Plotting Solar Panel Voltage and Power

Ever since I bought a cheap solar panel array from Harbor Freight, I’ve wanted to quantify its output. Most solar power instruments were designed for house-sized (or larger) arrays, overkill for my little panel. I had instruments from the world of remote-control hobbies to measure voltage and current, but they were only instantaneous values displayed on screen. I wanted more! This motivated my journey evaluating software tools like InfluxDB database, Grafana visualizer, Home Assistant, and ESPHome. Plus building on what I knew about hardware components like ESP8266 microcontroller, MP1584 buck converter, and INA219 sensor. All with the goal of building my own solar production monitoring system.

I had thought I could make the ESP8266+INA219 sensor node run exclusively on solar power, but I gave up on that and have built a temporary setup powered by old tired alkaline AA batteries. Enough to give me a graph!

This was a fairly sunny day, so cloud cover effects were minimal. The solar panel was connected to a MPPT charger. I could see the power levels (green line) climb smoothly from sunrise, reach a maximum at noon, then smoothly fading off to sunset. This was mostly as expected. The power curve is not symmetric because there are a few things blocking sunlight from the east, reducing morning power.

While the power curve was mostly expected, the voltage levels were not. Until this graph I didn’t know solar panels would jump up so high on voltage before producing meaningful amount of power. (I don’t know if the MPPT charger makes a difference here.) If I had known this, I probably wouldn’t have bothered trying to make a solar powered circuit turn on and off based on voltage. Or if I did, I would have tried anyway and knew why it failed. From this graph I learned that if I am to draw upon solar power, I need to base my decision on power and not on voltage. But in order to read power levels, I need power for my INA219! There’s probably a clever way to circumvent this Catch-22 without batteries, but I’m going to take the easy way out and incorporate a rechargeable battery.

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