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An SG Ready heat pump has two digital inputs, SG1 and SG2, that an energy management system or the grid operator uses to set its behaviour. The combination of the two inputs selects an operating state: limit power, run normally, or run in boost mode to store surplus electricity as heat. Since 1 May 2025, interface 1.1 of the German Heat Pump Association (BWP) applies, and a space-heating heat pump has three operating states instead of four. Older units and most web pages still describe four.
If you wire a heat pump to an energy manager or drive it over Modbus, you need to know the version, because state 4 behaves differently under 1.1.
Two inputs, SG1 and SG2, select the operating state
Smart Grid Ready is a certification label that the BWP grants to heat pump models with a defined load-management interface. The BWP Technical Interface Specifications 1.1 name two users: grid operators, who want to reduce load, and owners, who want to use more of their own solar power.
Each input is open (0) or closed (1), so two inputs give four combinations. The table shows what each combination means under the old and the new interface.
| SG1 | SG2 | Interface 1.0 (from 2013) | Interface 1.1 (from May 2025) |
|---|---|---|---|
| 0 | 0 | 2: normal operation | 2: normal operation |
| 0 | 1 | 3: boost, a recommendation to start | 3: boost |
| 1 | 0 | 1: utility lock, two hours at most | 1: limit power consumption |
| 1 | 1 | 4: definitive switch-on command | 1: limit power consumption |
Source: BWP, Technical Interface Specifications 1.0 and 1.1.
In state 3 the heat pump raises the target temperature in the buffer tank or the hot water cylinder. The surplus electricity then sits in the water as heat. How far it raises the setpoint is a manufacturer setting. Manufacturers may use their own names. NIBE calls the four states Cut off, Standard, Encouraged, and Ordered, which the BWP allows as long as the manual refers to SG1 and SG2.
Interface 1.1 drops operating state 4
Under interface 1.1, SG1 has priority over SG2. When SG1 is closed, state 1 applies, whatever SG2 does, so the definitive switch-on command of state 4 no longer exists for space heating. An energy manager that closes both inputs puts such a heat pump into state 1.
Under 1.1, SG1 has priority: when SG1 is closed, state 1 applies whatever SG2 does.
State 1 used to be a hard lock of two hours at most, matching the old German utility lock times. Under 1.1 it limits power consumption with no time limit. The manufacturer implements one of three options: 0 kW, a fixed manufacturer value, or a value the installer sets at commissioning, which only qualified staff may reach. The BWP recommends the German §14a EnWG values: 4.2 kW for a grid connection of up to 11 kW, and 40% of the grid connection power above that.
The heat pump may skip a requested state when safe operation requires it, and may finish a defrost cycle first.
The transition has a deadline. According to the SG Ready Amendment 2026, labels on interface 1.0 must move to 1.1 by 31 December 2026, or they "cannot be renewed from 01.01.27 onward". The label and the BWP database now show which interface version a model implements.
Wiring an SG Ready heat pump to an energy manager
An energy manager, inverter, or relay module switches SG1 and SG2 through two separate contacts. The heat pump only senses whether each input is closed.
The control voltage on those inputs differs by brand. The BWP lets the manufacturer choose anything from 3 to 250 V and requires it in the documentation. Some installer pages state 230 V as a fixed requirement, which is wrong as a rule: 230 V occurs, but the specification allows much lower voltages too. Pick a relay rated for the voltage in the heat pump manual.
The manufacturer sets the control voltage, anywhere from 3 to 250 V. The manual says which terminal supplies it.
Use one potential-free relay contact per input, rated for that voltage.
Three rules in the BWP specification apply to the wiring itself. A system component keeps state 1 and state 3 active for at least 10 minutes and then leaves them off for at least 10 minutes. A room thermostat may not lock the heat pump on room temperature alone. With several heat pumps, one unit or controller handles grid-supportive operation for the whole group.
SMA shows what this looks like in a panel. Its Sunny Home Manager switches an SG Ready heat pump through a Moxa ioLogik E1214, a Modbus I/O module with relay outputs. The module sits on a DIN rail, needs its own power supply, and connects to the router by network cable.
SG Ready over Modbus: write the inputs, read the state
Some heat pumps also expose the SG Ready inputs as Modbus registers. The Stiebel Eltron ISG documents them in its Modbus TCP/IP software documentation.
| Register | Type | Content | Values |
|---|---|---|---|
| 4001 | holding register | SG Ready on or off | 0 off, 1 on |
| 4002 | holding register | SG Ready input 1 | 0 open, 1 closed |
| 4003 | holding register | SG Ready input 2 | 0 open, 1 closed |
| 5001 | input register | current operating state | 1 to 4 |
Source: Stiebel Eltron, Modbus TCP/IP software documentation for ISG web (321798-44755-9770). Default IP 192.168.0.126, port 502, slave ID 1.
You read the holding registers with function code 03 and write them with function code 06; register 5001 is read with function code 04. The Modbus function codes reference explains the codes. Three things go wrong in the field.
- Inputs 1 and 2 swapped. The manual writes the wiring as "(input 2/input 1)". State 3 is (0/1) there, so input 1 is closed. Read the pair as SG1/SG2 and the heat pump goes to state 1 instead of state 3. An openHAB user asked about exactly this.
- Register 4001 left at 0. Without 4001 set to 1, writes to 4002 and 4003 do nothing.
- One register off. The ISG counts from 1. Enter 4002 in a Modbus client that counts from 0 and you write input 2 instead of input 1. The Modbus register map guide covers the offset.
The biggest difference from wired contacts is feedback. Over two contacts the energy manager sends a command and never learns whether the heat pump carried it out. Over Modbus TCP it reads the actual state back.
Register 5001 on the ISG still follows the four states of interface 1.0. Value 4 there means the heat pump goes straight to its maximum heating and hot water temperatures. Other brands use other addresses, so the manufacturer's register table always decides. For a Daikin example, see Daikin heat pump Modbus.
Germany's §14a and subsidy rules behind the label
SG Ready is a German label, and two German rules explain why manufacturers changed state 1. The label is also used outside Germany: NIBE describes it on its UK site, and Remeha in the Netherlands states that all its residential heat pumps are SG Ready.
The first rule is §14a of the Energy Industry Act. Under Bundesnetzagentur decision BK6-22-300 of 27 November 2023, new controllable consumers above 4.2 kW must accept dimming by the grid operator since 1 January 2024. The operator may dim, not switch off, and must leave 4.2 kW per device; directly controlled heat pumps above 11 kW keep their grid connection power times 0.4. Installations commissioned before 2024 stay exempt.
The second is the BEG subsidy. The KfW technical FAQ 8.23 requires an interface for grid-supportive operation, "for example" SG Ready, VHP Ready, or EEBUS under VDE-AR-E 2829-6. From 1 July 2027 new digital-interface requirements apply, and the KfW information sheet version 11.1 of 1 November 2026 says grid control is met "in particular" with EEBUS. Integration with an energy management system then follows the European Commission's Code of Conduct for Energy Smart Appliances.
Frequently asked questions
What is an SG ready heat pump?
A heat pump with the Smart Grid Ready label of the German Heat Pump Association (BWP). It has two defined inputs, SG1 and SG2. Through them an energy manager or the grid operator selects an operating state: limit power, run normally, or boost to store surplus solar power as heat.
How many operating states does SG Ready have?
Three for a space-heating heat pump on interface 1.1, valid since 1 May 2025: limit power, normal, and boost. The older interface 1.0 had four, with a definitive switch-on command as state 4. Many units in the field still follow 1.0, so check the interface version in the manual or the BWP database.
How do I connect a heat pump to SG Ready?
Wire two relay contacts from an energy manager, inverter, or relay module to the SG1 and SG2 terminals. The manufacturer sets the control voltage somewhere between 3 and 250 V, so choose a relay rated for it. Keep each state active for at least 10 minutes before switching again, as the BWP specification requires.
Is the SG Ready voltage always 230 V?
No. The BWP lets each manufacturer choose a control voltage between 3 and 250 V and requires it in the documentation. Some heat pumps use 230 V, and the specification also allows far lower voltages. Read the value in the manual before choosing a relay, because a contact rated for low voltage is not always rated for 230 V.
Can SG Ready be controlled over Modbus?
Yes, if the manufacturer exposes the inputs as registers. On a Stiebel Eltron ISG you set register 4001 to 1, write holding registers 4002 and 4003, and read the state from input register 5001. Other brands use other addresses, so follow the manufacturer's register table and check whether it counts from 0 or 1.
Who issues the SG Ready label?
The German Heat Pump Association (Bundesverband Wärmepumpe, BWP). It grants the label to heat pump models and to compatible system components such as inverters, energy managers, and FNN control boxes. Labels run to the end of each calendar year, and the BWP database shows which interface version each model implements.
Reading heat pumps over Modbus with ModbusCloud
The ModbusCloud Gateway reads a heat pump over Modbus RTU on RS485 or over Modbus TCP on the network. The portal keeps the history of each register for 365 days. An alarm rule with the "change" condition on a state register reports by email, in the portal, or by webhook when the heat pump changes state. Automations can write a register based on the energy price; register writing is off per gateway by default, and ModbusCloud switches it on on request. The template library has a template for the NIBE S and F series. More on the subject is in heat pump Modbus monitoring.
