Solar Street Light Sizing Is an Energy-Balance Problem
A catalog line such as 40 W solar street light looks like a design input. It is only an instantaneous load. The system must supply energy to the luminaire every night, store enough energy for low-sun periods, and replace what was used under the site’s actual solar conditions.
The central design question is not how many watts appear on the product label. It is how many watt-hours the project consumes on a required night and how reliably the photovoltaic array can recover that energy.
Assume a 40 W luminaire runs at full output for five hours and at 30% output for seven hours. Its LED-side energy use is:
40 × 5 + (40 × 0.30 × 7) = 284 Wh per night
If the 40 W figure represents LED-board power rather than complete fixture input, a 10% allowance for driver and controller losses raises battery-side demand to approximately 312 Wh. A full-output schedule would consume 480 Wh, about 54% more than the dimmed example. The battery and panel cannot be sized correctly until that operating profile is known.
Start with the required light, not the catalog wattage
Energy calculations should begin after the photometric requirement is established. Road width, mounting height, pole spacing, roadway classification, target illuminance or luminance, uniformity, glare limits, and optic distribution determine how much light the fixture must produce.
Two fixtures can both carry a 40 W label while producing different results on the road. One may meet the required uniformity at the planned spacing; another may leave dark areas and force a higher operating level. A fixture that works at one pole height or spacing may fail at another even though its electrical rating has not changed.
A defensible design therefore records:
- The required lighting class and minimum performance criteria.
- Pole height, spacing, road geometry, and optic distribution.
- The actual luminaire input power at each programmed dimming level.
- The photometric file and simulation result, often verified in DIALux or equivalent software.
- The hours of operation and any motion-sensor or adaptive-lighting rules.
The LED-board rating is not a substitute for actual input power. It may describe a maximum component rating rather than the power drawn by the complete luminaire at the required light level.
Convert the operating schedule into nightly watt-hours
The basic load calculation is:
Enight = Σ(Pi × ti) + Eaux
Here, Pi is the actual power drawn at operating level i, ti is the time spent at that level, and Eaux covers energy from the controller, sensors, communications equipment, and any conversion losses not already included in the power measurement.
Dimming percentage should not automatically be treated as an identical percentage of electrical power. Some drivers are close to linear; others have a minimum operating draw or a different efficiency curve at reduced output. The controller data or a measured load should determine the value used in the calculation.
Motion detection requires the same discipline. An average traffic estimate can produce an undersized system if the project must maintain a defined lighting level during a high-use period. The design schedule should reflect the service level that the municipality or road operator expects, including the longest credible high-output period.
This step also prevents misleading comparisons between supplier proposals. A 60 W lamp operating at full output for 12 hours consumes 720 Wh before losses. The same lamp operating for four hours at full output, four hours at 50%, and four hours at 20% consumes only:
60 × (4 + 2 + 0.8) = 408 Wh
Those two systems should not receive the same battery or panel simply because both are described as 60 W products.
Size the battery around usable energy
Autonomy means the number of nights the system must deliver the specified lighting service with little or no useful solar harvest. Three days of autonomy should mean three complete nights at the defined output, not merely three days until the battery reaches a low-voltage cutoff.
A useful starting relationship is:
Nominal battery energy = (Enight × autonomy days) ÷ (usable DoD × available-capacity factor)
The available-capacity factor accounts for conditions such as low temperature, end-of-life capacity, and any manufacturer-specific derating. Use the supplier’s discharge data where available rather than hiding every uncertainty inside a large, unexplained margin.
Using the 312 Wh example, three nights of autonomy, a 90% permitted depth of discharge, and a 90% effective-capacity factor produce:
312 × 3 ÷ (0.90 × 0.90) = approximately 1,156 Wh nominal battery capacity
At a nominal 24 V, that is roughly 48 Ah before selecting the next available pack size and checking the manufacturer’s voltage, temperature, BMS, and end-of-life specifications.
Battery chemistry changes the equation. For illustration, a lead-acid design that permits only 50% depth of discharge and has an 85% effective-capacity factor would require approximately 2.2 kWh nominal for the same three-night load. A lithium design may use a higher permitted depth of discharge, but the actual value still depends on the cell supplier, warranty terms, operating temperature, and charge and discharge limits.
Amp-hours alone are not enough to compare batteries. A 100 Ah battery at 12 V has roughly 1.2 kWh nominal energy; a 100 Ah battery at 24 V has roughly 2.4 kWh. Neither figure states how much energy the system can safely deliver. A credible specification must state nominal voltage, nominal watt-hours, usable depth of discharge, low-voltage cutoff, temperature limits, and the capacity expected at the end of the warranty period.
Charging efficiency belongs in the photovoltaic recharge calculation and should not be counted twice in the battery calculation. If the nightly load has already been converted to battery-side energy, apply battery capacity derating to the storage calculation and charging-path losses to the panel calculation.
Size the panel for the worst charging month
For a battery that is full at the beginning of the night, the minimum daily array estimate can be expressed as:
PV array power = daily energy to replace ÷ (worst-month peak sun hours × total solar-path efficiency)
Peak sun hours are equivalent hours at standard solar intensity, not the number of daylight hours. The solar-path efficiency should account for module temperature, soiling, wiring, controller and MPPT losses, orientation, battery charging losses, and other site conditions. A value such as 0.75 may be a reasonable preliminary assumption, but it is not a universal constant.
For 312 Wh per night, three worst-month peak sun hours, and 75% total efficiency:
312 ÷ (3 × 0.75) = approximately 139 W
That is a maintenance-array estimate before an explicit recovery margin. The final array may need to use the next standard size or a larger margin after checking shading, dust, controller limits, panel temperature, and the required time to restore autonomy.
The same load changes significantly with location. Using the same illustrative 75% efficiency:
- At 4.0 worst-month peak sun hours: 312 ÷ (4.0 × 0.75) = approximately 104 W.
- At 2.2 worst-month peak sun hours: 312 ÷ (2.2 × 0.75) = approximately 189 W.
A single catalog configuration cannot represent both sites accurately. Annual average solar radiation can also hide a seasonal failure if the system must operate through a cloudy winter or monsoon month. The design basis should identify the critical month and the source of the solar-resource data.
Panel nameplate power is another common source of confusion. A 150 W module is rated under standard test conditions, not under a hot, dusty installation with a nonoptimal tilt. The controller must also accept the panel’s operating voltage and current. A larger module connected to an undersized controller does not automatically deliver its nameplate energy.
Make autonomy and recovery agree
Battery autonomy and panel capacity are linked. A three-day battery with a panel sized only to replace one ordinary night may protect the lights during a short cloudy period but recover very slowly after the reserve has been used.
Suppose the 312 Wh system has a three-night reserve and the project requires a fully depleted battery to recover in two clear days. The array must collect energy for both the two nights that still need lighting and the three nights of reserve being rebuilt:
Required recovery energy = (3 × 312) + (2 × 312) = 1,560 Wh
At two recovery days, three peak sun hours per day, and 75% efficiency:
1,560 ÷ (2 × 3 × 0.75) = approximately 347 W
A 347 W array is not automatically the correct product selection. It illustrates why a recovery requirement can produce a much larger array than a simple daily-energy calculation. If the project permits four recovery days, temporary dimming, or a lower reserve target, the required panel size changes. Those operating rules must be written into the specification rather than left to an assumption.
The specification should expose every assumption
A supplier quotation becomes comparable when it shows the chain from lighting performance to energy balance. The essential fields are:
- Photometric load: required lighting performance, actual input watts at each dimming level, and the operating schedule.
- Nightly energy: watt-hours drawn from the battery, including controller and auxiliary loads.
- Autonomy definition: number of full-service nights, permitted depth of discharge, low-voltage behavior, and whether the calculation is based on new or end-of-life capacity.
- Solar basis: project coordinates, critical-month peak sun hours, panel orientation, shading, soiling assumptions, and temperature conditions.
- Recovery target: the number of clear days allowed to restore the battery after a low-sun event.
- Hardware limits: panel nameplate power and voltage, controller charging limits, battery nominal and usable watt-hours, BMS protections, and temperature restrictions.
- Verification: photometric simulation, energy-model assumptions, factory data, and commissioning measurements.
A proposal that lists only 100 W panel, 100 Ah battery, and three days of autonomy is not yet an engineering specification. The battery voltage and usable capacity are unknown, the panel’s site energy is unknown, and the nightly load may be full output or a heavily dimmed schedule. The labels cannot be compared until those missing variables are exposed.
Once the load is expressed in watt-hours per night, the rest of the design becomes traceable: the battery follows usable energy and autonomy, while the panel follows worst-month solar resource, losses, and recovery time. Teams building repeatable calculation sheets can use the sizing and design guides as a reference, but final values must remain tied to the project’s coordinates, lighting target, schedule, and warranty assumptions.