How to Design & Install a Solar PV System?

Design and installation of Solar PV Systems

Today our modern world needs energy for various day to day applications such as
industrial manufacturing, heating, transport, agricultural, lightning applications, etc.
Most of our energy need is usually satisfied by non-renewable sources of energy
such as coal, crude oil, natural gas, etc. But the utilization of such resources has
caused a heavy impact on our environment.
Also, this form of energy resource is not uniformly distributed on the earth. There is
an uncertainty of market prices such as in the case of crude oil as it depends on
production and extraction from its reserves. Due to the limited availability of non-
renewable sources, the demand for renewable sources has grown in recent years.
Solar energy has been at the center of attention when it comes to renewable energy
sources. It is readily available in an abundant form and has the potential to meet our
entire planet’s energy requirement. The solar standalone PV system as shown in fig
1 is one of the approaches when it comes to fulfilling our energy demand
independent of the utility. Hence in the following, we will see briefly the planning,
designing, and installation of a standalone PV system for electricity generation.

Planning of a Standalone PV system

Site assessment, surveying & solar energy resource assessment:

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Since the output generated by the PV system varies significantly depending on the
time and geographical location it becomes of utmost importance to have an
appropriate selection of the site for the standalone PV installation. Thus, the
following points must be considered for the assessment and selection of locations for
installation.
1. Minimum Shade: It must be made sure that the selected site either at
rooftop or ground should not have shades or should not have any structure
that intercepts the solar radiation falling on the panels to be installed. Also,
make sure that there won’t be any structural construction soon surrounding
the installation that might cause the problem of shading.
2. Surface Area: The surface area of the site at which the PV installation is
intended should be known, to have an estimation of the size and number of
panels required to generate the required power output for the load. This
also helps to plan the installation of inverter, converts, and battery banks.
3. Rooftop: In the case of the rooftop installation the type of roof and its
structure must be known. In the case of tilt roofs, the angle of tilt must be
known and necessary mounting must be used to make the panels have
more incidents of solar radiation i.e. ideally the radiation angle must be
perpendicular to the PV panel and practically as close as to 90 degrees.
4. Routes: Possible routes for the cables from an inverter, battery bank,
charge controller, and PV array must be planned in a way that would have
minimum utilization of cables and lower voltage drop in cables. The
designer should choose between the efficiency and the cost of the system.
To estimate the output power the solar energy assessment of the selected site is of
foremost significance. Insolation is defined as the measure of the sun’s energy
received in a specified area over a period of time. You can find this data using a
pyranometer, however, it is not necessary as you can find the insolation data at your
nearest meteorological station. While assessing the solar energy the data can be
measured in two ways as follows:
 Kilowatt-hours per square meter per day (KWh/m 2/day): It is a quantity
of energy measured in kilowatt-hours, falling on square meter per day.
 Daily Peak Sun Hours (PSH): Number of hours in a day during which
irradiance averages to 1000 W/m2.
Peak sun hours are most commonly used as they simplify the calculations. Do not
get confused with the “Mean Sunshine Hours” and “Peak Sun Hours” which you

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would collect from the meteorological station. The “Mean sunshine hours” indicates
the number of hours the sunshine’s were as the “Peak sun hours” is the actual
amount of energy received in KWh/m 2/day. Amongst all months over a period of year
use the lowest mean daily insolation value as it will make sure that the system will
operate in a more reliable way when the sun is least due to unsuitable weather
conditions.

Considerations for Standalone PV system

Calculation of Energy Demand
The size of the standalone PV system depends on the load demand. The load and
its operating time vary for different appliances, therefore special care must be taken
during energy demand calculations. The energy consumption of the load can be
determined by multiplying the power rating (W) of the load by its number of hours of
operation. Thus, the unit can be written as watt × hour or simply Wh.
Energy demand Watt-hour = Power rating in Watt × Duration of operation in hours.
Thus, the daily total energy demand in Wh is calculated by adding the individual load
demand of each appliance per day.
Total energy demand Watt-hour = ∑ (Power rating in Watt × Duration of operation in
hours).
A system should be designed for the worst-case scenario i.e. for the day when the
energy demand is highest. A system designed for the highest demand will ensure
that the system is reliable. If the system meets the peak load demand it will meet the
lowest demand. But designing the system for the highest demand will increase the
overall cost of the system. On the other hand, the system will be fully utilized only
during the peak load demand. So, we have to choose between cost and reliability of
the system.

Inverter & Converter (Charge Controller) Ratings

For choosing the proper inverter both the input and output voltage and current rating
should be specified. The inverter’s output voltage is specified by the system load, it
should be able to handle the load current and the current taken from the battery
bank. Based on the total connected load to the system the inverter power rating can
be specified.
Let’s consider 2.5 kVA  in our case, hence an inverter with power handling capacity
having a size of 20-30% higher than the power running the load should be chosen
from the market. In the case of motor load, it should be 3-5 times higher than the

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power demand of such an appliance. In the case of the converter, the charge
controller is rated in current and voltage. Its current rating is calculated by using the
short-circuit current rating of the PV module. The value of voltage is the same as the
nominal voltage of batteries.
Converter and Charge Controller Sizing
The charge controller rating should be 125% of the photovoltaic panel short circuit
current. In other words, It should be 25% greater than the short circuit current of
solar panel.
Size of solar charge controller in amperes = Short-circuit current of PV × 1.25
(Safety factor).
For example, we need a 6 numbers each of 160W solar panels for our system.
Following are the related date of PV panel.
Suppose the PV module specification are as follow.
 PM = 160 WPeak
 VM = 17.9 VDC
 IM = 8.9 A
 VOC = 21.4 A
 ISC = 10 A
The required rating of solar charge controller is = (4 panels x 10 A) x 1.25 = 50 A
Now, a 50A charge controller is needed for the 12V DC system configuration.
Note: This formula is not applicable on MPPT Solar chargers. Please refer to the
user manual or check the nameplate data rating for proper sizing.

Inverter Sizing
The size of Inverter should be 25% bigger than the total load due to losses and
efficiency problem in the inverter. In other words, It should be rated 125% than the
total load required in watts. For example, if the required wattage is 2400W, than the
size of inverter should be:
2400W x 125%
2400W x 1.25
3000 Watts.
So we need a 3kW of inverter in case of 2400W load.
Daily Energy Supplied to Inverter
Let us consider in our case the daily energy consumption by the load is 2700 Wh.
Note that the inverter has its efficiency, thus the energy supplied to the inverter
should be more than the energy used by the load, so the losses in the inverter can

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be compensated. Assuming 90% efficiency in our case, the total energy supplied by
the battery to the inverter would be given as;
Energy supplied by the battery to the inverter input = 2700 / 0.90 = 3000 Wh/per day.
System Voltage
The inverter input voltage is referred to as the system voltage. It is also the overall
battery pack voltage. This system voltage is decided by the selected individual
battery voltage, line current, maximum allowable voltage drop, and power loss in the
cable. Usually, the voltage of the batteries is 12 V so will be the system voltage. But
if we need higher voltage it should be multiples of 12 V. i.e. 12 V, 24 V, 36 V, and so
on.
By decreasing the current, power loss and voltage drop in the cable can be reduced,
this can be done by increasing the system voltage. This will increase the number of
batteries in the series. Therefore, one must choose between power loss and system
voltage. Now for our case let us consider the system voltage of 24 V.

Sizing of the Batteries

While sizing the battery some parameters are needed to be considered as follows:
1. Depth of Discharge (DOD) of the battery.
2. Voltage and ampere-hour (Ah) capacity of the battery.
3. The number of days of autonomy (It is the number of days required to
power up the whole system (backup power) without solar panels in case of
full shading or rainy days. We will cover this part in our upcoming article) to
get the needed Ah capacity of batteries.
Let us consider we have batteries of 12 V, 100 Ah with DOD of 70%. Thus, the
usable capacity of the is 100 Ah × 0.70 = 70 Ah. Therefore, the charged capacity that
is required is determined as follows;
Required charge capacity = energy supplied by the battery to the inverter
input/system voltage
Required charge capacity = 3000 Wh/ 24 V = 125 Ah
From this, the number of batteries required can be calculated as;
No. of batteries required = Required charge capacity / (100 × 0.7)
No. of batteries required = 125 Ah / (100 × 0.7) = 1.78 (round off 2 batteries)
Thus, 2 batteries of 12 V, 100 Ah are required. But due to round off 140 Ah instead
of 125 Ah is required.
Required charge capacity = 2 × 100Ah × 0.7 = 140 Ah

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Therefore, two 12 V, 100 Ah batteries in parallel to meet the above charge capacity.
But as the individual battery is of 12 V, 100 Ah only and the system voltage
requirement is of 24 V we need to connect two batteries in series to get the system
voltage of 24 V as shown in figure 2 below:

Battery Bank
So, in total there will be four batteries of 12 V, 100 Ah. Two connected in series and
two connected in parallel.
Also, the required capacity of batteries can be found by the following formula.

Sizing of the PV Array

Different sizes of PV modules available in the market produce a different level of
output power. One of the most common way to determine the sizing of the PV array
is to use the lowest mean daily insolation (Solar irradiance) in peak sun hours as
follows;
The total size of PV array (W) = (Energy demand per day of a load (Wh) / T PH) × 1.25
Where TPH is the lowest daily average peak sun hours of a month per year & 1.25 is
the scaling factor. With this the number of PV modules N modules required can be
determined as;
Nmodules = Total size of the PV array (W) / Rating of selected panels in peak-watts.
Suppose, in our case the load is 3000 Wh/per day. To know the needed total W Peak of
a solar panel capacity, we use PFG factor i.e.

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Total WPeak of PV panel capacity = 3000 / 3.2 (PFG)
= 931 WPeak
Now, the required number of PV panels are = 931 / 160W = 5.8.
This way, we need 6 numbers of solar panels each rated for 160W. You can find the
exact number of solar panels by dividing the WPeak by other rating i.e. 100W, 120W
150W etc based on the availability.
Note: The value of PFG (Panel Generation Factor) is varying (due to climate and
temperature changes) in different regions e.g, PFG in USA = 3.22, EU = 293,
Thailand = 3.43 etc.
Moreover, the additional losses should be considered to find the exact panel
generation factor (PGF). These losses (in %) occur due to :
 Sunlight not striking the solar panel straight on (5%)
 Not receiving energy at the maximum power point (excluded in case of
MPPT charge controller). (10%)
 Dirt on solar panels (5%)
 PV panels aging and below specification (10%)
 Temperature above  25°C (15%)

Sizing of the Cables

The sizing of the cables depends on many factors such as maximum current carrying
capacity. It should have a minimum voltage drop and have minimum resistive losses.
As the cables would be placed in the outdoor environment it should be water-
resistant and ultraviolet.
The cable must behave minimum voltage drop typically less than 2% as there is an
issue of voltage drop in low voltage system. Under sizing of the cables will result in
energy loss and sometimes can even lead to accidents. whereas the oversizing is
not economically affordable. The cross-sectional area of the cable is given as;
A = (ρIML / VD) × 2
Where
 ρ is the resistivity of the conducting wire material (ohm-meters).
 L is the length of cable.
 VD is the maximum permissible voltage drop.
 IM is the maximum current carried by the cable.
In addition, you may use this cable and wire size calculator. Also, use the proper
sized circuit breaker and rated plugs and switches.
Lets have a solved example for the above example.

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Example:
Suppose we have the following electrical load in watts where we need a 12V, 120W
solar panel system design and installation.
 An LED lamp of 40W for 12 Hours per day.
 A refrigerator of 80W for 8 Hours per day.
 A DC Fan of 60W for 6 Hours per day.
Now let’s find the number of solar panels, rating and sizing of charge controller,
inverter and batteries etc.
Finding the Total Load
Total Load in Wh / day
= (40W x 12 hours) + (80W x 8 hours) + (60W x 6 hours)
= 1480 Wh / per day
The required wattage by Solar Panels System
= 1480 Wh x 1.3 … (1.3 is the factor used for energy lost in the system)
= 1924 Wh/day
Finding the Size and No. of Solar Panels
WPeak Capacity of Solar Panel
= 1924 Wh /3.2
= 601.25 WPeak
Required No of Solar Panels
= 601.25 / 120W
No of Solar Panels = 5 Solar Panel Modules
This way, the 5 solar panels each of 120W will capable to power up our load
requirements.
Find the Rating and Size of Inverter
As there is only AC loads in our system for specific time (i.e. no additional & direct
DC load connected to the batteries) and our total required wattage is:
=  40W + 80W + 60W 
= 180W
Now, the rating of inverter should be 25% greater than the total load due to losses in
the inverter.
= 180W x 2.5
Inverter Rating & Size = 225 W

Find the Size, Rating & No of Batteries

Our load wattage and operational time in hours

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= (40W x 12 hours) + (80W x 8 hours) + (60W x 6 hours)
Nominal Voltage of Deep Cycle Battery = 12V
Required Days of Autonomy (Power by batteries without solar panel power) = 2
days.

[(40W x 12 hours) + (80W x 8 hours) + (60W x 6 hours) / (0.85 x 0.6 x 12V)] x 2 days

The required capacity of batteries in Ampere-hour = 483.6 Ah

This way, we need a 12V 500Ah battery capacity for 2 days of autonomy.
In this case, we may use 4 number of batteries each of 12 V, 125Ah connected
in parallel.
If the available battery capacity is 175Ah, 12 V, we may use 3 number of batteries.
You can get the exact number of batteries by dividing the required capacity of
batteries in Ampere-hour by the available battery Ah rating.
Required Number of batteries = Required capacity of batteries in Ampere-
hour / Available battery Ah rating

Find The Rating and Size of Solar Charge Controller

The charge controller should be 125% (or 25% greater) than the solar panel short
circuit current.
Size of solar charge controller in Amp = Short circuit current of PV × 1.25
PV module specification
 PM = 120 WPeak
 VM = 15.9 VDC
 IM = 7.5 A
 VOC = 19.4 A
 ISC = 8.8 A
The required rating of solar charge controller is = (5 panels x 8.8 A) x 1.25 = 44 A
So you can use the next nearest rated charge controller which is 45A.
Note that this method can’t be used to find the exact size of MPPT solar chargers.
Please refer to the user manual provided by the manufacturer or see the nameplate
rating printed on it.
Finding the Cable, CB, Switches & Plug Ampacity

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Use the following tools and explanatory posts with charts to find the exact amperage
rating of wire and cables, switches & plugs and circuit breakers.

 Cable Wire Size Calculator or How to Find the proper wire size with
ampacity.
 Find the Amperage rating of Switches & Plugs
 Find the Appropriate size and rating of circuit breaker.
Conclusion
The standalone PV system is an excellent way to utilize the readily available eco-
friendly energy of the sun. Its design and installation are convenient and reliable for
small, medium, and large-scale energy requirements. Such a system makes the
availability of electricity almost anywhere in the world, especially in remote areas. It
makes the energy consumer independent of the utility and other sources of energy
such as coal, natural gas, etc.
Such a system can have no negative impact on our environment and can provide
energy for long periods after its installation. The above systematic design and
installation provide useful guidelines for our need for clean and sustainable energy in
the modern world.

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