Components of a Typical Solar Power System and Power Rating.

Solar Energy System

A Solar Energy System is a renewable energy generating system that collects photovoltaic energy from the sun and converts it into usable electricity. Often found as roof-top PV arrays, these systems can range in size and are able to power different types of properties - such as residential, commercial, and utility-scale zones.

Photovoltaic systems can be used in multiple applications: standard arrays such as roof-top or ground mount serve to generate energy for a residence or building, while other non-traditional systems can be utilized to power other objects or function (such as space satellites, hand-held calculators or vehicles).

For most DC-coupled off-grid systems it really comes down to four main components – solar panels, charger controller, inverter and the battery bank. There is a lot more that can go into a solar system setup, but those are the four main pieces.

Solar Panels

Starting with the most obvious part of an off-grid solar system are the solar panels. Currently, the most cost-effective solar panels are those made up of 60, 72, 120 or 144 cells. Solar cells are the small squares that makeup the entirety of the panel itself. These days, monocrystalline panels are the industry standard in most systems. A typical 60 cell monocrystalline solar panel will be around 66” x 40” in size and output in the range of 300-330 watts, whereas a 72 or 144 cell panel will measure 80” x 40” usually and output around 360 watts or above. Knowing how many solar panels needed for an off-grid solar system varies. Once the number of panels is known it is time to connect the panels together into a solar array. The solar panels are connected in series to form a group, then the various groups of solar panels are connected in parallel and combined into a combiner box that sends the homerun to the equipment that manages the power. This method simplifies the solar array output down to a positive and negative wire.

 

So why use monocrystalline instead of polycrystalline panels? It really comes down to availability and money. Most of the time monocrystalline panels are used in off-grid solar systems as the industry has shifted to manufacturing those types of panels instead of polycrystalline. In the early days, affordability gave the advantage to polycrystalline panels because they were cheaper to manufacture. Since that time, monocrystalline has become mainstream and affordable, so there isn’t a real advantage to using polycrystalline any longer.

 

Now that we’ve discussed the types of panels you can expect to be using in an off-grid system, it is time to think about how to mount the solar panels. There are a variety of ways to mount solar panels to make sure they don’t get damaged or blown away. There are three main ways of mounting your solar panels and the option chosen is based on the individual application:

 

Roof Mount (Mounting the solar array on a home or other shelter structure)

Pole Mount (Mounting the solar array on pole secured in the ground by concrete)

Ground Mount (Mounting the solar array on concrete piers closer to the ground for stability)

Now that the solar panels are mounted and ready to go, it’s time to figure out what to do with the positive and negative wires coming from the solar array which leads us to the next component, solar charge controllers.

 

Charge Controller

The charge controller is the device that manages the flow of energy from the solar panels to the battery. Charge controllers make sure batteries are charged properly and are not overcharged which is important for the longevity of the battery bank. There are two main types of charge controllers, MPPT (Maximum Power Point Tracking) and PWM (Pulse Width Modulation).

PWM charge controllers use pulse modulation to turn on and off the rate at which the energy from the solar panels is being sent to the batteries. When using PWM charge controllers it’s essential for the nominal voltage of the panels match the nominal voltage of the batteries. For instance, if the system is using 12 volts panels the battery bank needs to be 12 volts. There isn’t a lot of control on managing the power coming from the panels using a PWM, it’s essentially dumping the power into the batteries. PWMs offer limited input compared to an MPPT controller.

MPPT charge controllers are different in the fact that the input voltage from the solar panels needs to be 30% over the voltage of the battery voltage (up to the limit of the charge controller), so it doesn’t matter as much what voltage solar panel is used with the system. MPPT charge controllers are more efficient due to their ability to track the maximum point of power coming from the solar panels and delivering it to the batteries. It takes a higher voltage / lower current input and converting it to lower voltage / higher current output for the same amount of power. Given this fact, MPPTs very accurately control the amount of power that is sent to the batteries which is important when batteries get full and trying to satisfy system loads. The main selling point of using an MPPT controller is their ability to capture the most power from the solar array at any given moment contrary to limited input of a PWM controller. It is possible for a PWM to deliver as much power as an MPPT, but it will never deliver more power than an MPPT. For those reasons, MPPTs are usually the norm when choosing a charge controller for a solar system design.

Inverter

The next component in an off-grid solar system design would be an inverter. In nearly all off-grid solar systems, the inverter is a battery-based inverter. The inverter’s purpose is to take DC power that is stored in the battery bank and converting it to usable AC power and sending it to your loads so it can be used in the same manner as plugging into an AC outlet in a home. Inverters come in different sizes which can accommodate smaller loads or larger loads depending on the off-grid loads required. Another consideration is making sure the inverter can handle all the loads running simultaneously in the system. When all the system loads that are present in the off-grid system are added up, it will determine that maximum amount the inverter needs to be able to handle.

Another important fact is that the inverter needs to match “voltage-wise” with the system in which it is being used. For instance, a 12-volt inverter cannot be used with a 24-volt battery bank – it must be used with a 12-volt battery bank. Unlike charge controllers, the voltage on an inverter cannot be changed as it is fixed and must be matched with the battery voltage of the system. Given that information, it’s important to choose an inverter wisely when designing a system especially if expanding the system are in the plans. Choosing an inverter is an important decision to make correctly in the beginning due to the cost associated with them.

In most off-grid systems we choose to use inverter chargers. Notice we said inverter “charger”. So, we already know what a regular inverter does. What does an inverter charger do? Basically, the inverter charger acts the same as a regular inverter, but doubles as a charger. That means that the inverter not only has an output, but it also has an input. This is important because this allows the system to use an external power source such as a gas generator to power the system loads and stops drawing power from the battery bank. Once the system loads are satisfied the excess power that is being input into the system from the external power source is then being used to charge the battery bank. Going with an inverter charger allows redundancy in the system which is needed if there a several cloudy days and the solar array cannot provide enough power to charge the battery bank.

Batteries

The last main component in the solar system is the battery bank, which is one of the most important considerations and the most expensive. In the solar power industry, there are two main battery chemistries – lead acid and lithium. With lead acid there are different battery options, meaning there are multiple ways of constructing the battery. The two main types of lead acid batteries used in solar are flooded lead acid batteries and sealed AGM batteries.

 

A flooded battery is a standard wet cell lead acid battery which is usually the most cost-effective battery up front. The batteries themselves are relatively inexpensive, but there comes necessary maintenance that is required to prolong the life of the battery. Maintenance such as checking water levels in the battery, checking the specific gravity are required to prevent the battery from being ultimately destroyed. Also, regular equalization charges should be done as well to help loosen build-up that may have become solidified and adhered to the plates within the battery. Another consideration when thinking about using flooded lead acid batteries is off gassing. Under certain conditions when lead acid batteries are being charged, hydrogen gas is a byproduct which requires ventilation for the battery bank. Lack of ventilation can pose a dangerous situation when dealing with hydrogen gas fumes, so it must be taken seriously. Being that these types of batteries are cost effective many people tend to use them for their solar applications. Many that are new to off-grid solar tend to burn through their first set of batteries due to battery neglect. If a maintenance-free battery is desired and a cost-effective lead acid battery option is needed, AGM batteries may be the perfect fit.

 

AGM means Absorbed Glass Mat – which refers to the fiberglass mats between the plates where the electrolyte is absorbed. These batteries are completely sealed and there’s little to no maintenance involved. Life expectancy, charge cycles and size/weight in an AGM battery are consistent with those of a flooded lead acid battery. Due to the lack of maintenance of AGM batteries, there is an increase in price compared to flooded batteries which offsets the concern of destroying the batteries due to lack of maintenance in their flooded counterpart. One of the drawbacks of an AGM battery is that if it’s abused there isn't much that can be done to revive the battery once the damage has been done. AGM and flooded batteries offer a cheaper price point up front than lithium batteries, but their lifespans are significantly shorter.

 

Lithium Iron Phosphate (LiFePO4) is the chemistry makeup of lithium batteries. Lithium batteries are significantly different than flooded and AGM batteries in several different ways, not only in size/weight, but also how they can be charged and discharged. Lithium Iron Phosphate is an extremely safe chemistry which means it does not off gas and can be stored without the need for ventilation. Lithium batteries are completely maintenance-free and do not need to be fully charged unlike lead acid batteries. LiFePO4 chemistry is also designed specifically for a significant amount of charging cycles. These characteristics make lithium batteries extremely advantageous for off-grid solar applications. Another advantage is that lithium batteries have a built-in BMS (battery management system). The BMS is constantly monitoring the operating state of the battery. This means if the battery is being over-discharged or if the battery is too hot or cold, the BMS will force the battery to shut down until those parameter violations have been resolved. Think of BMS as a level of protection for the batteries, which makes it difficult to damage them.

 

Another advantage of lithium is that you can stack or expand an existing battery bank without affecting the lifespan of the existing bank. Adding batteries to an existing lead acid battery bank will ultimately result in failure of the entire battery bank. Lithium batteries can also be purchased in 12v, 24v and 48v variations so you can stack them easily in a nominal system voltage. This is important because if a battery is forced into shut down mode by the BMS, the entire bank does not shut down. Loads may have to be reduced to accommodate the inoperable battery, but the remaining batteries will still be online.

 

In all facets, lithium batteries are significantly superior to lead acid batteries. Depth of discharge, the number of charge cycles, safe chemistry and a built-in BMS deal a knockout blow to lead acid batteries in the long run. Not to mention lithium batteries also charge faster and deliver a substantial amount of power continuously without damaging the battery. Also, all reputable manufacturers are offering warranties on lithium batteries for around 10 years which is substantially more than warranties given on lead acid batteries. One more advantage is that space needed for a lithium battery bank is much less than that of a lead acid battery bank, which also reduces overall weight, which is roughly a 75% reduction.

 

FACTORS THAT DETERMINE THE POWER RATING OF A SOLAR POWER SYSTEM AND HOW THE COMPONENTS OF THE SOLAR SYSTEM ARE CHOSEN TO MEET THE POWER RATING.

Your personal energy use:

Your utility bills are a gold mine of information. Each bill shows your total energy used on a month-by-month basis, with a comparison to the same month from the previous year. You can even see day-to-day and hour-to-hour consumption. With this data, we can easily calculate how many solar panels you will need by multiplying your hourly energy consumption by the sunlight hours that strike your available roof area, and then dividing that number by a panel’s wattage.

Roof area and shading:

The amount of roof space available to solar panels, and the amount of sunlight that strikes that area are key to determining energy output. Fortunately, here in the Valley of the Sun, we enjoy more than 300 sunny days every year. So for us, the most important thing is to avoid shading. Trees, adjacent buildings, chimneys, and other structures can all reduce panel output.

Panel efficiency:

Not all solar panels are created equal. Most commonly used residential panels offer wattages ranging from 150 to 370 watts per panel, depending on size. Higher-grade panels create more electricity from less surface area than lower-grade panels. So, if roof space is limited, or shading structures cannot be removed, a higher-efficiency panel may cost a bit more, but will produce more electricity from less space.

Buyback programs through your utility provider:

In the vast majority of cases, solar works in partnership with the grid, which is essential to ensuring that your home has electricity even at night. Both electricity purchased from the utility, and electricity generated by your solar panels is measured by the same meter. The meter runs forward when being supplied by the grid, and backward when your solar PV system is generating excess electricity. In utility districts that support energy buyback programs (net metering), the homeowner has an opportunity to earn energy credits for their excess power. You can draw from these credits when you’re using conventional power, such as at night. With that said, determining how your local power utility provider calculates these credits is a critical factor in determining how many panels you buy. The more credits you want, the more panels you will need.

SOLAR PV SYSTEM SIZING

1 Determine power consumption demands

The first step in designing a solar PV system is to find out the total power and energy consumption of all loads that need to be supplied by the solar PV system as follows:

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

Add the Watt-hours needed for all appliances 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.

2 Size the PV modules

Different size of 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 Inverter sizing

An inverter is used in the system where AC power output is needed. The input rating of the inverter should never be lower than the total watt of appliances. The inverter must have the same nominal voltage as your battery.

For stand-alone systems, the inverter must be large enough to handle the total amount of Watts you will be using at one time. The inverter size should be 25-30% bigger than total Watts of appliances. In case of appliance type is motor or compressor then inverter size should be minimum 3 times the capacity of those appliances and must be added to the inverter capacity to handle surge current during starting.

For grid tie systems or grid connected systems, the input rating of the inverter should be same as PV array rating to allow for safe and efficient operation.

4. 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:

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

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

4.3 Divide the answer obtained in item 4.2 by 0.6 for depth of discharge.

4.4 Divide the answer obtained in item 4.3 by the nominal battery voltage.

4.5 Multiply the answer obtained in item 4.4 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)

5. 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 190W 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

So the solar charge controller should be rated 40 A at 12 V or greater.