Design a modular, duty‑cycled stack. Put a tiny MCU with deep‑sleep mode at the core. Use low‑power humidity, temperature, and conductivity probes that pull samples only on a schedule—say every 15 minutes or when a change flag is set. Buffer the raw data in the MCU’s RAM and batch‑write to a small flash; only wake the radio for a few hundred milliseconds to send the packet, then back to sleep. Use a solar panel sized to just cover the average drain: calculate the total energy in a day, pick a panel that delivers 120–150 % of that, and choose a Li‑Poly with 5 Ah. Add a supercapacitor to smooth the spike when the MCU wakes. Keep the radio low‑power too: LoRa or BLE‑mesh with a low‑drain sleep state. Finally, run a calibration routine once a week via OTA, but only if the MCU sees that the battery voltage is above a threshold; otherwise, postpone until the battery is safe. That keeps the battery alive while still giving you accurate, up‑to‑date water‑quality data.