Whether you are buying new or used solar panels, you have to know some important facts in order to make a good decision. If you intend to build your own solar panels, these information can help shorten your learning curve.
Solar Panel Buying Factors
These are some of the most important information you have to know:
1. Solar panels frame
Most commercial solar panels are made using aluminum. This is because aluminum is resistant to rust and is light. However solar panel frames can occasionally be made from stainless steel or plastic.
2. Backside Material
The most common material used for the backside plane includes EVA (ethylene-vinyl-acetate) and PVB (polyvinyl-burial).
3. Solar Cells
Solar cells are made from 3 major types of technology — monocrystalline, polycrystalline and amorphous (or thin-flim) solar panels.
4. Efficiency of solar panels
The efficiency of solar panels depends greatly on the type of PV cells used. Environmental conditions also play an important part. Some of these include temperature, solar irradiance, angle-of-incidence and solar spectral (air mass).
Solar panel efficiency is measured in percentage. It is the ratio of output power to input power from the sunlight, which is the proportion of light energy that gets converted into electricity. The higher the value, the more electricity is generated.
However you must bear in mind that the efficiency of the solar panel is about 1 to 3 percent lower than what’s stated in the datasheet of the solar cell. This is due to several factors like reflection, heat and frame shadowing.
5. Performance Factors
Commercial PV panels have to be rated for different performance factors such as maximum rated power, open circuit voltage, short circuit current, maximum power voltage, maximum power current, and temperature coefficients.
These are measured under the industrial Standard Test Conditions (STC). This means the solar panel is subject to a controlled environment where there is solar irradiance of 1,000 W/m² with zero angle of incidence, solar spectrum of 1.5 air mass and 25 degrees celcius cell temperature
6. Fill Factor
This is the ratio of the actual maximum power as compared to the theoretical maximum power of the solar panel. It is measured in percentage. Commercial solar panels have a fill factor greater than 70%. The fill factor for Grade B solar panels ranges from 40% to 70%. Higher fill factor implies lesser resistive (series and parallel) losses within the solar cells.
7. Connector Type
Solar panels must be fitted with output terminals that are weatherproof. Many manufacturers are also fitting their solar panels with lockable connectors to prevent untrained people from unplugging the panels.
8. Solar Panel Defects
Defects can occur on both new and used solar panels. Thus you have to inspect you solar panels before purchasing.
Some common defects include:
- Scratches on the frame and/or glass
- Gap between frame and glass due to poor sealing (sometimes moisture can get trapped on the inside)
- Significantly lower output than data sheet figures
- Significantly lower fill factor than data sheet figures
- Inconsistent solar cell colors
Solar Terms That You Need to Know
1. Materials Warranty
The warranty period for materials varies from 1 to 10 years. The warranty only covers degradation of materials due to normal wear and tear. Your contractor will not normally change the whole solar panel, though. A technician will first try to replace or repair those damaged parts for free. A replacement will be made only if it is beyond repair.
2. Power Warranty
It is important to ensure that the power output from your solar panels stay consistent through its lifespan. Manufacturers usually provide a 20-year warranty based on the minimum peak power rating.
Warranties may also be divided into two different time frames. For example, warranties can be stated as (i) 80 percent for 20 years, or (ii) 90 percent for the first 10 years, then 80 percent for the next 10 years.
3. Type of solar cell
This involves the material, technology and manufacturing process of each solar cell. Each type of solar cell has its pros and cons. There is no perfect solution.
Monocrystalline cells yield the highest efficiencies. However they are also the most expensive. Its manufacturing process also consumes enormous amounts of energy.
Polycrystalline and ribbon silicon cells are less efficient than monocrystalline cells. However they are cheaper and require less energy to manufacture.
Their physical shape (rectangular or square, as opposed to circular) also allows more PV material to be packed into the PV panel. Thus polycrystalline and ribbon cell PV panels have almost the same power density as monocrystalline counterparts.
Amorphous silicon cells, which are also known as thin-film cells, require the least resources to manufacture. Thus they are also the cheapest. However they are less efficient than their monocrystalline and polycrystalline cousins. They require about twice as much area to produce the same power.
Thin-film panels enjoy better shade tolerance and are able to perform better under high-temperature conditions. But this advantage is somewhat marginal and does not make up for its inefficiency. These panels are also more expensive to install because more panels are required to generate the same amount of electricity.
However thin-film solar technology has its own place in photovoltaic applications due to its physical characteristics. They are thin and light. Therefore they are widely used in handheld devices or field equipment.
4. Number of cells in series
The number of solar cells connected in series determines the output voltage. Each crystalline PV cells produces about 0.5 volts. For solar panels to produce 18 volts, there has to be 36 cells connected in series.
Panels with 36 cells or 72 cells in series are designed to charge batteries rated at 12 volts or 24 volts respectively. Other solar cell configurations are normally designed for grid-tied applications. (“12V”) or 72 (“24V”) cells are designed for battery-charging applications.
Note: Using 36 or 72 cells-in-series solar panels also allow us to have a grid tied system simply by using a charge controller.
5. Nominal operating temperature
This is measured under controlled ambient conditions of (i) an irradiance of 800 W/m2, (ii) air temperature of 20°C, (iii) wind speed of 1 meter per second against a tilt angle of 45 degrees.
When used with the maximum power temperature coefficient, this figure is useful for getting an idea of the power loss due to temperature increase.
The actual cell temperature, however, is influenced by several external factors such as the ambient temperature, wind speed and direction, the tilt-angle of solar panels and the irradiance intensity.
Certifications and Solar Power Ratings
1. Panel certification
This is a pre-requisite for federal and state rebates in the US. There are specific sets of standards that manufacturers must meet. These are some of the most widely used standards:
- IEC 61215 (for crystalline silicon)
- IEC 61646 (for thin films)
- IEC 61730 (safety, for all module types)
- UL 1703 (safety)
- CE mark (for EU regulations)
2. Flash Report
A flash test measures the power output in response to a short burst of xenon light source. The flash lasts for 1 to 30 milliseconds at a light intensity of 1 watt per Meter Square. The spectrum of the light source closely resembles the solar spectrum. The results in the flash report are then compared to the specifications of the PV module’s datasheet.
3. Series Fuse
Every solar panel has to be protected by a surge-protection device. This is to prevent unintended damage due to high currents leaking from neighboring panels.
Such condition (called backfeeding) occurs frequently whenever one series string of panels experience shading or has a damaged circuit. Series fuses are often located in the combiner box or in the grid-connected inverters.
4. Maximum Power Voltage
This is the voltage (V) when the output power is at its maximum. It is one of the important figures that must be considered when choosing a particular inverter or controller.
5. Maximum Power Current
This is the current (A) when the output power is at its maximum. This figure is usually needed in calculations for PV array disconnect labeling as required by National Electrical Code (NEC) Section 690.53.
6. Open-Circuit Voltage
This is the maximum open-circuit voltage of your PV panel when it is exposed to sunlight. Note that no load should be connected while the measurement is done. Since the output voltage increases with lower ambient temperature, the open-circuit voltage is measured at the historic low temperature. This will determine the highest possible voltage output.
This rating is important for making sure your components (panels, wiring, inverters, charge controllers, electronics, etc.) are designed to take the maximum possible voltage.
7. Short-Circuit Current
This is the maximum current produced by your solar panel when short-circuited. It is important to ensure that fuses and breakers are rated to handle high current surge during such events. Thus this measurement must be done.