Solar System Sizing Guide
Goal
Size your battery to survive N cloudy days, and size your panel to fully recharge that battery in one average sunny day.
Step 1 —- Measure Your Device's Actual Current Draw
Use a USB power meter or a multimeter in series with the power supply. Do not rely solely on datasheet figures —- real-world draw depends on firmware, radio duty cycle, and accessories.
Typical average current values:
| Device | Avg Current |
|---|---|
| ESP32 repeater (no display, no BT) | ~ |
| nRF52840 repeater (RAK4631 / T114) | ~ |
| Pi Zero 2W gateway | ~ |
| Pi 3B+ gateway | ~ |
Step 2 —- Calculate Daily Energy
Formula: (mA × 24 hours) / 1000 = Ah/day
Example: 40 mA × 24 = 0.96 Ah/day for an ESP32 repeater.
Step 3 —- Size the Battery for 3–3 - 5 Days Without Sun
Multiply daily energy by the number of cloudy days you want to survive:
0.96 Ah/day × 5 days = 4.8 Ah minimum
For LiFePO4, do not discharge below 20% —- usable capacity = rated × 0.8. So a 6 Ah LiFePO4 provides 4.8 Ah usable. That covers our 5-day ESP32 example exactly.
Step 4 —- Size the Solar Panel
A solar panel produces its rated wattage for approximately 4–4 - 6 "peak sun hours" per day at a typical US latitude on a clear day.
Formula: panel watts / panel voltage × peak sun hours × 0.8 efficiency = Ah/day charged
Example: 5 W panel, 6 V output, 5 peak sun hours, 0.8 efficiency →
5 / 6 × 5 × 0.8 = 3.3 Ah/day
More than enough to recharge the 0.96 Ah/day ESP32 repeater, even in winter.
Step 5 —- Account for Worst-Case Latitude and Season
Latitude >45° (northern US, Canada) in winter may have only 2–2 - 3 peak sun hours. Always size the panel for your winter minimum, not the summer average.
Quick Reference Table
| Device | Avg mA | Ah/day | Recommended Battery | Recommended Panel |
|---|---|---|---|---|
| nRF52840 repeater | 10 mA | 0.24 Ah | 3 W minimum | |
| ESP32 repeater | 45 mA | 1.1 Ah | ||
| Pi Zero gateway | 125 mA | 3.0 Ah | 20 W | |
| Pi 3B+ gateway | 350 mA | 8.4 Ah | 40 Ah LiFePO4 | 50 W |