Step 1: Collect real current numbers
A node's energy use comes from four things: how much current it draws in deep sleep, and how much it draws while it wakes, reads sensors and talks over LoRa. Measure these on your own board with a current profiler or a series ammeter. Datasheet values for the chip alone are not enough, because the voltage regulator, sensors and leakage paths often dominate the sleep current of the finished board.
The table uses example values for a generic ESP32-S3 node with an external LoRa transceiver. They are illustrative and are not measured specifications of any e.FM product.
| State | Current | Duration per event |
|---|---|---|
| Deep sleep (whole board) | 10 µA | between events |
| Wake and read sensors | 50 mA | 1 s |
| Radio transmit | 120 mA | 0.5 s |
| Radio receive windows (Class A) | 15 mA | 2 s (conservative) |
Step 2: Calculate the daily charge
Charge per event: 50 mA × 1 s + 120 mA × 0.5 s + 15 mA × 2 s = 140 mA·s, which is about 0.039 mAh.
With one uplink every 15 minutes there are 96 events per day, so events use 96 × 0.039 ≈ 3.7 mAh. Sleep adds 0.010 mA × 24 h = 0.24 mAh. The total is about 4.0 mAh per day, or roughly 15 mWh at 3.7 V.
The uplink interval is the biggest lever you have:
| Uplink interval | Events per day | Daily charge |
|---|---|---|
| 5 minutes | 288 | ≈ 11.4 mAh |
| 15 minutes | 96 | ≈ 4.0 mAh |
| 60 minutes | 24 | ≈ 1.2 mAh |
Step 3: Size the battery
Decide how many days the node must run without any sun, for example 7. At 4.0 mAh per day that is 28 mAh. Then derate: only part of a cell's rated capacity is usable (plan on roughly 70 to 80 percent), capacity drops in the cold, and cells age.
In this example a common 1000 mAh LiPo cell holds about 800 mAh of usable charge, or around 200 days of operation with no sun at all. Autonomy is therefore not the limiting factor. Choose the cell by temperature range, cycle life and physical size instead.
One rule matters more than capacity: standard Li-ion and LiPo cells should not be charged below about 0 °C. For cold sites use a charger with a temperature cut-off, or a cell rated for low-temperature charging.
Step 4: Size the solar panel
Peak sun hours (PSH) are the hours per day of full 1000 W/m² sunlight that would deliver the same energy as the real day. Look up the value for your coordinates and panel tilt with a tool such as PVGIS or NASA POWER, and use your worst month, not the average.
Required panel rating (W) ≈ daily energy (Wh) ÷ (charging efficiency × peak sun hours × derating factor).
With 0.0147 Wh per day, 70 percent charging efficiency, 2 PSH in winter and a 0.5 derating factor for dust, angle and heat: 0.0147 ÷ (0.7 × 2 × 0.5) ≈ 0.021 W, about 21 mW. In practice nodes use a larger panel, often 0.5 to 1 W, because the physical size is small and it covers cloudy weeks and dirty glass.
What changes the result
- Uplink interval. Send only as often as the application needs. See the table above.
- Spreading factor. Airtime roughly doubles with each step up in spreading factor, and so does the energy per message. Use the lowest SF that gives a reliable link. In some regions, such as EU868, duty-cycle limits also cap how often you may transmit.
- LoRaWAN class. Class A opens receive windows only after an uplink. Class C keeps the receiver on. At 15 mA continuously that is 360 mAh per day, about 90 times the example node, so Class C is not a fit for small solar nodes.
- Sensor warm-up. Some sensors need seconds to stabilise. Power them through a load switch only while sampling.
- Temperature. Cold reduces battery capacity and charging ability.
Common mistakes
- Using the chip's datasheet sleep current instead of a board-level measurement.
- Ignoring the quiescent current of the voltage regulator and the leakage of pull-ups, LEDs and sensors left powered.
- Sizing the panel on summer sunshine.
- Charging a LiPo cell below freezing.
- Mounting the panel flat where dust or snow collects.
Where to go next
SolarNode is e.FM's LoRaWAN controller designed for solar operation with a LiPo backup. If you want an energy-monitoring setup planned around your own site, see our Energy Intelligence service, and see e.FM Cloud for dashboards of production, battery state and consumption.