A customer asks us: «can the Shelly Pro EM-50 send an order to a heat pump so it switches on with the solar surplus?». The short answer is yes, and it's one of the uses it's designed for. But it's worth understanding how it gives that order, because that's where the detail that saves you trouble lies.
Yes, it works: here's how it fits
The Shelly Pro EM-50 is the only Shelly meter with a built-in relay, and it measures both ways: it tells apart the energy you consume from the energy you export to the grid. With those two figures it can detect when you have spare solar production and, at that moment, close its relay.
That relay is a potential-free contact (a dry contact), which is exactly what the SG-Ready input of almost every current heat pump expects. You connect two wires from the Pro EM-50's relay to the heat pump's SG-Ready input and, when there's surplus, the unit takes the order and starts up to heat hot water or your heating using the sun you were about to export to the grid.
The best part: a single device measures and gives the order, with no extra modules, and with the logic running on the device itself (no dependence on the cloud or a PC left on).
The relay gives the order; it doesn't power the heat pump
This is the important point before wiring anything. The Pro EM-50 output is a dry-contact relay rated 2 A / 240 V: it's a signal relay, meant to command, not to supply power.
- Ideal case (SG-Ready): the SG-Ready input only needs you to close a contact; barely any current flows there. The Pro EM-50's relay goes straight to that input, with nothing in between. It's the most common and the simplest situation.
- If you had to switch real power (for example, cutting the supply to a heater element): the relay can't take that. In that case it drives the coil of a contactor sized for the load, and it's the contactor that switches the power.
What you must never do is hang the compressor or a heater straight off the meter's relay: those 2 A can't take it and you'd damage the unit. With SG-Ready, in most installations, you don't even need a contactor.
How it decides when to start the heat pump
The logic is done with a script running on the Pro EM-50 itself (Gen2 firmware, mJS scripting). Broadly:
- The script watches the energy exported to the grid in real time (the grid-tail channel).
- If the surplus exceeds a threshold (for example, the minimum power the heat pump needs) for a while, it closes the relay.
- The SG-Ready input takes the order and the heat pump starts up using the sun.
- When the surplus drops below the threshold for a while, it opens the relay.
That «for a while» and the margin on the way down —the hysteresis— matter: a heat pump shouldn't start and stop every few seconds because the compressor suffers. So it only switches on when the surplus holds (not on a single spike from a passing cloud) and switches off with some slack.
Before you fit it, check three things
To know whether your case fits directly or needs another part:
- Does your heat pump have an SG-Ready input? It's standard on current units (two potential-free contact inputs). If it doesn't, this simple route isn't available and you'd need to see what other control input it accepts.
- Single-phase or three-phase? The Pro EM-50 is single-phase. If your installation is three-phase, the equivalent is the Shelly Pro 3EM plus the Pro 3EM Switch Add-on, which provides the relay (the three-phase meters don't have one built in).
- Which SG-Ready mode do you want? The SG-Ready standard defines four states through the combination of two contacts. With one relay you activate one of those modes (typically «run recommendation» on surplus). To command all four states you'd need two contacts.
The rule of thumb: single-phase heat pump with an SG-Ready input → the Pro EM-50 solves it with one device.
FAQ
So the Pro EM-50 starts the heat pump on its own with the sun?
Yes: it measures the surplus and, via its relay, closes the heat pump's SG-Ready input when there's spare production. The unit does the rest according to what its SG-Ready mode allows.
Do I need a contactor?
To command over SG-Ready, usually not: the relay closes a signal, not a load. You only need a contactor if what you're switching is real power (a heater element, for example), and then the relay drives the contactor coil.
What if the heat pump has no SG-Ready?
Without SG-Ready there's no direct route. Some units accept other control inputs or integrate over Modbus; you'd have to check your heat pump's manual, and then the solution changes.
Can it modulate the heat pump by how much sun there is?
The relay is ON/OFF: it tells the heat pump «there's surplus, use it». The fine modulation (how much power it uses) is decided by the heat pump itself with its SG-Ready logic, not by the Shelly.
Does it work on a three-phase installation?
The Pro EM-50 is single-phase. For three-phase, use a Pro 3EM with the Switch Add-on, which adds the relay you need.
Does the logic depend on the internet?
No. The script runs on the Pro EM-50 itself, so surplus management works locally even if the internet drops. It also stores 60 days of history and has an internal clock.
Summary
- Yes, the Pro EM-50 works: it measures the surplus and closes its relay onto the heat pump's SG-Ready input.
- The relay is a signal (2 A): it goes straight to SG-Ready; for real power it drives a contactor, never the load directly.
- It decides by threshold with hysteresis, with a local script, so it doesn't cycle the compressor.
- Single-phase: on three-phase, Pro 3EM + Switch Add-on.
Tell us your heat pump model (whether it has SG-Ready) and whether your installation is single- or three-phase, and we'll confirm the exact wiring and the surplus threshold that suits you. Technical support and shipping available for your market.
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