Fronius Solar Display

An ESP32 (Arduino) based solar power display that reads photovoltaic production, grid import/export and home consumption from a Fronius inverter's Solar API v1 and shows them on a cheap 20×4 character LCD — no I²C backpack, no app, no website.

Put it somewhere you actually walk past: kitchen, hallway, living room. Its whole purpose is a glance that tells you whether now is a good moment to run the dishwasher.

This is v2. The v1 code is preserved on the v1 branch, along with its Fritzing sketch. The application logic is the same; v2 rewrites the hardware layer, fixes a set of latent memory bugs, adds OTA and makes the WiFi link actually survive a marginal signal. ARCHITECTURE.md documents all of it.

System requirements

• A Fronius inverter with Solar API v1 (Symo, Galvo, Gen24, Tauro …).
• A Fronius Smart Meter installed alongside your main electricity meter and connected to the inverter — without it there are no grid figures. Supplemental inverters not directly connected also work, provided they are registered via a Smart Meter.
• An IPv4 network, preferably a /24 (255.255.255.0) subnet.

Features

• PV production, grid import/export and home consumption on a 20×4 LCD.
• Maximum grid-voltage deviation (gV). European grids run 230 V with a 10% (23 V) limit.
• WiFi signal strength (RSSI) and inverter connection errors (E), errors self-clearing after five minutes.
• Configured over WiFi through a captive portal (WiFiManager by @tzapu) — no credentials in the source.
• Remembers the inverter's IP and MAC, and re-finds it by MAC when DHCP moves it.
• OTA updates over WiFi once the first USB flash is done.
• Survives a weak link: radio tuning (power-save off, HT20, 802.11b retained), a blocking boot-connect retry loop, and active reconnect supervision rather than trusting the stack.

Hardware

Board	ESP32-WROOM-32 DevKit (4 MB flash) — v2 was developed on a uPesy board
Display	HD44780-compatible 20×4 character LCD, 4-bit parallel, run at 5 V
Contrast	B10K potentiometer
Power	An external 5 V supply — see below
Input	The board's on-board BOOT button

Wiring

LCD pin	Signal	ESP32
4	RS	GPIO 13
6	E	GPIO 33
11	D4	GPIO 14
12	D5	GPIO 27
13	D6	GPIO 26
14	D7	GPIO 25
1, 5, 16	VSS, RW, backlight cathode	GND
2, 15	VDD, backlight anode	5V
3	V0	Potentiometer wiper (outer legs to 5 V and GND)

Pins 7–10 (D0–D3) stay open — 4-bit mode uses D4–D7 only.

All eight connections land on the DevKit's left header, so no wire crosses the board:

3V3 EN 36 39 34 35 32  33  25  26  27  14  12  GND  13  9 10 11  5V
                        E  D7  D6  D5  D4  ✗  rail  RS  ·  ·  ·  rail

Three things that are not optional:

• RW (pin 5) must be tied to GND. It puts the HD44780 in write-only mode, which is also what makes driving a 5 V display from 3.3 V GPIOs safe — the data pins stay inputs and never drive 5 V back into a non-5 V-tolerant ESP32.
• Leave GPIO 12 bare. It is the MTDI strapping pin: held high at reset it selects 1.8 V for the flash core and the module will not boot. It sits inside the run of pins we use, between D4 and GND, so it is easy to hit by miscounting. (v1 ran EN here and got away with it; v2 uses GPIO 33 instead.)
• Use an external 5 V supply. USB power is not enough. The LCD with its backlight plus the ESP32's transmit bursts sag the rail, and because contrast is ratiometric the display goes unreadable — which looks exactly like a wiring fault. USB is for flashing and serial.

Build & flash

PlatformIO. The first flash of a blank board must go over USB, because OTA needs firmware that already contains ArduinoOTA.

# USB — the default environment
pio run -t upload
pio device monitor -b 115200

# Over the network, once it is on WiFi
pio run -e esp32-wroom-32-ota -t upload

If esptool reports Wrong boot mode detected (0x13), this board's auto-reset isn't pulling GPIO 0 low: hold BOOT through the Connecting...... dots and release once writing starts.

⚠️ OTA_PASSWORD is "changeme" in src/main.cpp. Anyone on your LAN can reach port 3232. Change it, and keep upload_flags = --auth=… in platformio.ini matching.

First run

1. Power up. The display shows a splash, then tries saved credentials.
2. With none saved it raises an access point, SOLAR_POWER_DISPLAY. Join it and open 192.168.4.1.
3. Enter your WiFi details and the inverter's IPv4 address.
4. To wipe the configuration later, hold the BOOT button for three seconds.

Case

case/ holds a 3D-printable cap for the 20×4 module, under its own licence.

Documentation

ARCHITECTURE.md is the real reference: the runtime state machine, the WiFi supervision logic and why it exists, the Solar API quirks, the memory bugs fixed in the port, and a catalogue of the hardware traps this project has walked into — including the 0 Ω antenna resistor that cost 30 dB, and the serial-terminal DTR line that silently factory-resets the device.

Licence

Creative Commons Attribution-NonCommercial 4.0 International.