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Coupling a Heat Pump with PV: SG-Ready, Relay, or Direct Control?

Three paths, one decision, and clear answers about what each path can really do.

Why couple them at all?

After the EV, a heat pump is the biggest controllable load in the house, and heat can be stored without needing a battery: in the hot water tank, in a buffer tank, in the building's thermal mass. Heat up at midday with solar power, use less grid power in the evening: that's the whole idea. It stands or falls on whether, and how, the heat pump can be triggered from outside.

Path 1: SG-Ready, the standard contact

Most newer heat pumps carry the "SG Ready" label: two switching contacts through which an external system can request operating states, from "normal" through "recommended to switch on" to "switch-on command" (we're deliberately leaving out the lockout stage here). In practice, that means: an energy manager closes a relay, and the heat pump raises its target temperatures and pre-heats the tank with surplus power.

The limits: SG-Ready is a recommendation to the heat pump, not a power controller. How strongly it reacts (how much the temperature rises, which tanks are affected) is decided by its own control logic, and the configuration for that sits in the installer menu. And: it requires the two wires from the energy manager to the heat pump, a matter of minutes during installation, a small electrician's job for a retrofit depending on the cable run.

Path 2: Direct control, where the model allows it

Some manufacturers expose their control system via a local network interface (typically Modbus TCP, for example with IDM or Nibe). Then an energy manager can set target temperatures directly and with fine granularity: no relay, no extra wiring, and you also get to see more (actual temperatures, compressor status).

The limits: this is the premier league, but it's model-dependent, and the interface often has to be enabled on the device first. With cloud interfaces from some manufacturers, there's an additional catch: rate limits and internet dependency make them the second choice compared to local paths.

Path 3: Relay + heating element, the pragmatic retrofit

Where the heat pump itself isn't controllable (existing unit, no SG-Ready), a proven workaround remains: a heating element in the tank on a switchable outlet or relay. The energy manager switches it on when there's surplus, with a temperature sensor acting as the upper limit.

The limits: a heating element turns 1 kWh of electricity into 1 kWh of heat; a heat pump turns it into three to four times that. So the heating element only makes sense for surplus that would otherwise be fed into the grid; as a grid-power heater, it would be a waste. A well-designed system enforces exactly that (surplus-only mode) rather than leaving it to chance.

What always applies: safety before optimization

Whichever path you choose: frost protection and legionella protection are non-negotiable. An optimization may shift heating limits upward, but must never lower them below the safety limits. Legionella protection (a regular 60°C cycle in the domestic hot water tank, unless the device manages it itself) has to happen independently of any savings logic. If you're evaluating a system: ask exactly about this. The answer says a lot about the manufacturer's diligence.