How to Calculate Solar Panel Size for Solar Street Lights

Key conclusion

Solar panel size should be calculated from the measured nightly energy demand and the available solar resource, then adjusted for real system losses and the required recovery behavior. Dividing LED wattage by a fixed number is not a valid project method.

The calculation produces a preliminary array power. The proposed module must then be checked against controller voltage and current limits, available mounting area, orientation, shading, temperature and the battery charging requirement.

Inputs required before calculation

• Measured luminaire input power at each programmed dimming level.
• Hours spent at each output level.
• Controller, sensor and communication energy per night.
• System voltage and battery charging limits.
• Location-specific solar resource, preferably using a conservative design period.
• Panel orientation, tilt and credible shading conditions.
• Temperature, wiring, mismatch, soiling and conversion losses.
• Required battery recovery time after low-solar days.

Step 1: calculate nightly energy demand

Calculate each operating stage separately:

Lighting energy (Wh/night) = sum of [measured power at each stage (W) x time at that stage (h)]

Then add auxiliary energy:

Total nightly load = lighting energy + controller energy + sensor energy + communications energy

Use measured system input power where available. LED board wattage alone can exclude driver and control losses.

Step 2: select the solar-resource input

Peak sun hours are an energy equivalent, not the number of daylight hours. A location may have a long day but limited usable solar energy because of clouds, haze, shading, orientation or seasonal conditions.

Use a location-specific dataset and state whether the calculation uses an annual average, a low-solar month, or another project-defined design period. NREL’s PVWatts and National Solar Radiation Database are useful references for preliminary solar-resource and loss modeling, although an off-grid street light still requires its own battery and controller model.

Step 3: apply a transparent derating model

A convenient preliminary formula is:

Required PV power (W) = total nightly load (Wh) / [design peak sun hours (h) x combined delivery factor]

The combined delivery factor represents the modeled effect of temperature, module tolerance, wiring, soiling, orientation, controller conversion and other applicable losses. Do not mislabel one universal value as an MPPT correction coefficient. MPPT is a controller operating method; it does not replace a project-specific loss model.

List the individual assumptions used to create the combined factor. NREL’s PVWatts documentation shows why system losses are modeled explicitly and warns that performance predictions contain assumptions and uncertainty.

Worked example

This example demonstrates the method only. It is not a product recommendation.

• 30 W measured system input for 4 hours: 120 Wh
• 15 W measured system input for 4 hours: 60 Wh
• 9 W measured system input for 4 hours: 36 Wh
• Controller and sensor energy: 12 Wh
• Total nightly load: 228 Wh
• Design solar resource: 4.5 peak sun hours
• Combined delivery factor: 0.75, based on stated project loss assumptions

Preliminary PV power = 228 / (4.5 x 0.75) = 67.6 W

The result suggests that an array above 67.6 W should be evaluated. It does not automatically justify a specific commercial panel size. The next step is to check battery recovery, seasonal resource, panel tolerance, temperature, mounting area and controller limits. A project may select a larger array after those checks.

Battery recovery after low-solar days

A panel sized only to replace one average night’s energy may recover slowly after several poor-solar days. Recovery should be modeled separately:

Daily charging requirement = next-night load + planned recovery energy

The recovery period is a project decision. Faster recovery requires more available charging energy and may require a larger array or a different operating strategy. The controller’s maximum PV input voltage, charging current and thermal limits must not be exceeded.

Why oversizing is not unlimited

Additional panel capacity can improve recovery and low-solar resilience, but only within the electrical and mechanical design:

• open-circuit voltage must remain within the controller limit at the lowest expected temperature;
• short-circuit current and charging current must remain within rated limits;
• the pole, bracket and foundation must account for the panel’s projected area and wind load;
• the panel must not create shading or compromise luminaire aiming;
• the battery and BMS must accept the proposed charging current.

Common calculation errors

• Using advertised LED wattage instead of measured system input power.
• Using annual-average solar data for a project that must operate through a low-solar season.
• Adding arbitrary rainy-day multipliers without modeling the battery.
• Treating all losses as one unexplained fixed factor.
• Ignoring controller self-consumption and communication loads.
• Ignoring panel orientation, shading, soiling and temperature.
• Failing to check controller voltage and current limits.

Procurement data to request

• Panel model, rated power, tolerance, dimensions, Voc, Isc, Vmp and Imp.
• Controller model, PV voltage range, maximum PV current and charging current.
• Measured luminaire power at every programmed level.
• Battery nominal voltage, capacity, allowable charge current and BMS limits.
• Loss assumptions and solar-resource source used in the calculation.
• Mechanical drawing showing panel area and mounting geometry.

Sources and further reading

• NREL PVWatts Version 8
• NREL PVWatts V8 API documentation
• NREL System Advisor Model: photovoltaic models

Use this calculation together with the battery guide and rainy-day autonomy guide.