Editorial owner: MCL Solar Knowledge Center. Technical scope: Final sizing must be verified against the selected model, site coordinates, climate data, and contract documents. Last updated: July 16, 2026.
Key takeaways:
- Rainy-day backup is an energy-balance requirement, not a fixed number for every project.
- Battery autonomy and photovoltaic recovery after rain are separate calculations.
- Use worst-month peak sun hours, local temperature, the actual dimming schedule, and continuous auxiliary loads.
- State every assumption so the result can be checked during tender review and commissioning.
Step 1: calculate daily load
Daily load (Wh) equals the sum of LED input power multiplied by operating hours for every dimming period, plus camera, communications, sensing, and standby loads that run outside the lighting schedule.
Step 2: calculate rated battery energy
Rated battery energy (Wh) = daily load x autonomy days / (usable depth of discharge x discharge-path efficiency x temperature derating factor).
Battery capacity (Ah) = rated battery energy (Wh) / nominal battery voltage. Do not divide daily load by voltage without applying the project assumptions above.
Step 3: calculate photovoltaic power
PV power (W) = daily load / (worst-month peak sun hours x combined PV, controller, wiring, and soiling derating). Peak sun hours should come from long-term data for the project coordinates and the selected array tilt, such as NASA POWER or an equivalent authoritative source.
Step 4: check recovery after rain
After an autonomy event, the array must supply the current night’s load and recharge the energy deficit. Check the allowed recovery period separately. Increasing autonomy days in the battery calculation does not automatically prove that the selected panel can restore the battery within the required time.
Worked-method template
| Input | Project value to record |
|---|---|
| Coordinates and climate dataset | Location, source, years, and worst month |
| Lighting profile | LED input power and hours at each dimming level |
| Auxiliary load | Camera, modem, sensor, heater, or standby Wh/day |
| Battery assumptions | Chemistry, nominal voltage, usable DoD, efficiency, temperature factor |
| PV assumptions | PSH, tilt, controller, wiring, soiling, and module derating |
| Recovery requirement | Maximum days allowed to return to target state of charge |
Controller and battery notes
MPPT is not automatically mandatory for LiFePO4. The controller must provide the correct charging profile, protections, conversion efficiency, and voltage compatibility. Any benefit over PWM depends on array-to-battery voltage difference, temperature, irradiance, and operating point; it should not be presented as a fixed percentage.
Battery cycle life must state depth of discharge, C-rate, temperature, and capacity end condition. Cycle count alone cannot be converted into a guaranteed number of service years.
Limitations
This method supports preliminary sizing. Final values require the exact luminaire, controller, battery test data, site geometry, long-term climate data, and contractual performance criteria.
Separate autonomy from recovery time
Autonomy is the number of nights the approved load can be supplied from the defined usable battery energy under stated conditions. Recovery time is how long the charging system needs to restore the battery after that sequence. A system can have a large battery and still recover too slowly if the panel, controller or solar resource is insufficient.
For long rainy seasons, model a sequence of daily solar inputs rather than treating every rainy day as zero charging. State the source of the solar data, the dimming and low-state-of-charge logic, the minimum reserve, the temperature assumptions and the recovery criterion.