A quick guide to off grid solar power systems design

A QUICK GUIDE TO OFF
GRID SOLAR POWER
SYSTEMS DESIGN
(Based on the Udemy course Off Grid Solar Power
Systems Design 101)

Download this document at www.energyoneafrica.com
©Mathy Mpassy Isinki
__________________________________________________________________
This guide by Mathy Mpassy Isinki is based on the Udemy.com couse Off Grid Solar Power
Systems Design 101; Use the Code HILL101 to unlock 50% discount on the course.
Mathy is an accomplished off grid energy solutions business and technical sales professional
technically competent in the applications and installation of off grid photovoltaic systems, as well as
in hybrid energy solutions combining diesel generators and solar.
In addition to providing solutions design; customers development and project management services
to the off grid solar industry, Mathy contribute to better energy solutions through training, with the
vision of building a better future for under-served communities in off grid locations.

1. Electricity basics
Solar panels are DC electricity sources. DC electricity refers to direct current. In DC
electrical systems, the charge carriers’ flow or movement stays the same at all times. In
contradiction; alternating current or AC refers to electrical systems where the electric
charge carriers periodically reverse their moving direction. Diesel generators and the grid
for example are AC sources.
Electrical energy supposes the flow of electrons within a conductor. Electrical energy
comes from the conversion of other forms of energies: a generator for example convert
mechanical energy into electrical energy.
Electrical energy is measured in kWh; it is the product of power expressed in kW and time
expressed in hours. Therefore, Electrical energy is power over time. In our everyday life,
KWh is what we pay power companies for.
Electric power in watt is obtained by multiplying voltage expressed in volts and current
expressed in amps. Power is defined as the rate at which energy is supplied to a circuit or
a load; it is energy over an instant.
But, what is voltage and what is current? Electricity is the flow of electrons and the current
the amount of electrons passing in a single point of a circuit.
Making an analogy with a water hose, current can be compared with the quantity of water
and voltage with the force or pressure that is necessary to move water within the hose.
Just like in a water hose, if we increase the voltage, or the pressure, then the current or
the water flow will increase.
Resistance refers to the resistance to the flow of electricity through a conductor. It is
measured in ohm. Resistance reduces the voltage. Referring again to a water hose, an
increase in the pressure causes the flow to increase, but getting a kink in the hose,
increasing the resistance, will cause the flow to decrease.
Voltage drop in a circuit is calculated by multiplying current and resistance. The
relationship between voltage, current, and resistance is described by Ohm's law.
Together, a power source, conductors and loads make an electric circuit.
Electric circuits can be configured in series or in parallel to power several loads, such as
light bulbs, TVs, washing machines and others.
In a series circuit made of load 1 and load 2, the negative of the load 1 is connected to the

positive of the load2.
Current is constant in a series circuit and voltage ads.
In a parallel circuit, the negative of the loads 1 and 2 are connected together and the
positives, separate from the negatives are also connected together.
In a parallel circuit current ads and voltage remains constant.
For large-scale generation of solar electricity solar panels are connected together into a
solar array. The PV array consists of strings of solar panels, where string means that the
panels are connected in series.
A PV module or solar module or again solar panel itself, is made up of the interconnection
of solar cells.
Solar cells within a PV module can be connected in different ways: first, cells can be

connected in series. In a series connection the voltages add up.
2. Stand-alone PV systems configuration
What makes a stand alone PV system different from a grid connected system is that it’s
not connected to the grid.
The PV modules are responsible of producing enough energy to meet the load
requirement.
Excess energy is stored in a battery bank which in it turn provides electricity to the
connected loads when the sun is not available.
The brighter the sunlight, the more the voltage the PV modules produce, then, a charge
controller is used to prevent overcharging or excessive discharge of the battery bank.
The brighter the sun, the more the voltage but, what if a string of cloudy days happens?
Or, what if you draw more power than what the battery bank can support? Off course the

battery bank will get empty, the system will go down and you will run out of power. To
prevent this, the battery bank is oversized.
In the same goal, off grid solar PV systems are often supported by a generator set or a
wind turbine to enable continuous charging of the battery bank.
It is also advisable to reduce load demand: all the connected appliances must be the most
energy efficient.
Power from the PV modules up to the battery bank is DC. If AC loads has to be connected
to the system, an inverter is then necessary. The inverter is responsible of converting DC
power from the battery bank into AC power usable by most of appliances.
Four major issues arise when designing a system:
1. the load (power) required to be supplied by the system is not constant over the period
of one day;
2. the daily energy usage varies over the year;
3. the energy available from the PV array may vary from time to time during the day;
4. the energy available from the PV array will vary from day to day during the year.
Since the system is based on photovoltaic modules, then a comparison should be
undertaken between the available energy from the sun and the actual energy demands.
The worst month is when the ratio between solar energy available and energy demand is
smallest.
Basic system design process follows 4 steps:
Step 1. Determination of the energy usage that the system must supply.
Step 2. Determination of the battery storage required.
Step 3. Determination of the energy input required from the PV array
Step 4. Selection of the remaining system components.
Additionally, off grid PV systems design is influenced by budget and site constraints.
Again here, the importance of energy efficiency is revealed; making small changes on the
way you use energy could make a difference to the size of the system you need and
therefore help fit in the budget.
Solar modules produce more power when they are pointed directly to the sun. Designer
should inspect the site and find out to comply with this requirement. Additionally, suitable
areas should be found where controller, battery bank and inverter will be located.
Furthermore when designing a system, it must comply with local electrical standards
requirements.

3. Battery storage
A battery bank is necessary for off grid PV systems. It stores excess energy from the PV
modules and supply it back to the connected loads when the sun is not available.
It is recommended to use deep cycle batteries for off grid solar PV systems applications.
Deep cycle batteries have the capacity of being charged and discharged hundreds of
times before they wear out.
Lead-acid batteries are the most used. Car batteries don’t work with solar PV. Their thin
plates don’t allow them withstanding deep discharges.
Lead acid batteries are made of separate 2-volt compartments known as cells. The cells
are filled with sulfuric acid, serving as electrolyte, and, inside of each cell is a series of
thick, parallel lead plates. Insulators prevent short circuit between the plates and partition
walls separate each cell preventing sulfuric acid to flow from one cell to the next.
To build up the voltage, the cells are wired in series. Electricity can flow out or in the
battery through the negative and positive terminal.
All the cells are encased in a heavy-duty plastic case.
Lead acid batteries can be of two type: flooded or sealed. They work in two way;
converting electrical energy into chemical energy at the charging and reconverting
chemical energy into electrical energy at the discharge. The discharge, off course
corresponds to when batteries supplies power to the load.
The thickness of plates allows multiple deep discharges of a deep cycle battery. Charging
and discharging being the cycle.
Unlike flooded batteries which require to be filled before installing them, sealed batteries
are factory filled; they are delivered ready for use.
Sealed lead acid batteries are also known as maintenance free batteries because fluid
level never need to be checked and they never need to be filled with water.

Two types of sealed lead acid batteries are available: AGM standing for absorbed glass
mat and gel cell batteries.
The advantage of sealed batteries over flooded batteries is that they charge faster, they
do not release explosive gases, they are more tolerant to low temperatures and they have
lower self discharge rate.
Flooded lead acid batteries on the other hand are less expensive, they store more
electricity, they have longer lifespan and can be rejuvenated after they have been left
deep discharged for a long period of time.
The system might require more than what a single battery can supply. This might be the
voltage or the amp-hour storage capacity.
In such a case, installers have to wire the batteries series or parallel to build up the voltage
or the amps-hours.
Series connect Parallel connect
Series parallel connect

4. Controller
Batteries are often the bottle neck of off grid solar PV systems.
Overcharging and deep discharge are the main causes of their premature failure. A
charge controller is used to maintain the proper charging.
As we know, the brighter the sun, the higher the voltage
Oldest charge controllers just short or disconnect the solar panels when a certain
voltage is reached.
Nowadays controllers match better batteries behavior and offer a 3 stage charging
cycle: bulk charging followed by absorption and float.
2 types of controllers are in use. MPPT and PWM.
MPPT are more performant, providing 10 to 30% more power to the battery.
PWM can create interference in radios and TVs. This is a downside.
Both controllers type; PWM or MPPT offers battery low voltage protection: a disconnect
turning off whatever is connected to the load terminal until the battery reaches a usable
voltage.
A single LED indicator, a series of LED or a digital meter for the more sophisticated
allows to monitor the controller.

5. Inverter
Let’s assume a set of solar panels on a rooftop. The sun is shining and the panels are
working well converting sunlight into electricity. The brighter the sun, the more the voltage;
a controller is regulating the voltage from the panel, feeding the batteries with the proper
voltage. Up to here, the system is DC. Unfortunately, chances are very high that the
appliances you want to power are AC.
You now need a device which will be converting your DC power into the AC necessary
for your appliances to work and, that is the role of inverter: converting DC into AC.
We now know that if the sun is not available for a string of days, it might happen that your
batteries get empty. In such a case you will run out of power. The option for most off grid
systems is to install a back up generator. Generators are AC sources. They are
compatible with your appliances but they can’t directly charge the batteries. Then, instead
of using a conventional inverter, an inverter charger is more advisable: it is an all in one
solution combining the conventional role of an inverter with additional battery charging
features.
Inverters come in 3 basics output types: square wave; modified square wave and sine
wave.
Modified square wave inverters generally have good surge and continuous capability and
are usually cheaper than sine wave types. However, some appliances, such as audio
equipment, television and fans can suffer because of the output wave shape. Sine wave
inverters often provide a better quality power than the grid supply.
when selecting an Inverters
 input voltage should correspond to system voltage
 output voltage should refer to the voltage supported by your appliances
You should also consider:
 wave form;
 output power;
 surge capacity;
 stackability.
All those information are found at inverters specification sheets.
All theDC components of an off-grid PV system, meaning the PV array, the controller and
the battery bank must must operate at the same voltage, We refer to it as the system
voltage. The inverter input voltage is selected to match that voltage.

Some appliances, such as audio equipments, television and fans can suffer because of
the output wave shape. It is therefore important to collect information about any possible
implication of the wave form on the loads before selecting an inverter.
Additionally, the selected inverter should be capable of supplying continuous power to all
AC loads. In that view, you should de-rate the inverter and take into account it efficiency.
I would like also to mention that the inverter you will have selected should be capable of
providing sufficient surge capability to start any loads that may surge.
The last point we will touch about inverter is stacking. Stacking inverters allows them to
act as a single system. They will work together to charge batteries and provide power to
loads. The advantage is an increased output.

6. Load assessment
Off grid systems are sized based on the energy demand of the appliances to be
connected.
A system designer can only design a system to meet the power and energy needs of the
customer.
The system designer must therefore use load assessment process to understand the
needs of the customer.
Load assessment is critical. Undersized systems will not deliver what they are expected
to deliver; oversized systems will cost more than necessary.
You need to calculate the electrical energy usage with the customer and at the same time
educate him.
Many systems have failed over the years not because the equipment has failed or the
system was installed incorrectly, but because the customers believed they could get more
energy from their system than what the system could deliver. It failed because the
customers were unaware of the power and energy limitations of their systems.
The problem is that the customer may not want to spend the time determining their
realistic power and energy needs which is required to successfully complete a load
assessment form. They just want to know: How much for a system to power my lights and
TV?
.
Electrical power is supplied either from the batteries to produce DC or via an inverter to
produce AC. Electrical energy usage is normally expressed in watt hours (Wh) or kilowatt
hours ( kWh ).
To determine the daily energy usage for an appliance, multiply the power of the appliance
by the number of hours it will be operating per day. The result is the energy consumed by
that appliance per day. Remember Energy = Power multiplied by hours.
Appliances can either be DC or AC. You will have to undertake load assessment for each
type.

At the end of the load assessment you should had have identified DC and AC loads; you
should also have determined DC and AC appliances contribution to the load demand, this
the total watt for each type of appliances the last you should determine is the energy
demand for each type ofappliances.
Let us assume a house equipped with:
 12 DC lamps, 5W each. The lamps are on 6 hours per day.
 1 TV rated at 150W and used 6 hours per day also
 1 computer 150W also and on 4 hours per day
 1 fridge; 80W; used 12 hours perday.
We will go further in our design process using this house and its load as example.
First of all, load assessment. The goal is to determine power demand and energy needs.
Step 1 DC power demand: multiply the number of lamps, which is 12 by their wattage (5).
DC power demand is 60W
Step 2 DC energy demand: multiply power demand (60W) by usage time (6hours). DC
energy needs are 360Wh
Step 3 AC power demand: multiply the number of each type of AC appliance by its
wattage. AC power demand is 380W.
Step 4 AC energy needs: multiply the number of each type of AC appliance by its wattage
and its usage time. AC energy needs are equal to 2460Wh
Step 5 Total energy needs: sum up DC and AC energy needs, which will lead you to
2820Wh.

7. Energy savings
The bigger the energy needs, the bigger the system and the more you pay. As a designer
you should take the time to educate the customer to energy efficiency. Customers should
understand that making small change to the way they use energy could make the
difference to the size of the solar system they need. Energy efficiency allows to reduce
energy consumption without affecting comfort.
Energy is power over time. Acting on power or time will equally result on lower energy
consumption. Switching appliances when they are not in use is the simplest way of
reducing usage time.
Moreover, TV’s, DVD players, computers, stereo and game consoles still use energy in
standby more. It is advisable to use a power board. It supplies electricity to multiple
appliances and has the advantage of allowing to switch them all off from one location.
It is also advisable to track phantom loads by unplugging chargers and adapters when
not in use.
Sensors are suitable for commanding lights in low occupancy locations. With a sensor,
you are sure that the light is on only when there is a presence. Toilets and corridor lighting
are particularly indicated for sensors.
We have been talking of reducing usage time. If you want to reduce power demand, the
first thing to look at it is light.
Consider proposing retrofitting to your customers. They have the opportunity to change
old light globes to compact fluorescent lamps. They can also change CFL to LED.
Fridges and freezer perform more efficiently when
Temperature is settled between 4 to 5 degrees Celsius for fridges and between -15 and
-18 for freezers
They should be kept in a cool, well ventilated spot away from the oven and the sun.
Additionally, it is advisable to keep a five-centimeter gap around fridges and freezers.
The washing machine and dryer should be used on full load only. Additionally, wherever
possible, it is advisable to use a clothesline instead of a dryer.
Last but not least, always consider using a solar water heater.

8. PV array energy output estimate
If we consider that off grid PV systems have to be designed to meet the load demand,
load assessment should be the first step of the design process. Here the design’s steps:
1. Load assessment
2. Estimating PV array required energy output
3. Estimating PV array power output
4. PV array configuration
5. Controller selection
6. Designing battery storage bank
7. Inverter selection
Coming back to our example, we found out that AC energy needs are equal to 2460Wh
and 360Wh for DC energy needs. This was step 1
Step 2 of our design process is to estimate the required energy output from the PV
array.
We have to adjust energy needs, taking into account the various losses in the system. In
doing so, we want to estimate the minimum power that the PV array is expected to deliver.
As you can guess, losses come from efficiency of system components. Here we have
their various efficiencies.
The inverter is supposed to have an efficiency of 90%, while the combined
efficiency of the charge controller, battery and cables is 85%.
DC energy needs are adjusted accounting cables, charge controller and battery bank
efficiency.
.
AC energy needs are adjusted in two steps:
Step 1 Accounting cables, controller and battery bank efficiency,
Step 2 Applying inverter efficiency

Now the total of DC and AC energy needs should be transposed to the output of the PV
array. In doing so, we can get the actual amount of PV energy expected everyday by
the system.
From the load assessment in our example,
To estimate the PV array energy output covering the DC energy needs: we have to
adjust the 360Wh energy needs taking into account cables; controller and battery bank
85% efficiency. 360Wh divided by 0.85. The required PV array energy output is of
423.5Wh
Step 2, For the PV array energy output covering the AC needs, we first adjust AC needs
accounting controller, battery storage and cables efficiency before adjusting to inverter
efficiency. The required PV array energy output is calculated as 3215.6Wh.
After summing up the adjusted DC and AC energy needs we obtain 3639Wh; this is
what the PV array should deliver to meet the needs of the consumer.

8. PV array power output estimate
How much energy a PV module delivers depends on several factors, such as local
weather patterns, seasonal changes, and installation of modules. PV modules should
be installed at the correct ‘tilt-angle’ in order to achieve best year-round performance.
It is also important to know whether a PV system
is expected to be used all-year round or only during a certain period of a year.
The energy produced in winter is much less than yearly average and in the
summer months the generated energy can be more that the average.
In the PV language, 1 equivalent sun means the solar irradiance of 1000 W/m2.
This value corresponds to the standard, at which the performance of solar cells and
modules is determined. The rated parameters of modules
are determined at solar irradiance of 1 sun.
When solar irradiation data are available for a particular location than the equivalent sun
hours can be determined.
For example, in India the average annual solar
irradiation is 1645 kWh/ m2. 1 sun delivers 1000 W/m2= 1 kW/m2.
It means, the Indian average annual solar irradiation can be expressed in
1645kWhm2 = 1645 equivalent sun hours,
1kWm2
which also means that 1645 h/365 days = 4.5 h per day.

The map in illustration shows a rough estimation of the daily equivalent sun hours for
an average annual solar irradiation.
Coming back to our example;
We estimated the required daily PV array energy output at 3639Wh. Assuming that the
system as to be installed in India where ESH is 4.5, after dividing, the required PV array
power output is of 809W.
Now that you know the array power output, the choice of modules will be guided by
market availability and the performance.
Module performance are reflected by modules specifications sheets.
Back to our example; let’s assume that due to market availability you decide to use
120W solar panels.
We estimated the required PV array power output at 809W. Applying our Pmax of 120W
we can easily find that the minimum number of panels is 7.
How are you going to connect your panels? Series? Parallel or combination of the two?
What will be your system voltage? We’ll answer to hese questions in the next lecture.

9. Array configuration
How are you going to connect your panels? Series? Parallel or combination of the two?
What will be your system voltage? That is what we are going to find out now.
By connecting modules in series, their voltages are added. The total voltage will be the
sum of the voltage of all single modules. The current remains the same and will be
equal to the current of a single module.
If panels are connected in parallel, the current will be equal to the sum of the current of
all single modules and, the voltage will remain equal to the voltage of one single
module.
Array configuration is influenced by the system voltage and aesthetic.
Usually, system voltage is increased as the total load increases.
However, here is what is recommended:
 12Volts system voltage for energy demand bellow 1kWh ;
 24Volts for energy demand between 1kWh and 3 to 4kWh;
 48Volts for energy demand above 3 to 4kWh.
Applying this to our example; total energy demand was 2820Wh, which is above 1kwh
but bellow 3kWh. Our system should be 24Volts.
Our module voltage is of 12Volts. How can we configure them to get a 24Volts system?
First the strings: 2 modules in series to get the 24Volts system voltage, then the array
which should be made up of 4 parallel strings.
As you can see 4 strings of 2 modules are 8 modules. Due to system voltage, we have
to adjust the number of module from 7 to 8.

10. Controller selection
If we refer to the power flow, the next component of an off grid PV system is the
controller.
Controllers come in two types: PWM and MPPT. We will in this lecture learn how to
select a controller for our systems.
If you decide to work with a PWM, voltage and current are the selection parameters.
Whereas, voltage refers to array’s Voc and current refers to array’s Isc.
If using a MPPT, the array output power and the system voltage must be within the
range of the controller.
MPPT solar charge controllers allow to use high wattage off-grid systems. They are
selected based on the combination of wattage and battery voltages.
Battery storage charging current should also be considered when selecting a MPPT.
In our design example, our PV array is made of 4 parallel strings of 2 series 120W
panels. The array Voc is 43Vdc and current is 29.8A
Electrical standards request to oversize the controller. Mostly, a 125% over sizing factor
for current input and 120% for voltage.
In our example we decided to use 120W panels having Voc of 21.5V and 7.45A of Isc;
due to system voltage, we connected 2 x 120W modules in series, making 4 strings.
In series connections, voltage adds and current remains the same. Therefore,
Voc string = Voc module x 2 modules in seriesVoc string = 21.5V x 2Voc string = 43Vdc
Isc string = Isc moduleIsc string = 7.45AWe then connected our 4 strings in parallel;
making our PV array. In parallel connections voltage remains constant and current
adds.Isc array = Isc string x 4 strings
Isc array =7.45A x 4Isc aray =29.8AVoc array = Voc string
Voc array = 43Vdc
Voc and and Isc represent the highest voltage and current the PV array can produce. We
refer to them for controller selection.

11. Battery bank configuration
Battery bank sizing is critical. Under sizing the battery bank will reduce the system
usability and over sizing it will dramatically impact on the cost.
To size a battery bank, what you need to know first is the array energy output and
voltage.
You will then have to convert the energy needs from Watt hour to Amp-hour. Amp-hour
is energy measure for batteries. To convert Watt hour into Amps hours, you just divide
watt hours by system voltage.
Again, you will need to adjust energy needs in amp-hour, this time your goal is to take
into account the depth of discharge. Depth of discharge or DOD is a percentage
representing the usable capacity of a battery. It varies from 50 to 80% depending of the
manufacturer and type of battery.
It might happen that the sun is not fully available for a string of days. To prevent power
outage, the battery bank should be oversized to provide power despite absence of the
sun. In solar language, Days of autonomy is the number of days a system can supply
power without being recharged.
The final battery bank configuration will depend on the capacity or the amps hour of the
batteries available on the market.
As a designer, what you will do is to divide the overhaul battery bank size obtained after
all adjustments and days of autonomy over sizing by amp hour of the available
batteries. The number of batteries you get will be adjusted to take into account system
voltage: batteries voltage comes by multiple of two and the largest batteries are of 12V.
For better understanding, let’s come back to our example. Energy needs we got after
our load assessment was 3639Wh.
Our system voltage is 24Volts. 3639Wh is equal to 152 Ah.
The batteries we use have 60% depth of discharge. After adjustments, we go from
152Ah to 253Ah.
The client would like to have 3 days of autonomy. Our battery storage should then be of
760Ah.

12. Inverter selection
We have sized the PV array and the battery bank; we have selected a controller we now
need to select an inverter as it is the component which will make our PV system usable
by AC appliances.
When sizing the inverter you should keep in mind that:
One; it will have to supply enough power for all the AC appliances to run continuously.
Two; the inverter conversion efficiency should not affect the system and,
Three the inverter DC input voltage should match the system voltage.
From our example, after load assessment, we found that AC power demand was
equivalent to 380W;
After adjusting to avoid losses due to inverter conversion efficiency, assuming that
efficiency is 90%, the minimum inverter size is then 422W.
The think is that after sizing you have to go through inverters specifications and select
the inverter which is the most appropriate to your system.
Input voltage must correspond to system voltage;
Output voltage should match appliances nominal voltage, this might be 120 or 240Volts
depending of the country you are located;
Improper wave form may affect connected appliances;
The inverter should supply enough power for all appliances;
Check inverter surge capacity; the inverter must be capable to start appliances that may
surge.
Check inverter Stacking ability. This is an option of some inverters to be
connected together to work as a single unit.

©Mathy Mpassy Isinki
_________________________________________________________________
_
This guide by Mathy Mpassy Isinki is based on the Udemy.com couse Off Grid Solar Power
Systems Design 101; Use the Code HILL101 to unlock 50% discount on the course.
Mathy is an accomplished off grid energy solutions business and technical sales professional
technically competent in the applications and installation of off grid photovoltaic systems, as well
as in hybrid energy solutions combining diesel generators and solar.
In addition to providing solutions design; customers development and project management services
to the off grid solar industry, Mathy contribute to better energy solutions through training, with the
vision of building a better future for under-served communities in off grid locations.

A quick guide to off grid solar power systems design

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