Solar and battery
How to Size Solar Power for a Remote Sensor
A remote sensor needs enough stored energy to keep reporting through dark days, and enough panel output to refill that reserve. Here is a practical way to work through both sides.
Solar power for remote sensors starts with a question that a panel label cannot answer: how much energy must the whole node use between visits? A weather station, gate monitor or soil sensor may spend most of its time asleep, but its radio, controller and power electronics still draw energy. Size the load, battery and panel as a system before choosing parts.
This guide is for small, isolated, low-voltage devices. It is not a plan for connecting a system to a building supply or the grid. Have a licensed electrician handle any mains or grid-connected electrical work.
Start with daily energy, not panel wattage
List every powered part: sensor, microcontroller, radio, voltage regulator, charge controller and any status light or heater. A board's active current is not its all-day current. Record how long it sleeps, samples, transmits and waits for a response. If you have the hardware, measure current at the supply in each state, including the overnight idle period. Manufacturer data is useful for a first estimate, but the finished assembly may behave differently.
Multiply power in watts by time in hours to get energy in watt-hours. For each state, multiply its power by its hours per day, then add the results. If you measure current instead, multiply current in amps by the voltage at that measurement point to estimate watts. Do not add milliamp-hours measured at different voltages as if they were the same energy. Count the standby draw of anything left connected while the sensor sleeps.
Illustrative example: a node averaging 2 watts over a whole day uses 48 watt-hours per day. That is an example for the arithmetic, not a typical sensor specification. Your own measured average is the number that matters.
Decide how many dark days it must survive
The battery carries the sensor at night and when the panel cannot produce enough. Choose the number of low-sun days you want it to endure without a visit. Multiply that by daily energy use to find the minimum usable battery energy before allowing for cold, ageing and system losses. A battery's nameplate capacity is not always the energy you can safely take out of it. Check the discharge limits and temperature guidance for the exact battery chemistry and model.
Illustrative example: at 48 watt-hours per day, three days with no useful panel output consume 144 watt-hours. A nominal 12-volt, 20-amp-hour battery holds 240 watt-hours by simple multiplication, but that is not a promise of 240 usable watt-hours in the field. The allowable discharge, conversion losses and cold-weather performance still have to be checked. Do not treat the example as a shopping list.
Think about what a low-battery day should do. Can the node send fewer updates, stop a nonessential display, or alert you before it goes offline? Reducing its load may be more effective than fitting a larger panel into a shaded site.
Check panel output in the hardest season
A panel's rated watts describe output under specified test conditions, not continuous power at your location. Estimate the site's available sunlight for the month when the sensor must work but light is scarcest. Account for tree and building shade, the panel's direction and tilt, snow cover and the controller's conversion losses. Summer production should not be used to justify a winter deployment.
As a rough calculation, multiply panel rating in watts by estimated effective sunlight hours to get a gross daily watt-hour figure. Illustrative example: a 40-watt panel with two effective sunlight hours could produce 80 watt-hours before shade, wiring and charging losses. A node using 48 watt-hours daily would have less than 32 watt-hours of that gross figure left to refill a drained battery, and losses make the real margin smaller. That does not establish that the panel is large enough. Use location-specific monthly solar data and site observations to test the plan, particularly after several cloudy days.
A battery sized for three dark days does not help indefinitely if the panel only replenishes the average day's usage. Ask how quickly the system can recover its reserve when sunlight returns. If the numbers are tight, improve placement or reduce radio and sensor duty cycle before assuming a larger panel will solve a badly shaded location.
Match the charging and power path
Check that the panel's voltage and current suit the chosen charge controller, and that the controller supports the battery chemistry and its allowed charging temperatures. Verify that the sensor's power input matches the output after regulation. A nominal battery voltage is not necessarily its actual voltage throughout a charge cycle. Consider the controller's own standby draw in the daily load estimate.
Use appropriate overcurrent protection and weather-resistant cable routes and connections for the actual installation. Keep condensation, strain on connectors and access for maintenance in mind when selecting an enclosure. Some battery types must not be charged below their specified temperature; follow the manufacturer's instructions rather than assuming an enclosure alone makes winter charging safe. If the site involves building wiring or a grid connection, stop at planning and involve a licensed electrician.
Test the assembled node before deployment
Put the real sensor, radio, controller, battery and panel together and watch what happens when the node wakes and transmits. Check whether signal retries keep it awake longer than planned. Log battery voltage or a more useful state-of-charge measurement where the battery system supports one. A voltage reading by itself may not reliably show the remaining capacity under changing load and temperature.
Run a supervised low-light test, check the controller's behaviour near the battery's discharge limit, and confirm the node can recover when charging resumes. Keep notes on the measured daily use, site shade and any unexpected resets. A bench test cannot reproduce every winter condition, but it can expose a mismatched power path before the box is mounted out of reach.
What to send for a useful plan
Write down what the sensor measures, how often it reports, the radio type and the longest gap between site visits. Add the device and battery specifications, any current measurements, photos of the proposed panel location, its shade through the day, the cable route and what you have already tried. Say what success looks like, including whether missing an update is acceptable. That gives anyone helping you a real load, site and maintenance target instead of a guess about panel size.
Aegis Engineered Solutions works on device-level solar and battery setups, including panel placement, battery sizing and wiring-route planning. To discuss a remote sensor scope, email admin@aegisequity.ca or call (705) 341-3980.