Solar Traffic Sign System Sizing: Panel, Battery, and Power Consumption
solar traffic sign system, solar panel sizing, traffic sign power consumption
SOLAR POWER & BATTERIES
9/6/20264 min read
Solar Traffic Sign System Sizing: Panel, Battery, and Power Consumption
A reliable solar traffic sign system must generate enough energy during the day, store enough power for nighttime operation, and continue working through cloudy weather.
Choosing a larger solar panel or battery without calculating the actual load can increase cost and weight unnecessarily. An undersized system, however, may stop operating after several low-sunlight days.
Proper sizing starts with three questions:
How much energy does the sign consume each day?
How many days should the battery operate without solar charging?
How much usable sunlight is available during the worst part of the year?
Step 1: Calculate Traffic Sign Power Consumption
Start by listing every electrical component in the system, including:
LED light bars
Internally illuminated sign faces
Radar sensors
Pedestrian push buttons
Wireless communication equipment
Controllers, photocells, and monitoring devices
For equipment that operates continuously:
Daily energy use (Wh) = Power (W) × Operating hours per day
For example, a device averaging 5 watts over 24 hours uses:
5 W × 24 hours = 120 Wh per day
Activated equipment requires a different calculation. An RRFB may remain in low-power standby most of the day and consume higher power only when flashing. Daily consumption therefore depends on the number of pedestrian activations and the flash duration.
Radar-activated signs should be evaluated using expected traffic volume, activation settings, display brightness, and the amount of time the display remains active.
The average daily load—not only the maximum operating wattage—is what determines the required battery and solar-panel capacity.
Step 2: Determine Required Battery Capacity
The battery must operate the sign when the solar panel is not producing enough energy. This includes nighttime, cloudy weather, snow coverage, and other low-sunlight conditions.
A simplified calculation is:
Required battery energy = Daily energy use × Required backup days ÷ Usable battery percentage
Suppose a sign consumes 120 Wh per day and requires three days of backup:
120 Wh × 3 days = 360 Wh of usable energy
The battery must normally be larger than 360 Wh because most batteries should not be fully discharged.
For example:
AGM and GEL batteries are commonly sized using a conservative depth of discharge to support longer service life.
LiFePO₄ batteries can often provide a greater percentage of their rated capacity, subject to the manufacturer’s specifications and battery-management settings.
Additional capacity may be required in cold weather because available battery capacity decreases as temperature falls.
To convert watt-hours to amp-hours:
Battery capacity (Ah) = Required energy (Wh) ÷ Battery voltage (V)
A nominal 12-volt system requiring 600 Wh of rated battery capacity would need approximately:
600 Wh ÷ 12 V = 50 Ah
This is only a preliminary calculation. Battery chemistry, temperature, aging, discharge rate, controller cutoff voltage, and desired service life must also be considered.
Step 3: Size the Solar Panel
The solar panel must generate enough energy to operate the sign and replace energy removed from the battery.
A simplified solar panel sizing formula is:
Panel wattage = Daily energy use ÷ Peak sun hours ÷ System efficiency
If the system uses 120 Wh per day, receives three peak sun hours during the design month, and has an estimated overall efficiency of 70%:
120 Wh ÷ 3 hours ÷ 0.70 = approximately 57 W
A designer may select the next appropriate panel size above this result and add a suitable engineering margin.
“Peak sun hours” do not mean the number of daylight hours. They represent the equivalent number of hours during which solar irradiance averages 1,000 W/m².
Solar-panel sizing should be based on the project location and the lowest practical seasonal solar resource—not the annual average or summer conditions. The NREL PVWatts Calculator can help estimate monthly solar production for a specific location.
Account for Real-World Losses
A solar panel rarely produces its rated power throughout the day. Actual output may be reduced by:
Clouds, haze, rain, or snow
Shade from trees, buildings, or sign structures
Dirt and debris on the panel
Incorrect panel direction or angle
High panel temperature
Wiring voltage loss
Charge-controller efficiency
Battery charging losses
Battery aging
The U.S. Department of Energy notes that solar production varies with season, time of day, clouds, dust, shade, and other environmental conditions. Energy storage also involves conversion losses and is not 100% efficient. U.S. Department of Energy
These losses should be included in the design rather than assuming that a 40-watt panel will continuously provide 40 watts.
Consider the Operating Mode
The operating mode can significantly affect traffic sign power consumption.
Continuous Flashing
A sign that flashes continuously, 24 hours a day, generally requires more daily energy than an activated sign.
Push-Button Activation
An RRFB or pedestrian warning sign normally remains in standby until activated. Its energy consumption depends on the number of activations and the selected flash duration.
Radar Activation
A radar-activated sign may operate frequently on a busy roadway. The designer should estimate realistic daily traffic volume rather than sizing the system based only on light traffic.
Nighttime Illumination
An internally illuminated sign may operate for 10–14 hours per night, depending on the location and season. Winter nights are longer and often coincide with lower solar production, making winter the most important design period.
Match the Controller to the Battery and Panel
The solar charge controller must be compatible with:
System voltage
Maximum solar-panel voltage and current
AGM, GEL, or LiFePO₄ charging requirements
Battery temperature limits
Low-voltage disconnect settings
Maximum load current
Lead-acid batteries generally benefit from temperature-compensated charging. LiFePO₄ batteries require a compatible charging profile and should have a battery management system that prevents charging below the manufacturer’s minimum temperature.
Panel voltage must also provide enough charging headroom for the selected battery and controller. Matching only the panel’s wattage is not sufficient.
Avoid These Common Sizing Mistakes
Common problems include:
Using summer sunlight data for a year-round system
Ignoring controller and standby consumption
Selecting a battery based only on amp-hours
Failing to include cold-weather capacity reduction
Assuming the solar panel always produces rated power
Ignoring shade, snow, dirt, and panel orientation
Using an incompatible battery charging profile
Providing too little battery autonomy for consecutive cloudy days
Final Selection
A properly designed solar traffic sign system balances the load, solar panel, battery, controller, climate, and required operating time.
The best configuration is not always the system with the largest panel or battery. It is the system that can reliably support the traffic sign during the project’s most difficult operating conditions while remaining practical to install and maintain.
Zoodico can help evaluate traffic sign power consumption, solar-panel capacity, battery options, operating modes, and environmental conditions for RRFB systems, LED-enhanced signs, internally illuminated signs, radar signs, and other electronic traffic safety equipment.
Contact Zoodico Traffic Solutions to discuss your sign type, project location, activation method, required backup time, and solar-power requirements.
All calculations in this article are simplified examples. Final system sizing should use the actual equipment load, battery specifications, local solar-resource data, temperature range, and project requirements.
Solar Traffic Sign System Sizing Guide
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