# How to design and calculate Solar Street Light system?

In this blog we will try to help you understand how to design cost effective Solar PV System for Street Light.

First of all let’s start from basics: What is solar PV system?

Solar power system is one of renewable energy sources which uses solar PV modules to convert sunlight into electricity. The energy generated can be either stored or used directly, fed back into grid line or mixed with one or more other electricity or different renewable energy source. Solar system is the most reliable and clean source of electricity that can suit a wide range of applications such as residence, industry, agriculture, livestock, etc. Of course, if the first condition is achieved: Enough SUN hours.

Solar Street Light

includes different components that should be selected according to your system type, site location and applications. The main parts for solar street light system are solar panel, solar charge controller, battery, inverter, pole, LED Light.

Below we will briefly mention basic features of each part:

• PV module – converts sunlight into DC electricity.

• Solar charge controller – regulates the voltage and current coming from the PV panels going to battery and prevents battery overcharging and prolongs the battery life.

• Battery – stores energy for supplying to electrical appliances when there is a demand.

• Load – is electrical appliances that connected to solar PV system such as lights, wifi, camera, etc,

Now when you know the basics about all parts it is very useful to undersdand how to design and determine the best system for your solar street light project. In order to that you should:

1. Determine what is power consumption of your street light

The first step in designing a solar street light system is to find out the total power and energy consumption of LED light and other parts that will need to be supplied by solar power, such as WiFi, Camera etc. need to be supplied by the solar PV system.

How to calculate total consumption of your solar system? Simply follow the steps below:

1.1 Calculate total Watt-hours per day for each part used.

Add the Watt-hours needed for all parts together to get the total Watt-hours per day which

must be delivered to the appliances.

1.2 Calculate total Watt-hours per day needed from the PV modules.

Multiply the total appliances Watt-hours per day times 1.3 (the energy lost in the system) to get the total Watt-hours per day which must be provided by the panels.

Image: solar street light solutions from: www.engoplanet.com

2. What is the size of the Solar Panel needed for my Solar Street Light system?

Different size of solar PV modules will produce different amount of power. To find out the sizing of PV module, the total peak watt produced needs. The peak watt (Wp) produced depends on size of the PV module and climate of site location. We have to consider “panel generation factor” which is different in each site location. For Thailand, the panel generation factor is 3.43. To determine the sizing of PV modules, calculate as follows:

2.1 Calculate the total Watt-peak rating needed for PV modules

Divide the total Watt-hours per day needed from the PV modules (from item 1.2) by 3.43 to get

the total Watt-peak rating needed for the PV panels needed to operate the appliances.

2.2 Calculate the number of PV panels for the system

Divide the answer obtained in item 2.1 by the rated output Watt-peak of the PV modules available

to you. Increase any fractional part of result to the next highest full number and that will be the

number of PV modules required.

Result of the calculation is the minimum number of PV panels. If more PV modules are installed, the system will perform better and battery life will be improved. If fewer PV modules are used, the system may not work at all during cloudy periods and battery life will be shortened.

3. Battery sizing

The battery type recommended for using in solar PV system is deep cycle battery. Deep cycle battery is specifically designed for to be discharged to low energy level and rapid recharged or cycle charged and discharged day after day for years. The battery should be large enough to store sufficient energy to operate the appliances at night and cloudy days. To find out the size of battery, calculate as follows:

3.1 Calculate total Watt-hours per day used by appliances.

3.2 Divide the total Watt-hours per day used by 0.85 for battery loss.

3.3 Divide the answer obtained in item 3.2 by 0.6 for depth of discharge.

3.4 Divide the answer obtained in item 3.3 by the nominal battery voltage.

3.5 Multiply the answer obtained in item 3.4 with days of autonomy (the number of days that you need the system to operate when there is no power produced by PV panels) to get the required

Ampere-hour capacity of deep-cycle battery.

Battery Capacity (Ah) = Total Watt-hours per day used by appliances x Days of autonomy

(0.85 x 0.6 x nominal battery voltage)

4. Solar charge controller sizing

The solar charge controller is typically rated against Amperage and Voltage capacities. Select the solar charge controller to match the voltage of PV array and batteries and then identify which type of solar charge controller is right for your application. Make sure that solar charge controller has enough capacity to handle the current from PV array.

For the series charge controller type, the sizing of controller depends on the total PV input current which is delivered to the controller and also depends on PV panel configuration (series or parallel configuration).

According to standard practice, the sizing of solar charge controller is to take the short circuit current (Isc) of the PV array, and multiply it by 1.3

Solar charge controller rating = Total short circuit current of PV array x 1.3

Remark: For MPPT charge controller sizing will be different. (See Basics of MPPT Charge Controller)

Example: A house has the following electrical appliance usage:

One 18 Watt fluorescent lamp with electronic ballast used 4 hours per day.

One 60 Watt fan used for 2 hours per day.

One 75 Watt refrigerator that runs 24 hours per day with compressor run 12 hours and off 12 hours.

The system will be powered by 12 Vdc, 110 Wp PV module.

1. Determine power consumption demands

Total appliance use = (18 W x 4 hours) + (60 W x 2 hours) + (75 W x 24 x 0.5 hours)

= 1,092 Wh/day

Total PV panels energy needed = 1,092 x 1.3

= 1,419.6 Wh/day.

2. Size the PV panel

2.1 Total Wp of PV panel capacity

needed= 1,419.6 / 3.4

= 413.9 Wp

2.2 Number of PV panels needed= 413.9 / 110

= 3.76 modules

Actual requirement = 4 modules

So this system should be powered by at least 4 modules of 110 Wp PV module.

3. Inverter sizing

Total Watt of all appliances = 18 + 60 + 75 = 153 W

For safety, the inverter should be considered 25-30% bigger size.

The inverter size should be about 190 W or greater.

4. Battery sizing

Total appliances use = (18 W x 4 hours) + (60 W x 2 hours) + (75 W x 12 hours)

Nominal battery voltage = 12 V

Days of autonomy = 3 days

Battery capacity = [(18 W x 4 hours) + (60 W x 2 hours) + (75 W x 12 hours)] x 3

(0.85 x 0.6 x 12)

Total Ampere-hours required 535.29 Ah

So the battery should be rated 12 V 600 Ah for 3 day autonomy.

5. Solar charge controller sizing

PV module specification

Pm = 110 Wp

Vm = 16.7 Vdc

Im = 6.6 A

Voc = 20.7 A

Isc = 7.5 A

Solar charge controller rating = (4 strings x 7.5 A) x 1.3 = 39 A