Tools

Will it survive winter?

Most solar node guides size the panel for a sunny average day. This one sizes it for the worst month of the year where you live, then checks how many grey days the battery can ride out.

What's a "solar node"?

A small, self-contained device that powers itself. A small solar panel charges an 18650 or LiPo cell through a charger board, and the battery runs an ESP32 that wakes up, takes a reading and transmits it periodically, then goes back to sleep. Think weather stations, soil sensors, tank levels or a MeshCore repeater on a pole.

  1. Solar panel1–10 W, facing north
  2. ChargerLinear, PWM or MPPT board
  3. Battery18650, LiPo or LiFePO4 cell
  4. ESP32Wakes, reads, sends, sleeps
  5. RadioWi-Fi, ESP-NOW, LoRa or MeshCore

The panel has to put back everything the ESP32 uses, even in the darkest month, with enough stored in the battery to cover a run of cloudy days. That's what this tool checks. The example is a sensor node in Adelaide, and your average current comes straight from the battery life calculator.

Your node

W
Includes dirt, heat and wiring losses
mA
mAh
e.g. two 3000 mAh 18650s in parallel
%
days
Five is a sensible minimum for southern Australian winters
Worst month
–
Your node uses
–
Sun on the panel, worst month
–
Winter margin
–
Battery alone lasts
–

Recommended

Month by month

Harvest covers the loadHarvest falls shortDaily load
Show the monthly figures

How it works

Monthly sun comes from the Bureau of Meteorology's sunshine-hour averages for each city, converted to solar energy with the FAO-56 Angström–Prescott method. It's then worked out for a north-facing panel at your chosen angle using the Erbs diffuse model and the isotropic-sky model (Duffie & Beckman). Results land within about 10–15% of measured winter values, so treat them as a solid first estimate and keep the 30% margin. Shade, dust and a cold battery will all eat into it. Li-ion shouldn't be charged below 0 °C, which matters for Hobart and alpine sites.

Not sure which charger to use? Download the free one-page solar charger guide.