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)	~40–50 mA
nRF52840 repeater (RAK4631 / T114)	~8–12 mA
Pi Zero 2W gateway	~100–150 mA
Pi 3B+ gateway	~300–400 mA

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 3–5 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 4–6 "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 2–3 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	3–5 Ah LiFePO4	3 W minimum
ESP32 repeater	45 mA	1.1 Ah	6–10 Ah LiFePO4	5–10 W
Pi Zero gateway	125 mA	3.0 Ah	15–20 Ah LiFePO4	20 W
Pi 3B+ gateway	350 mA	8.4 Ah	40 Ah LiFePO4	50 W