One of the characteristics of a properly designed solar street light is that the correct light intensity is provided by the system. A system with high lumens result in wasted energy at the same time a  system with low lumens result in poor performance. Just like any system design, a solar street light design calculation must be done to achieve optimum performance. This can only be achieved with proper calculation techniques. For solar street light design, experts start with photometric data where things like the location of the project, environmental factors such as dirt and temperature are considered.

The reason for solar street light design calculation is to achieve the right level of illuminance for the light unit. Illuminance defines the amount of lighting distributed per unit area, usually, it is measured lumens per square meter (lux) or lumens per square foot (footcandles). You have to check your city requirements regarding lighting efficacy before proceeding with your calculations to make sure you meet the requirements. For example, engineers in countries like the U.S. and Canada will likely follow the values established by the Illuminating Engineering Society of North America (IESNA) on illuminance. It may not be possible to perfectly get the established values, however, slight variations of the values in your light design are acceptable.

You can do the lighting calculation by hand, however, in the modern-day design that is practically impossible and besides it will take a substantial amount of time to do. A better way to it is the use of software in your lighting calculations work. This allows you to emphasize on the best choices to make, leaving the repetitive tasks to the computer to take care of.

Fundamental Lighting Calculations: The Lumen Method

For an easy hand calculation method for the projection of the illumination attained by a planned lighting system, the lumen method can prove effective to use. However, using the method for a very big project divided into several areas or complex room geometries is practically impossible. The lumen method is described in the steps below:

  1. Determine the RCR

The room cavity ratio or RCR is the ratio of the horizontal area of a room to the vertical area of that same room. The formula for this differs based on the geometry of the room.

  1. Find out the surface reflectances

Firstly, reflectance can be defined as the fraction of light reflected from the surface, this is greatly influenced by the color and texture of the surface.

  1. Find the photometric data

For any lighting product bought, there are specifications that pronounce the spatial distribution as well as the total lighting output. These are basic data needed to carry out your solar street light design calculation process.

  1. Establish the Coefficient of Utilization

Using the values got for surface reflectances as well as room cavity ratio, the coefficient of utilization (CU) can be established. This points out the efficiency of the lighting output that is distributed to the working plane.

  1. Determine the Maintenance Factor

This solar street light design calculation helps determine the lighting system dilapidation over time and the effect of the accumulation of dirt. When the system degrades and accumulates dirt over time, the lighting output is affected as it becomes reduced resulting in the illuminance going below the established value. The maintenance factor (MF) helps account for this.

  1. Utilize the Lumen Method Formula

The lumen method formula can be used in two ways, which are to determine the illuminance gotten from a certain combination of light fixtures or the number of luminaires needed to achieve a certain value of illuminance. The formula is given below:


= Number of luminaires

= Required illuminance

= Lamps per luminaire

= Lumen output per lamp

= Coefficient of utilization

= Maintenance factor



Overview of DIALux

Out of all lighting design software you can use for solar street light design calculation, DIALux stands out from the rest. Apart from being a powerful software package, it is also free to use. What the software company does is that it levies lighting manufacturers to put their products including the 3D models and photometric information into their database as a substitute for putting a fee on the software. This helps make the design procedure easier as there are readily available photometric files for any planned light system.


Another plus is the flexibility of the software as it is used in various project designs such as outdoor locations, single-room units, and whole building structures. Additionally, fill formats such as DWG from AutoCAD can be used directly, this eliminates the need for stressful conversion of files. A fundamental difference between using the lumen method and software for calculations is the ability of the software to calculate lighting variation in the evaluated area, however, Lumen method calculation can only account for the average value.

The use of software enables you to make changes to your solar street light design calculation in areas with too much darkness or brightness. DIALux presents a 2D plans and 3D models of the illuminance variations of the street light design area.


Making the right solar street light design calculation is very crucial as this can influence the success of your lighting project. Using lighting design software makes it easier to get accurate calculations. Additionally, this software is not restricted to new projects only, it can also be used during the renovation of previous projects.