Solar panels are only useful if you know what they are actually producing. This ESP32 solar power monitor measures panel voltage, charging current and battery level, and shows the readings on a small display, so you can spot a weak panel, a tired battery or a loose connection before it becomes a real problem.
How It Works
A small solar panel charges a battery through a charge controller module. The ESP32 reads the panel voltage and the battery voltage through simple voltage divider circuits, and reads the charging current with a low-cost current sensor module. It calculates approximate power from voltage and current, updates an OLED display every few seconds, and can optionally send the readings to a phone dashboard over Wi-Fi.
Components Required
- ESP32 DevKit board
- Small solar panel (example: 6V panel for learning builds)
- TP4056 or similar lithium battery charging module
- 18650 lithium cell with holder (or a small sealed lead-acid battery with a matching charger)
- ACS712 current sensor module (example: 5A version)
- 0.96 inch OLED display (I2C)
- Resistors for two voltage dividers (example values: 100k and 10k, to be checked against your panel voltage)
- Blocking diode, jumper wires, breadboard
Circuit Connections
- Solar panel positive through the blocking diode to the charger module input; panel negative to charger ground.
- Battery to the charger module output, observing polarity carefully.
- Panel voltage divider output to ESP32 GPIO34; battery voltage divider output to GPIO35.
- ACS712 output to GPIO32, VCC to 5V, GND to GND.
- OLED SDA to GPIO21, SCL to GPIO22.
- Keep the ESP32 powered from a stable 5V or 3.3V supply, separate from the raw panel output.
Calibration Matters
Divider resistor values must suit your own panel and battery voltages, so treat the example values above as a starting point only. Measure the real panel voltage with a multimeter in bright sun, then choose divider values that keep the ESP32 input safely below 3.3V. Compare the displayed readings against the multimeter, adjust the scaling factor in your code, and recheck after a few days of sun and shade.
Build Steps
1. Wire the dividers and current sensor on the breadboard first, without the solar panel connected.
2. Power the ESP32, confirm the OLED shows sensible voltages, and calibrate against a multimeter.
3. Connect the panel and battery through the charger module, and watch the readings rise in sunlight.
4. Add a simple alert in code, for example a warning when battery voltage falls below your chosen low threshold.
5. Mount the circuit in a weatherproof box if it will live outdoors, with the panel outside and the electronics protected from rain.
Safety Note
This project uses low-voltage DC only and does not involve mains electricity. Even so, lithium batteries need care: use a proper charger module, never short-circuit a battery, and stop immediately if a cell swells or gets hot. If you are unsure about battery wiring, ask an experienced maker to check your circuit before first use.
What will you monitor first — a garden light battery, a gate opener, or a small off-grid setup? Share your readings in the comments.
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