Fronius Symo 3.7-3-S

Fronius · photovoltaic inverter, wall-mounted · Fronius Symo 3.7-3-S

what it is

A string inverter for a photovoltaic installation: it converts direct current from the panels into three-phase alternating current fed into the grid. Casing 645 × 431 × 204 mm, weight 16 kg, IP65 rating — it may also be mounted outside the building, on a wall or on a pole.

One maximum power point tracker and three pairs of terminals on the DC side, connected in parallel internally. The DC disconnect switch is built in, on the left side of the casing. Forced, regulated cooling. Night-time consumption below one watt.

what we read from it

The inverter has two local interfaces: Solar API, i.e. JSON data over plain HTTP, read-only, and Modbus TCP following the SunSpec standard. Monitoring requires neither a manufacturer intermediary nor the cloud. The communication card does not support SNMP at all.

• Power output at this moment, separately on the DC and AC sides.
• Voltage and current on each of the three phases, and grid frequency. Often this is the only measurement of this kind in the building — the inverter doubles as a power quality meter.
• Daily, yearly and total yield.
• Operating state code together with whether the device is feeding the grid or idle.
• Extreme values — maxima and minima for the period, which you do not have to compute yourself.
• Voltage, current and power on the panel side — via Modbus SunSpec, for the single power point tracker.

For alerting: at night the inverter goes to sleep and Solar API stops responding. This is normal behavior, not a failure, and monitoring must tell the difference — otherwise the photovoltaic installation reports a failure every night.

what it looks like

An upright box, taller than it is wide, with a light upper cover and a dark lower cover. The display module sits low, on the left side — a liquid-crystal display, a column of three status LEDs next to it and four touch keys below.

There is not a single network port on the closed casing. Two RJ45 ports for the Solar Net network, the communication card's LAN port, a USB port and a potential-free contact sit under the removable dark cover, in the communication compartment. The DC and AC terminals are deeper, in the connection compartment.

The rating plate with the serial number is on the underside of the device, not on the front or back — when inventorying the installation you have to look at it from below.

what to watch out for

• The second panel input in Modbus returns a "not available" value, and that is expected. The 3.7-3-S has one power point tracker; the three pairs of terminals on the panel side are connected in parallel. The SunSpec table has two slots for inputs, but the second is always empty.
• Panel power and power fed into the grid are two different numbers. They differ by the conversion losses.
• Without a Smart Meter, grid power, load power and self-consumption are empty, not zero. On the phases the inverter then reports only voltage and current, without active power.
• The name assigned to the inverter arrives encoded as HTML entities (S instead of the letter S) — it has to be decoded before display.
• Network controllers show the vendor as "Fronius Schweissmaschinen" — the inverters share a hardware address prefix with the same company's welding machines. That it is an inverter is confirmed by the Solar API response.
• State code 7 means normal operation; values 0–6 are startup and standby phases.

what we cannot read

• Production of individual panel strings — this variant has one power point tracker, so there is no per-string breakdown.
• The state of individual panels, or anything about shading. The inverter sees the sum, not the cause.
• Building consumption without a separate meter — the inverter says how much it produced, not how much was consumed on site.
• Inverter temperature — in the SunSpec registers the temperature field is marked as unsupported.

full parameter list

A full parameter list for this model is not here yet — the ones described above are those we have read and verified. A missing list does not mean the device exposes nothing more.