1. Engineering Photovoltaic Systems I
Part I
This is the second presentation in a series covering solar photovoltaic energy. Today, we are going to talk about basic engineering of photovoltaic systems.
Instructors Notes:
Feel free to use all or parts of this presentation but please follow the Creative Commons Attribution-NonCommercial-ShareAlike 2.5 License which can be found here:
Original Presentation by J. M. Pearce, 2006

2. Outline Part I What is a photovoltaic system Cell, Module, and Array
BOS
Structure
Electronics
PV System Design Basics
Hybrid Systems
First we are going to outline a photovoltaic system, which includes solar cells, modules, and arrays. We will go over what you need to determine the balance of systems (BOS), which includes the structure and electronics, in order to get useable electricity from the solar cells. We will go over the photovoltaic system design basics and look at some examples of hybrid systems for homes, businesses and schools.
Engineering Photovoltaic Systems

3. The Cell, The Module and The Array
Here we have drawings of the cell, the module and the array. The cell is a single cell, and the example shown here is crystalline silicon. When you combine single cells together, you have a module. This is the actual electronic device you can buy off the shelf at places such as Home Depot. The modules are wired in series and in parallel to get an array. To power a modern American home, you cannot use a single module, you would need an entire array comprising 4-5kW.
The PV cell is the basic unit in a PV system. An individual PV cell typically produces between 1 and 2 W, hardly enough power for the great majority of applications. But you can increase the power by connecting cells together to form larger units called modules. Modules, in turn, can be connected to form even larger units known as arrays, which can be interconnected for more power, and so on. In this way, you can build a PV system to meet almost any power need, no matter how small or great.
Modules or arrays, by themselves, do not constitute a PV system. You must also have structures on which to put them and point them toward the sun, and components that take the direct-current (dc) electricity produced by the modules or arrays and condition the electricity so that it may be utilized in the application. These structures and components are referred to as the balance-of-system (BOS).
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Engineering Photovoltaic Systems

4. Balance of System (BOS)
The BOS typically contains;
Structures for mounting the PV arrays or modules
Power conditioning equipment that massages and converts the do electricity to the proper form and magnitude required by an alternating current (ac) load.
Sometimes also storage devices, such as batteries, for storing PV generated electricity during cloudy days and at night.
The balance of system components (BOS) are comprised of the additional electrical equipment required for the system to operate properly and the structural support. The balance of systems usually contains structures for mounting the PV arrays or modules. In some the modules are located directly on the roof and we will see examples of this in a moment. There is significant savings doing this to avoid infrastructure such as concrete and metal poles used to support the structure of modules shown in the above photo. This is power conditioning equipment that massages and converts the electricity to the proper form and magnitude required by an alternating current (ac) load is also necessary. The electricity produced from solar cells is DC and then it goes through an inverter in order to make it AC. An inverter is shown in the second picture. The third picture is a battery stored system. When you are not on the grid, you need to store the energy in some way and you can store it in batteries for instances such as cloudy days or at night. Even if the array is connected to the grid, sometimes users want a battery backup for safety reasons.
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Engineering Photovoltaic Systems

5. Three Types of Systems
Stand-alone systems - those systems which use photovoltaics technology only, and are not connected to a utility grid.
Hybrid systems - those systems which use photovoltaics and some other form of energy, such as diesel generation or wind.
Grid-tied systems - those systems which are connected to a utility grid.
There are basically three types of photovoltaic systems. The stand alone systems use photovoltaics technology only, and are not connected to a utility grid. The usually have some form of backup. The second type of system is the hybrid system. Hybrid systems are made up of PV and some other forms of energy production, such as wind or diesel generation. Finally, grid-tied systems do not necessarily need backup at all and are tied directly to the utility grid. The grid stores the energy. For example, if you have a PV system on your house, and it produces more energy than needed, the extra will go back onto the grid. So during cloudy days or at night, you can pull the energy from the grid into your home.
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Engineering Photovoltaic Systems

6. Stand Alone PV System Water pumping Engineering Photovoltaic Systems
This is an example of a stand alone PV system. To pump water in India as shown here, you have the PV arrays which produce electricity, which travels through a disconnect switch and linear current booster to provide power to the water pump. This system is common in underdeveloped countries and agricultural applications. It is very simple to design and has no storage.
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Engineering Photovoltaic Systems

7. Examples of Stand Alone PV Systems
PV panel on a water pump in Thailand
Here are some examples of stand alone PV systems. The bottom picture is a water pump that is providing water for cows in Wyoming. It actually will fill up a reservoir of water and only keep providing electricity to the pump when it is necessary. You can also see there is a tracking system so that it tracks the sun as it follows it’s arc in the sky and allows more sunlight to be collected.The University of Wyoming and several rural electric companies have set up rural demonstration projects to study the durability, maintenance requirements, and useful life of solar-powered water-pumping systems. Because of the vast, rural spaces in Wyoming, PV is a feasible solution to power needs in remote areas where traditional utility infrastructure is not economically viable. A medium-size solar-powered system costs about $12,000. The depth of the well and the amount of water needed on a case-by-case basis determine the size of the system.
The other picture is a water pump that provides water for a farm in Thailand.
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PV powers stock water pumps in remote locations in Wyoming
Engineering Photovoltaic Systems

8. Examples of Stand Alone PV Systems
Communications facilities can be powered by solar technologies, even in remote, rugged terrain. Also, if a natural or human-caused disaster disables the utility grid, solar technologies can maintain power to critical operations
Here are some examples of stand alone PV systems. Communications facilities can be powered by solar technologies, even in remote, rugged terrain. Also, if a natural or human-caused disaster disables the utility grid, solar technologies can maintain power to critical operations. It is very common to see these used for communication stations.
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Engineering Photovoltaic Systems

9. Examples of Stand Alone PV Systems
This exhibit, dubbed "Solar Independence", is a 4-kW system used for mobile emergency power.
while the workhorse batteries that can store up to 51 kW-hrs of electricity are housed in a portable trailer behind the flag.
The system is the largest mobile power unit ever built
Here is another example of a stand alone system. This exhibit, dubbed "Solar Independence", is a 4-kW system. It can provide power for one or two homes. It can store up to 51 kW-hrs of electricity that is stored in the portable trailer to the right of the flag. It is made to look like a flag to encourage American patriotism. This is one of the largest mobile power units ever built. It can be used for demonstrations at schools.
The flag's field of blue consists of PV panels that generate enough electricity to power 1-2 homes. Red and white stripes make it look like the flag. The system was designed by NREL's Ben Kroposki and set up by a host of volunteers from the National Center for Photovoltaics and the Public Affairs Office
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Engineering Photovoltaic Systems

10. Examples of Stand Alone PV Systems
Smiling child stands in front of Tibetan home that uses 20 W PV panel for electricity
PV panel on rooftop of rural residence
This is an example of very small PV systems to provide electricity for indoor lighting in rural residence in Tibet. In America we tend to forget that there are still many places around the world that cannot get electricity easily. These PV systems help rural families like this get some electricity for lighting and refrigeration.
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Engineering Photovoltaic Systems

11. Hybrid PV System Engineering Photovoltaic Systems
This is a block diagram of a hybrid PV system. The PV array produces electricity, which goes through the blocking diode and the fused disconnect to get to the maximum power tracker. If you need electricity immediately, it runs though the inverter and transformer and goes to your load. Otherwise, it is stored in a battery. Then if it became dark out, you can draw the energy from the battery. If it is dark for a very long time, you can draw the energy from the diesel generator.
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Engineering Photovoltaic Systems

12. Examples of Hybrid PV Systems
Ranching the Sun project in Hawaii generates 175 kW of PVpower and 50 kW of wind power from the five Bergey 10 kW wind turbines
This example of a hybrid system is a ranch project in Hawaii where you have both wind and solar PV energy needed for the farm. There is 175 kW of PV power and 50 kW of wind power.
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Engineering Photovoltaic Systems

13. Examples of Hybrid PV Systems
A fleet of small turbines; PV panels in the foreground
Here is another example of a wind/PV hybrid project. The fleet of small turbines capture power for the New Life Evangelistic Center in New Bloomfield, Missouri.
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Engineering Photovoltaic Systems

14. Examples of Hybrid PV Systems
PV / diesel hybrid power system - 12 kW PV array, 20 kW diesel genset
This system serves as the master site for the "top gun" Tactical Air Combat Training System (TACTS) on the U.S. Navy's Fallon Range.
This is an example of a PV/ diesel hybrid project. There is 12 kW PV array and 20 kW diesel generator housed behind it. This system serves as the master site for the “top gun” training for the U.S. Navy.
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Engineering Photovoltaic Systems

15. Grid-Tied PV System Engineering Photovoltaic Systems
Most of us are interested in grid-tied systems because most of us are connected on grid system houses, so if you don’t have a ranch or a cottage up in the country, you get your electricity from a grid. For this type of system it is very feasible to put a solar array on your roof. Here you have solar panel array on your roof, sun shines on it and produces a DC voltage (like a battery). Then it runs through the inverter and turns into AC Voltage (like the electricity that comes from a wall outlet). The inverter is usually monitored in some way. Usually you’ll have a panel inside your home. AC voltage goes straight to the main utility breaker panel. From there it can be used to power any electrical device in your home. In cases, lets say, at lunch time, if you’re at work, and the solar cell is doing really good because it’s a sunny day out, you’re going to be producing a lot of electricity, and then it can be fed back into grid and spin your utility meter backwards. You then only pay for the net amount of electricity you use. This is called net metering. You are only paying for the net amount of electricity you are consuming. Some solar users purposely make more electricity than they use so they can receive money back from the electric companies.
Base definitions for grid tied solar photovoltaic systems:
Solar Panels convert sunlight directly into electricity. The Inverter converts the solar electricity (DC) into household current (AC) that can be used to power loads in the house. The System Monitor is an easy-to-read digital meter that shows the homeowner the amount of electricity generated both cumulatively and daily. The Utility Meter tracks power usage and production, spinning forward when electricity is used from the grid, and spinning backwards, generating a credit, when the solar system creates more electricity than is used in the house.
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Grid-Tied PV System
Engineering Photovoltaic Systems

16. Examples of Grid Tied Systems
National Center for Appropriate Technology Headquarters
This is the National Center for Appropriate Technology Headquarters. As you can see it is solar designed and has solar panels on the roof. Solar power fills some of the power needs at the headquarters in Butte, Montana. The photovoltaic system was installed on NCAT's roof by Solar Plexus of Missoula; it consists of 60 Shell SP75 modules. Each module is rated at 75 watts and is flush-mounted on the south-facing, third-floor roof; total module area is about 480 ft2. Peak electrical output of the system is 4.5 kilowatts. Thirty modules make up each of the east and west arrays. Each array is electrically connected to produce direct current (DC) at 48 volts. The output from each array can be disconnected for comparison purposes. The system's DC electrical output is connected to a Trace SW4048UPV utility intertie inverter. The inverter, connected to the utility grid, senses the utility's voltage and frequency and produces alternating electricity at the utility's voltage and frequency. The inverter is designed to operate only when utility electricity is available. Whenever the utility goes down, the inverter shuts down and will only turn back on when the utility line comes back on. This is for safety and now all modern inverters have this feature. The system provides about 15% of the NCAT building's electrical consumption.
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Engineering Photovoltaic Systems

17. Examples of Grid Tied Systems
The world's largest residential PV project
This is the world’s largest residential PV project. It has PV for every single apartment. The system produces over 180 kWp in this 500 home development kW systems on 32 pitched roofed houses and 3.06 kW systems on 32 flat roofed houses in Amersfoort, Netherlands.
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Engineering Photovoltaic Systems

18. Designing a PV System Determine the load (energy, not power)
You should think of the load as being supplied by the stored energy device, usually the battery, and of the photovoltaic system as a battery charger. Initial steps in the process include:
Calculating the battery size, if one is needed
Calculate the number of photovoltaic modules required
Assessing the need for any back-up energy of flexibility for load growth
Stand-Alone Photovoltaic Systems: A Handbook of Recommended Design Practices details the design of complete photovoltaic systems.
So if you would like a PV system of your own, you can actually design it yourself. This can also be used for an exercise during class or do it for your own home. So the first thing you want to do is determine your load. This is the energy, not the power. You can think of this as supplied by the stored energy device, usually the battery, and of the photovoltaic system as a battery charger. The steps in this process are the following. You need to calculate your battery size, calculate the number of photovoltaic modules you’ll need, and then assess the need for any back-up energy of flexibility for load growth. A great place to get this information is Stand-Alone Photovoltaic Systems: A Handbook of Recommended Design Practices details the design of complete photovoltaic systems.
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Engineering Photovoltaic Systems

19. Determining Your Load
The appliances and devices (TV's, computers, lights, water pumps etc.) that consume electrical power are called loads.
Important : examine your power consumption and reduce your power needs as much as possible.
Make a list of the appliances and/or loads you are going to run from your solar electric system.
Find out how much power each item consumes while operating.
Most appliances have a label on the back which lists the Wattage.
Specification sheets, local appliance dealers, and the product manufacturers are other sources of information.
Your first step in designing your PV system is determining your load. Any device that consumes electrical power are called loads. You want to examine your own power consumption. Its important to note, that you want to reduce your power needs as much as possible. That is because it will reduce the size of your PV system and reduce your initial costs. You will want to make a list of things you will run on your system and try to reduce it as much as possible. For example, change your lighting from incandescent to CFL. So after you’ve found every means to reduce energy use, figure out how much power each appliance or load uses and how much time you are using it for. Most appliances have the power it uses on the back label. Another means of finding how much power a device uses, you can look off specification sheets, local appliance dealers, and the product manufacturers are other sources of information.
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Engineering Photovoltaic Systems

20. Determining your Loads II
Calculate your AC loads (and DC if necessary)
List all AC loads, wattage and hours of use per week (Hrs/Wk).
Multiply Watts by Hrs/Wk to get Watt-hours per week (WH/Wk).
Add all the watt hours per week to determine AC Watt Hours Per Week.
Divide by 1000 to get kW-hrs/week
Then you need to calculate your AC and DC loads. Sometimes you have more complicated systems that are half AC and half DC. But here we are going to work with just the AC systems. So first you want to list your AC loads, like your hair dryer, refrigerator, computer. Then you would want to list the wattage and hours used per week. For example, say I use my 250 Watt computer for an hour a day every week day, that adds up to five hours a week. Then I would multiply 250 Watts by 5 hrs a week and I would get 1250 Watt-hours per week. Next I would divide by 1000 to get the kW-hrs/week. As you can see kW-hrs is how the electric companies measure your energy use and that is what you pay for on your bill. The other thing to note here is that, for a refrigerator, it is not running all the time so you should not calculate it running 24 hrs/ 7 days a week. For devices like this the manufacturers will have a good estimate of how much energy it uses per year.
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Engineering Photovoltaic Systems

21. Determining the Batteries
Decide how much storage you would like your battery bank to provide (you may need 0 if grid tied)
expressed as "days of autonomy" because it is based on the number of days you expect your system to provide power without receiving an input charge from the solar panels or the grid.
Also consider usage pattern and critical nature of your application.
If you are installing a system for a weekend home, you might want to consider a larger battery bank because your system will have all week to charge and store energy.
Alternatively, if you are adding a solar panel array as a supplement to a generator based system, your battery bank can be slightly undersized since the generator can be operated in needed for recharging.
Now you need to determine the batteries needed for your system. Now if you are planning on doing a grid-type system, you can skip this step. But if you need battery back for places such as hospitals, or you are not tied to the grid, you need to first determine how long you want the electricity to be provided if there is no sunlight. This is usually expressed as “days of autonomy" because it is based on the number of days you expect your system to provide power without receiving an input charge from the solar panels or the grid. You also need to consider usage pattern and critical nature of your application. If its not very important to you to have electricity every single day, then its okay to use a small number for the days. If you are installing a system for a weekend home, you might want to consider a larger battery bank and a smaller PV array because your system will have all week to charge and store energy. Alternatively, if you are adding a solar panel array as a supplement to a generator based system, your battery bank can be slightly undersized since the generator can be operated in needed for recharging.
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Engineering Photovoltaic Systems

22. Batteries II
Once you have determined your storage capacity, you are ready to consider the following key parameters:
Amp hours, temperature multiplier, battery size and number
To get Amp hours you need:
daily Amp hours
number of days of storage capacity ( typically 5 days no input )
1 x 2 = A-hrs needed
Note: For grid tied – inverter losses
Once you have determined your storage capacity, you are ready to consider the following key parameters. So after you know how many days you want to go just off the battery, you want to determine the number of daily amp hours. You do this by taking the number of days of storage capacity and multiply it by the daily amp hours and then you get the total amp-hours needed for your battery.
Note if you are installing a grid-tied system you will need to take into account the efficiency of the inverter to convert battery loads into AC loads.
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Photovoltaics Design and Installation Manual --SEI
Engineering Photovoltaic Systems

23. Temperature Multiplier
Temp oF 80 F 70 F 60 F 50 F 40 F 30 F 20 F
 Temp oC 26.7 C 21.2 C 15.6 C 10.0 C 4.4 C -1.1 C -6.7 C
Multiplier
Batteries work better at lower temperatures than at higher temperatures so you want to select the closest multiplier for the average ambient winter temperature your batteries will experience.
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Select the closest multiplier for the average ambient winter temperature your batteries will experience.
Engineering Photovoltaic Systems

24. Determining Battery Size
Determine the discharge limit for the batteries ( between )
Deep-cycle lead acid batteries should never be completely discharged, an acceptable discharge average is 50% or a discharge limit of 0.5
Divide A-hrs/week by discharge limit and multiply by “temperature multiplier”
Then determine A-hrs of battery and # of batteries needed - Round off to the next highest number.
This is the number of batteries wired in parallel needed.
Next you want to determine the discharge limit for the batteries. This is usually between Normally for acid batteries you never want to discharge them completely. So an acceptable discharge is somewhere around 50 percent. So you divide the number of amp-hrs/week by discharge limit and multiply by “temperature multiplier” that we received from the last slide. If you end up with a fraction – round up to the next whole number of batteries to deliver the necessary current.
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Engineering Photovoltaic Systems

25. Total Number of Batteries Wired in Series
Divide system voltage ( typically 12, 24 or 48 ) by battery voltage.
This is the number of batteries wired in series needed.
Multiply the number of batteries in parallel by the number in series –
This is the total number of batteries needed.
Now to get the total number of batteries, you will be needing to think about the voltage. Remember, Power = Voltage x Current. You take your system voltage and divide by the battery voltage. For example you have a 48 volt system and you divide by 12 voltage battery, you get that you need 4 batteries. So now you need to wire 4 batteries in series and then you multiply the number of batteries in parallel by the number in series and this is the total number of batteries needed.
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Engineering Photovoltaic Systems

26. Determining the Number of PV Modules
First find the Solar Irradiance in your area
Irradiance is the amount of solar power striking a given area and is a measure of the intensity of the sunshine.
PV engineers use units of Watts (or kiloWatts) per square meter (W/m2) for irradiance.
For detailed Solar Radiation data available for your area in the US:
After you have figured out your batteries or you have skipped the other steps because you are already attached to the grid, you can then determine the number of PV modules you will need. First you want to find the Solar Irradiance in your area. Irradiance is the amount of solar power striking a given area and is a measure of the intensity of the sunshine. PV engineers use units of Watts, or kiloWatts, per square meter (W/m2) for irradiance. And if you want your irradiance for any location in the US, go to this website:
Engineering Photovoltaic Systems

27. How Much Solar Irradiance Do You Get?
You can get the basic idea of the average irradiance over the United States in W-hrs per square meter per day from this map, which is the average daily solar radiation from
It should be noted that this map is meant to give you a feel for solar irradiance. The best data and easiest to use to design a system is available in table/spreadsheet form. For non-tracking arrays, you want the global insolation on a surface tilted at the latitude facing south (for northern hemisphere).
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Engineering Photovoltaic Systems

28. Solar Radiation
On any given day the solar radiation varies continuously from sunup to sundown and depends on cloud cover, sun position and content and turbidity of the atmosphere.
The maximum irradiance is available at solar noon which is defined as the midpoint, in time, between sunrise and sunset.
Insolation (now commonly referred as irradiation) differs from irradiance because of the inclusion of time. Insolation is the amount of solar energy received on a given area over time measured in kilowatt-hours per square meter squared (kW-hrs/m2) - this value is equivalent to "peak sun hours".
On any given day the solar radiation varies continuously from sunup to sundown and it is also known to change spectrum. It also depends on cloud cover, sun position and content and turbidity of the atmosphere. The maximum irradiance is available at solar noon which is defined as the midpoint, in time, between sunrise and sunset. Insolation , now commonly referred as irradation, differs from irradiance because of the inclusion of time. Insolation is the amount of solar energy received on a given area over time measured in kilowatt-hours per square meter squared. This value is equivalent to "peak sun hours". In short, if you determine the number of peak sun hours you have for your area, it makes the math a little bit easier to deal with. Therefore, peak sun hours corresponds directly to average daily insolation given in kW-hrs/m2.
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Engineering Photovoltaic Systems

29. Peak Sun Hours
Peak sun hours is defined as the equivalent number of hours per day, with solar irradiance equaling 1,000 W/m2, that gives the same energy received from sunrise to sundown.
Peak sun hours only make sense because PV panel power output is rated with a radiation level of 1,000W/m2.
Many tables of solar data are often presented as an average daily value of peak sun hours (kW-hrs/m2) for each month.
Peak sun hours is defined as the equivalent number of hours per day, with solar irradiance equaling 1,000 W/m2. That gives the same energy received from sunrise to sunset. Peak sun hours only make sense because PV panel power output is rated with a radiation level of 1,000W/m2. In other words, six peak sun hours means that the energy received during total daylight hours equals the energy that would have been received had the sun shone for six hours with an irradiance of 1,000 W/m2. Many tables of solar data are often presented as an average daily value of peak sun hours (kwh/m2) for each month.
In other words, six peak sun hours means that the energy received during total daylight hours equals the energy that would have been received had the sun shone for six hours with an irradiance of 1,000 W/m2.
This is how you would determine your irradiance. Therefore, peak sun hours corresponds directly to average daily insolation given in kwh/m2.
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Engineering Photovoltaic Systems

30. Calculating Energy Output of a PV Array
Determine total A-hrs/day and increase by 20% for battery losses then divide by “1 sun hours” to get total Amps needed for array
Then divide your Amps by the Peak Amps produced by your solar module
You can determine peak amperage if you divide the module's wattage by the peak power point voltage
Determine the number of modules in each series string needed to supply necessary DC battery Voltage
Then multiply the number (for A and for V) together to get the amount of power you need
P=IV [W]=[A]x[V]
Next you want to calculate energy output of a PV Array. You have already determined the total number of Amp-hrs per day and you will want to increase this by 20% for battery losses then divide by “1 sun hours”, from the map on an earlier slide, to get total Amps needed for array. Then divide your Amps by the Peak Amps produced by the solar module you have chosen to use. You will need to pick out a certain type of solar module and determine the peak amperage if you divide the module's wattage by the peak power point voltage. The power (P) equals the current (I) times the voltage (V). Dividing P by V gives you the current in Amps. Next, determine the number of modules in each series string needed to supply necessary DC battery Voltage. For example, if you need 12V and your peak power voltage is 4V you need 3 panels in series. Finally you can determine the total power for your PV system by multiplying the number of amps and the number of volts together. P=IV [W]=[A]x[V].
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Engineering Photovoltaic Systems

31. Charge Controller
Charge controllers are included in most PV systems to protect the batteries from overcharge and/or excessive discharge.
The minimum function of the controller is to disconnect the array when the battery is fully charged and keep the battery fully charged without damage.
The charging routine is not the same for all batteries: a charge controller designed for lead-acid batteries should not be used to control NiCd batteries.
Size by determining total Amp max for your array
If your system has a battery, you’ll need a charge controller. Charge controllers are included in most PV systems to protect the batteries from overcharge and/or excessive discharge. The minimum function of the controller is to disconnect the array when the battery is fully charged and keep the battery fully charged without damage. The charging routine is not the same for all batteries: a charge controller designed for lead-acid batteries should not be used to control NiCd batteries. To determine the size of charge controller use the total Amp maximum for your array. The basic criteria for selecting a controller includes the operating voltage and the PV array current. Controllers are critical components in stand-alone PV systems because a controller failure can damage the batteries or load. The controller must be sized to handle the maximum current produced by the PV array.
There are two types of controllers: series and shunt. Series controllers stop the flow of current by opening the circuit between the battery and the PV array. Shunt controllers divert the PV array current from the battery. Both types use solid state battery voltage measurement devices and shunt controllers are 100% solid state.
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Engineering Photovoltaic Systems

32. Wiring
Selecting the correct size and type of wire will enhance the performance and reliability of your PV system.
The size of the wire must be large enough to carry the maximum current expected without undue voltage losses.
All wire has a certain amount of resistance to the flow of current.
This resistance causes a drop in the voltage from the source to the load. Voltage drops cause inefficiencies, especially in low voltage systems ( 12V or less ).
See wire size charts here:
V=IR or
R = V/I
Wiring is also important. You want to select the correct size and type of wire to really make your PV system work properly. If your wiring is too small, you are going to lose a lot of your electricity to resistance losses. The size of the wire must be large enough to carry the maximum current expected without undue voltage losses. All wire has a certain amount of resistance to the flow of current. This resistance causes a drop in the voltage from the source to the load. Voltage drops cause inefficiencies, especially in low voltage systems. You can go to this website for sizing charts for wires based on the size of your PV array. The voltage (V) drop is given by the resistance (R), measured in Ohms in the wire times the current (I) flowing through the wire. V=IR is called Ohm’s Law – one of the most useful rules in electronics.
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Engineering Photovoltaic Systems

33. Inverters
For AC grid-tied systems you do not need a battery or charge controller if you do not need back up power –just the inverter.
The Inverter changes the DC current stored in the batteries or directly from your PV into usable AC current.
To size increase the Watts expected to be used by your AC loads running simultaneously by 20%
For AC grid-tied systems you do not need a battery or charge controller if you do not need back up power, just the inverter. The Inverter changes the DC current stored in the batteries or directly from the PV into usable AC current which is the most common type used by most household appliances and lighting. To size increase the Watts expected to be used by your AC loads running simultaneously by 20%. You most likely will not have all of your appliances running continuously at the same time but this will account for surges in the system.
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Engineering Photovoltaic Systems

34. Books for Designing a PV System
Steven J. Strong and William G. Scheller, The Solar Electric House: Energy for the Environmentally- Responsive, Energy-Independent Home, by Chelsea Green Pub Co; 2nd edition, 1994.
This book will help with the initial design and contacting a certified installer.
If you want to get more information about PV system design, I recommend reading The Solar Electric House: Energy for the Environmentally- Responsive, Energy-Independent Home.
Engineering Photovoltaic Systems

35. Books for the DIYer
If you want to do everything yourself also consider these resources:
Richard J. Komp, and John Perlin, Practical Photovoltaics:  Electricity from Solar Cells, Aatec Pub., 3.1 edition, (A layman’s treatment).
Roger Messenger and Jerry Ventre, Photovoltaic Systems Engineering, CRC Press, (Comprehensive specialized engineering of PV systems).
In addition, if you want to do everything yourself also consider these resources: Practical Photovoltaics:  Electricity from Solar Cells for a laymen’s treatment of the subject and Photovoltaic Systems Engineering for a comprehensive specialized engineering account of PV systems.
Engineering Photovoltaic Systems

36. Photovoltaics Design and Installation Manual
Photovoltaics: Design & Installation Manual by SEI Solar Energy International, 2004
A manual on how to design, install and maintain a photovoltaic (PV) system.
This manual offers an overview of photovoltaic electricity, and a detailed description of PV system components, including PV modules, batteries, controllers and inverters. Electrical loads are also addressed, including lighting systems, refrigeration, water pumping, tools and appliances.
There is also an excellent manual by SEI – Solar Energy International. This textbook manual is on how to design, install and maintain a photovoltaic (PV) system. This manual offers an overview of photovoltaic electricity, and a detailed description of PV system components, including PV modules, batteries, controllers and inverters. Electrical loads are also addressed, including lighting systems, refrigeration, water pumping, tools and appliances. The manual includes chapters on sizing photovoltaic systems, analyzing sites and installing PV systems. The manual also includes detailed appendices on PV system maintenance, troubleshooting, and insolation data for over 300 sites around the world. It is used as the textbook in SEI's PV Design and Installation Workshop.
Source:
Engineering Photovoltaic Systems

37. Solar Photovoltaics is the Future
Solar photovoltaics is the future and the future is now!

38. Acknowledgements
This is the second in a series of presentations created for the solar energy community to assist in the dissemination of information about solar photovoltaics.
This work was supported from a grant from the Pennsylvania State System of Higher Education.
The author would like to acknowledge assistance in creation of this presentation from Heather Zielonka, Scott Horengic and Jennifer Rockage.
Engineering Photovoltaic Systems