

you could set your desired target to 60% of your combined max power. so you would never feed in more than 60%, but would be able to use more locally, if needed

https://opendtu-onbattery.net/firmware/mqtt_topics/#power-limiter-topics

edit:
i'm not quite sure, if onbattery can handle 10 inverters. i know there was a time, where you only could have 5, but i think, that was changed. but, not to sure. so you might test it, maybe :)