How to Size a Solar Lighting System
Panel Size, Battery Capacity, and Runtime Explained
Solar lighting systems must be carefully designed to ensure reliable operation every night. Unlike grid-powered lighting, solar lighting systems rely on stored energy from batteries that are charged during the day by solar panels.
Proper system sizing requires balancing three major components:
• Solar panel capacity
• Battery storage capacity
• LED lighting power consumption
Understanding how these components work together is essential for designing reliable solar street lights, park lighting, and pathway lighting systems.
Solar Lighting System Components
A typical solar lighting system includes four primary components.
Solar Panel
The solar panel captures sunlight during the day and converts it into electrical energy.
MPPT Charge Controller
The MPPT controller regulates energy from the solar panel and charges the battery efficiently.
Battery Storage
The battery stores energy produced during the day so the lighting system can operate at night.
LED Lighting Fixture
The LED fixture uses stored battery energy to produce light during nighttime operation.
In modern solar lighting systems, LiFePO4 batteries are commonly used because of their long lifespan and deep-cycle capability.
For a deeper explanation of solar lighting battery technologies including the differences between Lead-Acid, NiMH, and LiFePO4 batteries, read our guide:
Lead-Acid vs NiMH vs LiFePO4 Batteries: Which Battery Is Best for Solar Lighting Systems
Battery Capacity Calculation
Battery capacity determines how long the solar lighting system can operate at night and how many backup days are available during cloudy weather.
A practical battery sizing formula is:
Battery Capacity (Wh) = LED Power (W) × Night Operation Hours × Days of Autonomy ÷ Allowable Depth of Discharge
For LiFePO4 batteries, many solar lighting systems are designed around 80% maximum depth of discharge (DoD).
Example
LED power: 40W
Night runtime: 12 hours
Autonomy requirement: 3 days
Allowable DoD: 80% (0.8)
Battery capacity calculation:
40 × 12 × 3 ÷ 0.8 = 1800 Wh
This means the solar lighting system requires approximately 1800 Wh of nominal battery capacity to maintain reliable operation.
Solar Panel Sizing
Solar panels must generate enough energy during the day to support the lighting load and recharge the battery.
A simplified daily sizing formula is:
Solar Panel Size (W) = Daily Energy Consumption ÷ Sun Hours ÷ System Efficiency
Where:
Daily Energy Consumption = LED Power × Night Operation Hours
Example
LED power: 40W
Night runtime: 12 hours
Daily energy consumption:
40 × 12 = 480 Wh per day
Average sun hours: 4.5 hours
System efficiency: 90% (0.9)
Panel size calculation:
480 ÷ 4.5 ÷ 0.9 ≈ 118 W
In practical solar lighting design, additional margin is usually added for winter conditions, panel aging, dust accumulation, and cloudy weather. For this reason, a designer might select a 130W–150W solar panel for this application.
Solar Lighting Autonomy
Autonomy refers to the number of days a solar lighting system can operate without receiving sunlight.
Autonomy is important for maintaining operation during:
• cloudy weather
• winter conditions
• storms
• seasonal daylight changes
Typical autonomy values used in solar lighting design include:
Pathway lighting: 2–3 days
Park lighting: 3–4 days
Street lighting: 3–5 days
Higher autonomy improves reliability but increases battery capacity requirements.
Why LiFePO4 Batteries Are Commonly Used
Modern solar lighting systems frequently use LiFePO4 batteries because they offer several advantages for daily charge and discharge cycles.
Key benefits include:
• long cycle life
• deep discharge capability
• stable performance in outdoor environments
• low maintenance requirements
• high safety and thermal stability
These characteristics make LiFePO4 batteries well suited for solar street lights, park lighting, pathway lighting, and off-grid lighting applications.
Conclusion
Proper solar lighting system design requires balancing solar panel capacity, battery storage, and lighting load.
Accurate sizing ensures reliable operation throughout the year while minimizing maintenance and replacement costs.
By using appropriate panel sizing, battery capacity calculations, and reliable battery technology such as LiFePO4, solar lighting systems can provide dependable lighting for parks, streets, pathways, and other outdoor infrastructure.
Contact the 360 Solar Lighting team for project support: https://360solarlighting.com/contact-us/