Plenty of heat pump spec sheets carry a small "SG Ready" logo. It sounds like a smart grid protocol, and people expect an app, a cloud service or at least a network port behind it. What you actually get is two terminals on the controller board. You close or open them with a relay, and the heat pump changes the way it runs.
That simplicity is the point. If you have solar panels, a home energy manager or a dynamic tariff, SG Ready is often the simplest way to tell the heat pump "use the power now", without reverse engineering a cloud API. In 2025 the rules behind the logo also changed, so older guides that talk about four modes are partly out of date.
Who defines SG Ready
SG Ready is a label run by the German heat pump association BWP (Bundesverband Wärmepumpe) through its service company. Manufacturers apply for it per model or model range. Swissolar's planning guide for heat pumps and PV notes that BWP defined the label in 2013. Under the 2020 regulations it was awarded for the German, Austrian and Swiss markets.
It is not a data protocol. There is no bus, no addresses and no measured values. The heat pump only sees the state of two digital inputs, which the specification calls SG1 and SG2. Whatever closes those contacts, a PV inverter with a relay output, an energy manager or a smart relay on your Wi-Fi, is up to you.
The current rules: three states, not four
The technical interface description version 1.1 has applied since 1 May 2025, together with the label regulations 3.0. Version 3.1 of the regulations has applied since 25 February 2026. According to the interface document, a space heating heat pump must support three operating states:
| SG1 | SG2 | State | What the heat pump should do |
|---|---|---|---|
| 1 | 0 or 1 | State 1 | Limit power consumption |
| 0 | 0 | State 2 | Normal operation |
| 0 | 1 | State 3 | Boost, store surplus electricity as heat |
State 1 must be configurable. The manufacturer has to implement at least one of these options: 0 kW, a fixed positive power set by the manufacturer, or a positive power set by the installer. For the last two, the document recommends following §14a of the German Energy Industry Act: a limit of 4.2 kW for a grid connection of up to 11 kW, or 40% of the connection power above that. The installer setting has to be accessible only to qualified personnel. The heat pump may also finish a running cycle or defrost before it drops into state 1.
State 3 means the heat pump runs harder for space heating, domestic hot water or both, for example by raising target temperatures in the buffer or hot water tank. That is the useful one for solar surplus: you turn extra kilowatt-hours into a warmer tank instead of exporting them.
SG1 has priority over SG2, and the document says SG1 is mainly meant for distribution network operators. In practice SG1 often lands on the same terminal that used to be labelled "EVU", the German shorthand for the utility lockout.
A few other details from version 1.1 are useful when you wire things up:
- The control voltage on the inputs is set by the manufacturer anywhere between 3 V and 250 V, and it must be stated in the documentation. Do not assume a dry contact or 230 V until you have read the manual.
- Manufacturers can use their own terminal names, as long as the documentation links them to SG1 and SG2.
- A room thermostat alone may not block the heat pump based on room temperature.
- The heat pump may skip a requested state if that is needed for safe operation.
- For domestic hot water heat pumps, state 1 disables the optional electric booster heater. That part is optional unless the unit counts as a controllable consumer under German rules (more than 4.2 kW of connection power).
What changed compared to the old four modes
Most forum posts and many installation manuals describe the earlier version of SG Ready. The regulations 2.0 from 2020 listed four modes:
- 1:0 - a "hard" lockout of up to 2 hours, at most three times a day, compatible with utility lockout.
- 0:0 - normal operation.
- 0:1 - a switch-on recommendation, a boost the heat pump may follow.
- 1:1 - a definitive start command, where the manufacturer could offer variants such as compressor only or compressor plus electric heaters.
Interface 1.1 drops the forced-on state and the two-hour limit on lockout. 1:1 now means the same as 1:0: limit consumption. That matters when you build your own control logic. On a heat pump with the old firmware, closing both contacts may start the compressor and the electric heater element. On a heat pump that follows 1.1, the same combination tells it to hold back. The Bosch/Buderus documentation for the Compress 5800/6800i still describes four functions, including forced boost on Ext1=1 and Ext4=1, which shows how long old behaviour stays in the field.
There is also a transition period. Under the 2026 amendment, labels have to be updated to interface 1.1 by 31 December 2026, or they cannot be renewed from 2027. Between January and December 2026, applicants must explain how devices already in operation can be updated. Regulations 3.1 add a QR code on the label to check it, and the implemented interface version shown on the label and in the BWP database.
The safe approach: check which version your heat pump implements, and treat 1:1 as undefined until the manual tells you otherwise.
Wiring it to a home energy manager
The minimum setup for solar surplus is one relay on SG2. Open means normal operation, closed means boost. You do not need SG1 at all unless you want to limit consumption yourself.
The relay has to switch whatever the heat pump expects on its inputs. Swissolar's guide describes the SG Ready inputs as potential-free contacts, but since the manufacturer defines the control voltage, check the manual before choosing a relay. Most vendors also have to unlock the function in the controller menu.
Some real examples:
- Enphase IQ Energy Router with Vaillant aroTHERM plus. Enphase's guide wires its SG Ready relay to the EVU contact X106/S21 and to the X41/FB and X41/OT inputs. The router uses only states 2 and 3, and Enphase stresses that state 3 is a recommendation: the router can never force the heat pump on or off. SG Ready also has to be activated on the controller.
- Bosch Compress 5800/6800i and Buderus WLW176/186i. The community documentation maps SG Ready to external inputs Ext1 and Ext4. To see the photovoltaic menu, you set "External input 4" to "Photovoltaic system" in the system settings. Then you choose what the boost does: raise the room setpoint, heat hot water to the Comfort stop temperature, or cap compressor power during surplus. The same documentation describes using EMS-ESP to trigger the input in software instead of a physical contact.
- evcc. The open source energy manager has a charger type called
sgready-relay. Its example uses a Shelly 1 as the boost relay and a Shelly 3EM to measure the heat pump, with thresholds of 1300 W surplus for 5 minutes to switch on and 150 W grid import for 20 minutes to switch off. evcc can also drive a second relay for limiting consumption.
The evcc documentation adds two warnings. First, the heat pump decides for itself whether and how far it follows a request, based on its state and configuration. Second, add a separate energy meter, because the figures many heat pumps report internally are estimates or leave out some components.
Timing: do not chatter the contacts
A heat pump compressor does not like short cycles, and the specification knows this. Interface 1.1 requires compatible system components, such as energy managers, to hold state 1 and state 3 for at least 10 minutes once they activate them, and not to activate them again for another 10 minutes after they end.
If you write your own automation, build that in. A cloud passing over the panels should not toggle the relay. Use an averaged surplus value with a delay, as in the evcc example, and add a minimum on time and off time. Many heat pumps also have their own minimum run times, and the evcc example notes that you should check them in your manual.
When SG Ready is the wrong tool
SG Ready gives you three coarse states. You cannot set a flow temperature, request a specific power level or read any data through it. Swissolar's guide lists the alternatives:
- The utility lockout input (EVU contact). Many heat pumps have it alongside or instead of SG Ready. It only blocks.
- A dedicated PV input on some models, which works like a single boost contact.
- Modbus or IP interfaces, which allow variable setpoints and readings but are specific to each manufacturer.
- SmartGridready, a Swiss label with a richer data model. In 2023 Swissolar noted that only a few devices supported it.
If your heat pump has a documented Modbus or local API, that gives you finer control. SG Ready still has one big advantage: it is a contact on a terminal block, so it keeps working without the manufacturer's app, its cloud or its next firmware update.
Before you connect anything
The SG Ready terminals sit on the heat pump's control board, often next to mains wiring, and on some models the inputs themselves may carry mains voltage. Ask the installer to bring the contacts out to a terminal block or a DIN rail relay, and do the automation on the low voltage side.