A node on a roof is only useful if it's still running in February. This is the arithmetic, with real numbers, so you can size it instead of guessing.
Step 1: know your current draw
Average current depends mostly on the chip and the role:
| Board | Idle average | Notes |
|---|---|---|
| RAK 4631 (nRF52) | ≈ 15-30 mA | Screen off, GPS off. This is why it's the roof board. |
| T-Echo (nRF52) | ≈ 20-40 mA | E-paper costs nothing between refreshes. |
| Heltec V3 / T-Beam (ESP32) | ≈ 80-150 mA | Wi-Fi or Bluetooth on pushes it higher. GPS adds 20-40 mA. |
Transmit bursts are short (hundreds of mA for a fraction of a second); for a budget, the average is what matters. Measure yours with a USB meter if you can - one evening tells you more than any table.
Step 2: daily energy
mA × 24 h = mAh per day RAK at 25 mA → 600 mAh / day ESP32 at 120 mA → 2,880 mAh / day
Step 3: how much battery
You want to survive the longest run of dark days you'll see. In Oklahoma, three or four overcast days in a row is a normal winter week. Aim for 4 days of autonomy and don't drain lithium below about 20%:
600 mAh/day × 4 days ÷ 0.8 = 3,000 mAh → one good 18650 for a RAK 2,880 × 4 ÷ 0.8 = 14,400 mAh → four to five 18650s for an ESP32
This is the whole argument for nRF52 on solar in one line.
Step 4: how much panel
A panel's watt rating is for perfect sun. In winter at 36° north you'll get roughly 3 peak-sun hours a day on a decent day, and a solar charger loses another 15-20%. So a panel delivers about:
panel W ÷ 5 V × 3 h × 0.8 = mAh recovered per sunny day 5 W → ≈ 2,400 mAh/day 10 W → ≈ 4,800 mAh/day
The panel must refill what you used and catch up after the cloudy stretch. For a RAK using 600 mAh/day, a 5-6 W panel has a comfortable 4× margin. For an ESP32 at 2,880 mAh/day you want 10 W or more, tilted, facing south - and you'll still lose it some Januaries.
Build notes
- RAK and T-Beam boards have a solar/battery charger on board; feed the panel into the solar input (usually 5-6 V max - check), not USB.
- Use a real 18650 with a protection circuit, or a small LiFePO4 pack if it'll bake in the sun.
- Height first. A node at 60 ft on a 3 dBi antenna beats one at 20 ft on 6 dBi, every time.
- Set the role thoughtfully. ROUTER is for nodes that genuinely have the best view in the area and always-on power; it relays everything and stays awake. A modest rooftop node is often better as CLIENT so it doesn't become a bottleneck. Ask in your state group before choosing ROUTER.
- Turn off Bluetooth, Wi-Fi (unless it's your MQTT uplink), screen and GPS on a fixed node. Set the position manually once.
Worked example, Turkey Mountain style: RAK 4631 at ~25 mA, one 3,500 mAh 18650, 6 W panel facing south. Survives a five-day overcast stretch and recovers in two sunny ones. Total parts around $90, most of it the enclosure and antenna.