What this estimate does and does not do
This calculator estimates the solar panel and battery size for a small remote IoT sensor node, such as an ESP32, Arduino-class logger, LoRa sensor, wildlife camera trigger, or low-power telemetry device.
It is not a full solar engineering design. It does not check charge-controller current limits, lithium protection circuits, temperature charging limits, panel open-circuit voltage, battery chemistry constraints, cable voltage drop, shading, enclosure heat, or safety compliance. Use the result for early sizing, then validate with measured current draw and field testing.
Formula used
daily load mAh = average current mA × 24
daily load Wh = average current A × load voltage × 24
required panel W = daily load Wh / (peak sun hours × system efficiency) × safety factor
required battery Wh = daily load Wh × autonomy days / (usable DoD × battery efficiency)
required battery Ah = required battery Wh / battery voltageThe calculator also estimates the approximate panel charging current at battery voltage:
panel charge current A ≈ required panel W / battery voltageWorked example
For a node averaging 25 mA at 3.3 V:
daily mAh = 25 × 24 = 600 mAh/day
daily Wh = 0.025 × 3.3 × 24 = 1.98 Wh/dayWith 4 peak sun hours, 70% system efficiency, and 25% panel margin:
required panel = 1.98 / (4 × 0.70) × 1.25 = 0.88 WFor 3 days autonomy on a 3.7 V battery, 80% usable DoD, and 90% battery round-trip efficiency:
required battery Wh = 1.98 × 3 / (0.80 × 0.90) = 8.25 Wh
required battery Ah = 8.25 / 3.7 = 2.23 AhHow to use the result
- Measure real current draw if possible; datasheet current is often wrong for the full system.
- Use worst-month peak sun hours, not annual average, for year-round unattended devices.
- Oversize the panel if the node is shaded, dirty, behind glass, or hard to access.
- Choose battery chemistry and charging electronics that match the deployment temperature.
- Verify that your charge controller can handle the panel voltage and current.
- Field-test the node through cloudy periods before relying on it remotely.
Assumptions and limitations
- Average current should include all operating modes and regulator losses unless those losses are included in system efficiency.
- Peak sun hours already account for solar resource, but not all local shading or orientation losses.
- Battery Ah is given at the battery voltage, not necessarily at the load voltage.
- The “suggested” panel and battery values round up to common planning sizes, not specific products.
- Very low-power nodes can be dominated by charger quiescent current and regulator leakage.
- Very high-power nodes may need larger wiring, fuses, MPPT controllers, thermal design, and safety review.
Frequently asked questions
How do you size a solar panel for an IoT node?
Estimate daily load in watt-hours, then divide by peak sun hours and system efficiency. Add a safety margin for clouds, panel aging, dirt, temperature, and converter losses.
How do you size the battery?
Multiply daily watt-hours by desired autonomy days, then divide by usable depth of discharge and battery round-trip efficiency. Convert watt-hours to amp-hours by dividing by battery voltage.
What are peak sun hours?
Peak sun hours are the equivalent full-strength solar hours per day. They are not the same as daylight hours. Use worst-month or winter values for unattended remote nodes.
Should I use average current or active current?
Use average current over a full day, including sleep, sensing, transmit, regulator, and idle current. If you only use active current, the estimate may be too high or too low depending on duty cycle.
Why add a safety margin?
Small solar nodes are sensitive to shade, dirt, clouds, temperature, charger losses, and battery aging. A margin helps prevent the battery from slowly drifting down over time.