Live Demo — Solar Autonomy Days Calculator
Enter your lamp load and battery details to see how many days the system survives without sunshine, or find the battery size for a target autonomy.
Solar Autonomy Days Calculator
Free professional solar autonomy calculator for solar lighting systems. Input total lamp power, daily lighting hours, system voltage, battery capacity, depth of discharge, battery efficiency and wiring efficiency to determine how many days the system can operate through consecutive rainy days without solar input, or back-calculate the battery capacity needed for a target autonomy.
Key Features
Load Parameters
Set total luminaire, controller and device power (W) plus daily lighting hours to compute daily load consumption in Wh.
Battery & System Parameters
Choose system voltage, battery capacity, depth of discharge, battery efficiency and system wiring efficiency.
Autonomy Days Result
Instantly see how many days the battery bank can power the load with no effective sunshine before recharging.
Reverse Capacity Calculation
Enter a target autonomy in days to back-calculate the exact battery capacity in Ah your design requires.
Rainy Days Reference
Built-in reference table recommends 2-3, 3-5 or 5-7 autonomy days depending on the regional climate type.
Chemistry-Aware DoD
Follows recommended depth of discharge for lithium (80-90%) versus lead-acid (50-70%) battery chemistry.
Technical Specifications
| Platform | HTML5 Web App | Android (via WebView) |
| Core Calculation | N = C × V × DOD × η_bat × η_sys / E_load |
| Total Lamp Power | 1 W and above (luminaires + controller + devices) |
| Daily Lighting Hours | 0.5 - 24 h/day |
| System Voltage | 12V / 24V / 48V |
| Battery Capacity | 1 - 1000 Ah |
| Depth of Discharge | 10% - 100% (lithium 80-90%, lead-acid 50-70%) |
| Battery & System Efficiency | 50% - 100% each |
| Target Autonomy | 1 - 30 days |
| Industry Standards | AS/NZS 4509.2, IEC 61427-2 |
| Output | Autonomy days for a battery + required battery capacity (Ah) for a target autonomy |
Frequently Asked Questions
How is the autonomy in days calculated?
Autonomy days divide usable stored energy by the daily load: N = C × V × DOD × η_bat × η_sys / E_load, where E_load is lamp power × lighting hours. For a 12V 150Ah LiFePO4 battery at 90% DoD feeding a 600Wh/day load, the result is about 2.3 days.
How do I get the daily load E_load?
Daily load is total lamp power times daily lighting hours (E_load = P × h). Total power includes luminaires, controller and any other devices; lighting hours should reflect the night schedule, which may include dimmed-equivalent hours instead of full-power hours.
Why is the reverse capacity calculation included?
Engineers usually design from a reliability target: "the light must survive 3 rainy days". The reverse formula C = E_load × N / (V × DOD × η_bat × η_sys) solves directly for the battery capacity that achieves the target autonomy, so you can pick a commercial Ah rating immediately.
What does the autonomy result really mean?
It is the number of consecutive days a fully charged battery can keep the load running with no solar input. Longer autonomy improves reliability in bad weather but increases battery cost, so the reference table helps balance design: 2-3 days for sun-rich regions, 3-5 for cloudy regions and 5-7 for persistent rainy regions.
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