Solar Site Assessment

Friday, March 10, 2017
In the process of selection, design and installation of an appropriate solar PV system for your home or business, a professional site assessment is a crucial part. Cost and power output of your potential PV System significantly depend on it. The output of a PV module is directly proportional to the amount of sunlight strikes on it.

A proper site assessment significantly contributes to determine and identify a number of factors: the site capacity of generating energy, shading issues, how much revenue will be generated from the power plant, return on investment (ROI), rebates and incentives, structural and electrical concerns. The site assessment also significantly contributes to save money in basic energy efficiency improvements and more lucrative methods to organize a solar project on-site. After reviewing the report, you will be able to make well-versed decisions about your project. The site assessment report will help you to bring your goals, budget and energy needs together with the unique solar opportunities at your location. Every site is different and needs evaluation specific to the site. 

Electricity is produced from PV Modules produce when photons on solar cells and knock available electrons loose and into motion. Photons are small packets of energy contained in sunlight. When fewer photons strike on the solar cell, for example due to poor orientation or haze, fewer electrons are put into motion. As a result, little amount of electricity is produced. But if there is shade in the site even with little amount, it can cause shutting down the production completely in some cases.

Modules with built-in bypass diodes contribute to minimize the effects of partial shading. But, even a row of cells with shade can disable the module. Impact of shading requires careful site planning and design considerations for solar PV arrays. Whether it is a neighbor’s multistory home or trees on your property, most sites should be considered at least some shade.  While wide-open, dawn-to-dusk exposure is ideal, PV system designers generally shoot for a shade-free solar window from 9 a.m. to 3 p.m. (“solar time” for all days/months of the year). Majority of solar radiation is available during these hours. However, it may be effected by local climate variations. For example, in some locations, early morning fog can shift the “prime” solar window toward sunnier afternoon hours.

If forecasting shading throughout the year has been done by sight alone from various barriers like tall trees, nearby buildings, roof dormers and even chimney, then it can be challenging which requires many observations over the course of the year. But some tools like the Solar Pathfinder, the Acme Solar Site Evaluation Tool and the SunEye, can assist you to assess shading on your site throughout the year quickly with one site visit. Each tool has different technique and price. But, the job can be done by all these tools.  They can be used at a proposed array location for the evaluation.

Historical solar radiation and weather data for your latitude and longitude and the constantly changing sun elevation angle are considered by these tools. To provide additional data for accurate shade compensation calculations due to tall trees, nearby buildings etc. digital photos are taken at the site. Depending on the solar panel tilt (up and down angle), azimuth (right and left orientation) and whether a tracking system is to be employed, some modifications are done. Wire runs, connections, fuses and breakers, inefficiencies of the inverter and snow shading etc. are also needed to be considered for the final output of power production of potential solar PV system.

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How to design a solar pv system

Wednesday, May 11, 2016
In order to design a solar pv system, you need to follow following steps:

1. Determine the amount of electricity (kWh) consumption and the electricity rate

It is important to determine the amount of electricity consumption per year for better understanding the economics of installing a solar system. On your utility bill, your electricity usage is summarized wherein natural gas usage should be excluded. As the path of the sun varies with the seasons, a solar PV system would offset your electricity demands mostly in the summer months. It should be also considered that most utility companies have a tier rate structure based on consumption patterns and state mandates. A small PV system significantly helps to reduce your electricity demands for moving you to a different tier, minimize your rates and saves your money.

2. Determine your available roof space (sq-ft)

Knowing your available roof space is crucial for determining the quantity of solar panels you can actually use. Based on the size of your house, you should draw a diagram of your roof on a piece of paper. You can also use the free tool Google SketchUp or a free CAD program DraftSight. After determining the size of your roof, it is important to know how much of your roof faces towards the equator (South in the northern hemisphere [USA, Canada, etc.], north in the southern hemisphere [Australia, South America etc.]). Solar irradiance is strongest coming from the equator and you will want to maximize your system efficiency.

3. Determine your solar radiation data and calculate the amount of energy you could produce

In this case, you should use the official government statistics available on your area. PV Watts’ or Solmetric's solar calculators also can be used.

4. Determine shading issues

You should be careful if there are obstacles like trees or buildings in the vicinity of your roof which may cause shade. Shading decreases the output of your system and may also damage the solar cells. Solar Pathfinder or Solmetric Sun Eye should be used for detailed shading analysis. 

5. Choose an appropriate solar panel

Various types of solar panels are available in the market. Therefore, it is very important to choose an appropriate solar panel carefully. The solar rating and the size (surface area) of the module are the most important factors. Although the solar ratings might be the same for some modules, the voltage and current output might vary. Therefore, it is advised to choose a lower voltage module for smaller projects having capacity less than 4 kW. Factors like financial limitations and color are also should be considered.

6. Choose an inverter

It is very crucial to choose an inverter out of the many available in the market. You can consider to use micro inverters which are easy to install and can generate AC power directly from the solar panel. If you want to use a central inverter, you should use the string sizing tools. The most important factors for choosing appropriate sized inverter are:

  • The number of strings in the system.
  • The maximum input current.
  • The voltage on a string.
  • The minimum ambient temperature during day light time when the system is supposed to run.
  • The maximum ambient temperature of the location.

7. Select a racking system to mount your solar PV system on your roof

It should be remembered that the racking system will consist 10% to 25% of the total costs. You should decide carefully which racking system is most suitable for your budget and for your roof. The racking system is one of the key components which protects both the roof and the modules and may last for at least 20 years. A minor mistake with the racking can permanently damage the roof or injure people or property.

Different types of racking and mounting systems are available in the market for roof mounted solar systems. Check the manufacturer's websites, read the installation manual. Be aware of the plan and codes how you are going to mount the racking system on the roof. Know about flashing solutions.

Using an inexpensive racking solution does not mean lower quality, it might require more effort to install it or the materials may be different. Additionally, as the solar PV field matures unique racking solutions, such as hanging panels vertically along an exterior wall, become more applicable.

8. Know your local and state incentives; calculate how much money you could save

You should check the websites of your local and state incentives provider. There are also a number of other financial calculators available online. Most importantly, you should estimate your electricity generation and compare it to your utility rate. It should be remembered that different tier rates might apply based on your energy consumption. Know about solar programs of your local utility company and their applications process. Implement the benefit of tax cuts and incentives.

9. Prepare your documents 

There are different permitting processes in different location. In order to receive a construction permit or to apply for an incentive, you must obtain the equipments' (solar panel, inverter, racking) data sheets, submit a roof drawing and electrical drawing. You can draw those on your own using the programs mentioned above, or ask a professional to draw them for you. In order to determine the wire sizes, voltage drops, AC and DC disconnect, electrical background and knowledge is required. If you are not confident enough to do so, you should contact an electrical engineer.

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How to troubleshoot a solar photovoltaic (PV) system

Friday, December 4, 2015
We expect smooth power generation from a solar photovoltaic (PV) system while sun shining. When the sun is out but the system doesn't generate electricity as per required capacity, then we consider the system as problematic. Solar photovoltaic (PV) system occasionally requires troubleshooting as like as other energy systems. The key to your success depends on the strategy you choose for identifying the trouble source. Typically, three problems occur in solar photovoltaic (PV) system. They are-an array problem, inverter problem, or load problem. Concentrating on common troubleshooting problems and solutions can ensure that your system is taking the advantage of summer's sunny days.

Components of a solar photovoltaic (PV) system

Cell, module and array: A typical photovoltaic (PV) cell produces a small electrical output ranges from 0.5W to 2W approximately. As these devices are electrical, we can boost overall output level by wiring them in series and parallel strings. Wiring PV cells in this method is called a module. Some manufacturers now manufacture “power modules,” which can generate 190W or more power. Under full sun shine conditions, a typical 190W module which is connected to a load might generate the voltage and current of approximately 27V and 7A respectively. When modules are wired in series and parallel strings, then it is called an “array.” The output of an array is designed in such a way that the output of an array can meet almost any electrical requirement of small or large scale system.

Combiner box: Desired voltage and current can be gained by wiring modules into an electrical string. All strings are combined into one electrical output in the combiner box which is then fed to the inverter.

Inverter: Inverter converts the DC output into AC as per requirement of any photovoltaic (PV) system. Electric utility grid connected inverters generate AC which is identical to the power generated by the electric utility. These inverters sense the waveform characteristics and generated voltage of the electric utility and generate the same type of AC.

How to troubleshoot the problem of an array

The input voltage and current level of the inverter's from the array need to be checked and recorded. If the array is not generating required DC electricity, check all switches, fuses, and circuit breakers. Blown fuses should be replaced and the breakers and switches need to be reset. A specious surge might have passed through, blowing or tripping the protective devices. Loose or dirty connections and broken wires in the inverter should be checked. All connections should be clean and make them tighten. All damaged wires should be replaced. Check the array visually for obvious damage to the panels and wiring.

There are fuses for each module or sub-array string in many combiner boxes. While troubleshooting, all these fuses should be removed and the current reading and open-circuit voltage should be recorded for each circuit string. Low output voltage indicates that some panels in the series string are disconnected or defected which requires replacement. Defective bypass or blocking diodes in the modules might need to be replaced. Wrong wiring connecting the modules in the string to the combiner box, junction box or the inverter may cause low voltage. Undersized wire may cause this problem. This problem should be rectified by upgrading the wire size for the current level.

During overcast or cloudy conditions, a damaged panel or defective bypass diode can produce low output current. One or more parallel connections between modules in the string might be loose, broken, or dirty — or some parallel connections in the module might be loose, broken, or dirty. Replace a damaged module or one with internal parallel connection problems. Defective diodes should be replaced. All connections need to be tightened and clean. Shades on the array decreases output current significantly. So, in order to obtain full current output from the string, it is required to remove the source of shades from array.

Dirty modules decrease the output current. In order to restore the output current of array, these modules need to be washed. After washing, the output current should be checked again.

How to troubleshoot the problem of an inverter

First of all, operating DC input voltage and current level of the inverter should be checked by a volt meter and DC ammeter respectively. After then, these data should be recorded. On the AC side, check the inverter’s output voltage and current level. A blown fuse, broken wires or a tripped breaker can cause the insufficient output power from the inverter.

Some inverters have LED displays as indicators. It is necessary to check whether these LEDs are blinking properly or not. Properly blinking LEDs should indicate the actual condition of the inverter.

True-rms reading type volt meter can be used to measure the voltage and current to measure. After measuring, the kilowatt output should be recorded. You should record the total kilowatt hours generated since it first started up which is displayed by the inverter. You can compare the PV system’s production since the last inspection by using the recorded data.

AC load side of the inverter should be measured, because load on the inverter might have too high demand of a current. In this scenario, inverter should be upgraded or loads should be reduced.

Before starting the inverter again, any ground faults should be checked and repaired after shutting down the power. Inverter can sense the voltage and frequency of the electric utility. Typically, it generates AC electricity at the same voltage and frequency. The AC output current output of the inverter fluctuates with the level of solar input on the array. If internal disconnects sense that the electric utility voltage is high or low, then it will shut down the inverter. If this problem remains, then you should contact the concerned authority of electric utility to rectify the problem. Inverter problems could also be caused by a problem on the array side of the inverter, which trips one of the internal disconnects.

How to troubleshoot a load problem

All load switches should be checked first. It should be checked whether they are turned off or placed in the wrong position. You should check and ensure that the load is plugged in. Next, the fuses and circuit breakers should be checked. If there are tripped breakers or blown fuses, the cause should be located and the faulty component should be replaced or fixed. If there are no tripped breakers or blown fuses and the load is a motor, then there might be an open circuit in the motor or an internal thermal breaker might be tripped. In this scenario, you need to plug in another load and observe its operation.

Any loose connections and broken wires should be checked. All bad wiring should be replaced. After shutting down the power, any ground faults should be checked and repaired. Fuses need to be replaced and the switches should be reset. If they blow or trip again, there might have short circuit problem, which must be located and repaired.

If the load does not operate properly, the system voltage should be checked at the load’s connection point. Too small or too long wire feeding the circuit may cause the low voltage which needs to be upgraded to reduce the voltage drop. The load might also be large enough for the wire size in the circuit. In this scenario, the size of wire need to be upgraded or the load on the circuit should be reduced.

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How to maintain solar panels

Sunday, November 29, 2015
Solar panels play a significant role for generating electricity. Solar panels generate electricity when exposed to the sunlight. The efficiency of solar panels and generating electricity is strongly influenced when sun exposure to the solar cell is reduced. Dust, dry areas and building up other particles like droppings of birds have significant impact on the amount of generating electricity by solar panels. Cleaning solar panels regularly can significantly increase the overall efficiency of solar panels. Typically, solar panels are self-cleaning. Insufficient care for solar panels can decrease the efficiency of solar panels. Therefore, it is very important to maintain solar panels very carefully.

How to clean solar panel glass
  • Before starting cleaning, you have to shut down your entire system as per instructions or guidelines of your manual provided by your solar panels manufacturer.
  • Cleaning solar panels from the ground is safe. In order to stay on the ground safely, you need a good quality squeegee with a plastic blade and soft brush on one end and a cloth covered sponge on the other coupled having a long extension. For reaching water stream to panels, you should use a hose with a suitable nozzle. If it is not possible to clean panels from the ground, never attempt to access your rooftop unless you have necessary training and relevant safety equipment. If you don’t have any training, you should complete the cleaning process by a suitably qualified professional.
  • You should clean your solar panels early morning or in the evening and on an overcast day. Using water while beating down the sun on solar panels can quickly evaporate which causes smearing dirt. Falling dew on solar panels overnight makes dirt soften which helps you to clean panels using less water and energy. So, typically it is ideal time to clean panels early morning.
  • For removing slab on materials, never use harsh abrasive products or metal objects. Scratching the glass on a solar panel spreads shadow which decreases the performance of a solar panel. Don't use corrosive powder as it risks scratching the panels. Using detergents may smear the glass of a solar panel. So, it should be avoided.
  • If your solar panels are dry, you should dismantle all loose materials before approaching panels with water. It will help you to clean quickly and easily.
  • In some installation scenarios, oily splash may occur. For example, if you live adjacent to and downwind of a major roadway wherein vehicles like trucks move most frequently or you live near an airport.  If you notice appearing oily splash on your panels, then you should use isopropanol as a spot-cleaning element.
  • You can easily remove the most persistent dirt exists on the glass of a good quality solar panel by washing gently with a soft brush or plastic cleanser or a prickly cloth covered sponge using clean water.
  • Use rainwater and mineral-rich water as a final rinse, after then you should compress dry. If your available water is hard or mineral-rich, you must compress well, because mineral-rich water can form deposits on glass as it dries.
If you want to obtain the highest efficiency possible, then  you need to install a special ‘PID (Potential Induced Degradation) coating’ on the glass of your solar panels. This may be in the form of a spray or by mixing it with the water for the maintenance of your solar panels. In this way a filter exists on the panels, so that the electric charge quickly ends up in your installation. It should be noted here that the PID coating works properly only when a PID box is installed appropriately.

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Inverter for solar panels

Tuesday, November 17, 2015
Inverter is a power converter which converts Direct Current (DC) into Alternating Current (AC). As most of modern appliances operate on 120 volts AC, an inverter plays a key role for your solar power system. It can convert the low voltage DC to the 120 volts AC and also can charge the batteries if connected to the utility grid.

Basically, an inverter can supply three types of power. They are as follows:

Usual or typical power: An inverter has to supply usual or typical power on a steady basis which is continuous rating. It is comparatively lower than the surge power. For example, after the first few seconds, a refrigerator pulls this power to start up the motor.

Surge or peak power: Supplying maximum power by an inverter for short time period is called surge or peak power. Typically, it ranges from few seconds up to 15 minutes or so. For example, we can say about electric motors like pumps which require higher startup surge while running.

Average power: This power is comparatively lower than typical or surge power. Typically,  it is not any factor while choosing an inverter. While estimating required battery capacity,  average power is only useful factor. Inverters need to be sized for the typical continuous and maximum peak load. For example, running a small television or a pump for 20 minutes during a one hour period, the average should be approximately 300 watts only, even though the pump requires 2000.

Types of inverters:

There are various types of inverters are available in the market. Usually, the size of inverters are rated in the range from 50 watts up to 50,000 watts, although, sometimes units larger than 10,000 watts are used in solar photovoltaic systems or in household. Various types of inverters are as follows:

Square Wave Inverters: These types of inverters are not desirable at all. They produce inefficient square wave which is horrendous for running appliances. They are comparatively very cheap and the size of these inverteres are typically 500 Watt or less. These types of inverters should not be considered for a home or solar photovoltaic system.

Modified Sine Wave Inverters: These are most economical and popular inverter. Usually, they produce an AC waveform in between a square wave and a pure sine wave. Modified Sine Wave Inverters are also called Quasi-Sine Wave inverters. They are not too expensive and perform well in almost all household appliances. Most computers perform well with a Modified Sine Wave inverter. But, it is not good for appliances which use timer or motor speed controls.

True Sine Wave Inverters: Among all inverters, a True Sine Wave inverter produces considerably a pure sine wave. These type of inverteres are comparatively very expensive. Practically, it can run all kind of AC equipment perfectly. Most of True Sine Wave power inverters are controlled by computer. They can automatically turn on and off as per requirement of  AC loads. If you need to supply automatic power to a normal home using a wide variety of electrical devices, it is recommended to use a True Sine Wave inverter. It has been observed that most appliances operates more efficiently and smoothly with a True Sine Wave inverter consuming less power.

Grid Tie Inverters: Grid Tie Inverters are suitable for the system which is connected to grid power supplied by utility company. Whatever electricity your solar panels produce, by using a grid power inverter, you can expect reduced electricity bill from your utility company. You can also sell back your excess power produced by solar panels to tour utility company. In this system, a much smaller battery bank should be installed in order to cover short term outages from a few minutes to an hour or so. If there is no frequent long term power outages and back-up power requirement, no batteries are required indeed. 

Installing two inverters in a system is known as stacking which can provide more power or higher voltage. You can increase the output voltage by stacking two compatible inverters in series. This would be the technique to use to provide 120/240 volts AC. On the other hand, if you want to increase the power, you should configure them in parallel. For example, if two 2000 watt inverters are connected in parallel, then it will provide 4000 watts (4KW) of power.
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How to size wire for solar panels

Saturday, November 7, 2015
It is very important to size wires correctly for reaching energy from your solar panels to battery bank without any serious power loss. For example, we can say about flowing water through a pipe, if the size of the pipe is smaller, then very little amount of water can pass through it. Following steps are necessary for sizing wire for solar panels:

First Step: First of all, you have to decide the required voltage for your system. Typically, it is 12, 24 and 48 volts. Solar panels need the required size of wire which can last long. The simple equation is that if the voltage is higher, then solar panels need the smaller size of wire in order to carry the current smoothly and safely. In the power equation (P=V x I) of a circuit, it has been observed that the Power (Wattage) "P" is equal to the Voltage "V" times the current I. Therefore, it is realized that if the voltage increases the current decreases, because V x I always equal to P. If, the total amount of current is very small. then you will need the wire which is small in size. Therefore, a higher system voltage is chosen as a thumb rule. You have to remember that all your equipment must run with your specified system. For example, if you choose the system voltage 24 volts, then your solar panels, battery bank, inverter and solar charge controller will require be 24 volts.

Second Step: In this step, you have to determine the maximum currents (amps) are produced by your solar panels. This can be determine by multiplying the rating of one panel with the quantity of panels in your array. For example, if two 12 volt panels are connected in series to increase the voltage to 24 volts, then two panels should be count as one. This is done in this way, because in a series circuit, the current remains the same, but the voltage increases. For example, we can say about 12 solar panels rated at 12 volts and 6 amps. If you need the system voltage of 24 volt, then you should wire 2 panels in series to create required 24 volts for your system. You should do this 6 times. When 6 pairs are wired in parallel, 6 times 6 amps currents are added which provides you total current of 36 amps. This is the maximum amps your wires will carry.

Third Step: Now, you need to determine the distance in feet from your solar panels to the solar charge controller and battery bank location. Never double the distance, even though indeed you will be running two wires, one negative and one positive.

Fourth Step: Due to the resistance of the wire, there will be a transmission loss of the electrical power from your solar panels to your equipment location. You can't avoid this. Typically 3, 4 and 5 percent floss factors are considered for 12, 24, and 48 volt systems respectively. Based on copper wire using the standard AWG (American Wire Gauge) sizes, 00, 000, and 0000 gauges are generally referred as 2/0, 3/0 and 4/0. There are comparatively larger in size. A 4/0 size wire is fairly large. If you use this in a 48 volt system with a 5% loss factor, then you can expect the flow of current of 100 amps over 250 feet. This is considered as a very large system. Whatever gauge wire you use, you have to ensure that it is capable to carry the required amount of current produced by the system.

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Solar Charge Controller

Monday, October 26, 2015
Solar charge controller is a device which regulates voltage and/or current to protect batteries from overcharging in the solar photovoltaic system. It has been observed that most solar panels of 12 volts have output voltage of 16 to 20 volts approximately. So, if there is no regulation the batteries will be damaged due to overcharging. In order to get fully charged, most batteries require around 14 to 14.5 volts. A solar charge controller regulates the rate of adding and subtracting of electric current from batteries. It helps to prevent batteries from overcharging and deep discharging. It is also protective against over voltage. This feature of solar charge controller contributes to enhance the lifespan of batteries significantly.

Types of Solar Charge Controller

Various shapes, sizes, features, and price ranges of solar charge controllers are available in the market. They are within the range of 4 amps to up to the 60 to 80 amp MPPT programmable controllers having computer interface. The full abbreviation of MPPT is Maximum Power Point Tracker. It's an electronic DC to DC converter which optimizes the match between the photovoltaic array (solar panels) and the battery bank or utility grid. They convert a higher voltage DC output from solar panels (and a few wind generators) down to the lower voltage required to charge
batteries. If any system requires currents over 60 amps, then two or more 40 to 80 amp units are connected in parallel.

Basically, solar charge controllers are three types. They are as follows:

Relay based charge controllers: These types of charge controllers depend on relays or shunt transistors to control the voltage in one or two steps. When a specific voltage is reached, they necessarily disconnect the solar panel.

PWM charge controllers: PWM stands for Pulse Width Modulation. It is often used as float charging method. Using a very rapid "on-off" switch, it sends out a series of short charging pulses to the battery. The controller always checks the state of the battery to determine how quickly and how long (wide) send pulses should be sent. When the battery is fully charged with no load, it provides "tick" every few seconds and sends a short pulse to the battery. The controller may turn into "full on" mode or the pulse would be almost continuous when battery is discharged. The state of charge on the battery between pulses is checked by the controller and adjusts itself each time. These type of charge controllers now very much industry standard.

Maximum power point tracking (MPPT) charge controllers: MPPT charge controllers are high frequency DC to DC converters. They take DC input from the solar panels, turn it into high frequency AC and convert it back down to a different DC voltage and current for matching the panels to the batteries perfectly. MPPT's can operate at very high audio frequencies, usually in the range of 20-80 kHz. High frequency circuits are designed with very high efficiency transformers and small components.

Some linear (that is, non-digital) MPPT's charge controls are also available in the market. They are comparatively cheaper than the digital ones in terms of price and design. Somehow they can improve efficiency. But overall efficiency of these charge controllers can vary a lot and sometimes lose their "tracking point" when a cloud passed over the panel.

MPPT's are most effective under these conditions:
  • Solar panels work better at cold temperatures, but without a MPPT you are losing most of that. Cold weather is most likely in winter - the time when sun hours are low and necessary power is required to recharge batteries most.
  • MPPT puts more current into the battery when the state of charge is lower in the battery. This is another condition when the extra power is most required.
  • When panels are 100 feet away and 12 volt battery is charging, the power and voltage drop are considerable unless very large wires are used. But, it's too much expensive. On the other hand, when four 12 volt panels are connected in series for getting 48 volts, there is very little power loss and the controller can convert that high voltage into 12 volts at the battery. That is, when a high voltage panel setup feeds the controller; comparatively smaller wires should be used.
Most controllers are designed by various type of indicator, a simple LED, a series of LED's or a digital meter. At present, built in computer interfaces are used in controllers for monitoring and control.  The simplest one has a couple of small LED lamps which shows the available power and charge in the system. Both voltage and the current coming from the panels and the battery voltage are displayed in the meter of this charge controller. Amount of current is being pulled from the load terminals are also displayed in some charge controllers.

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