SOLAR 4 RVS
OFF-GRID, EXTRA LOW VOLTAGE SOLAR PANEL WIRING GUIDE
Please read this document in full before commencing installation.
To maximise the efficiency and longevity of your new solar panels, please ensure you follow the instructions in this guide carefully. Failure to do so WILL result in shortened panel lifespan AND void your warranty.
While it seems simple to connect solar panels to your battery, common installations practices such as using non-waterproof connectors as permanent connection points CAN damage the solar panel AND void your warranty, please continue reading the guide for more details.
Whilst due care has been taken to ensure the recommended wiring practices will provide a safe and efficient installation, we cannot guarantee these methods will be suitable in every scenario due to variations in cable, connectors, solar charge controllers, environment, and installation practices. It is the duty of the installer to verify the installation is installed to local standards and refer to each manufacturer’s instructions.
This guide is limited to 12 and 24V battery systems.
Contents
Safety. 3
Selecting an Appropriate Solar Panel based on the Specifications. 4
Series, Parallel and Combination Wiring Installations. 4
Installation Type 1 – Parallel Wiring. 5
Installation Type 2 – Series Wiring. 6
Installation Type 3 – Series/Parallel Combination Wiring. 7
Paralleling and Series of Different Solar Panels. 8
Cable Size. 8
Solar Array Performance. 8
Bypass, Blocking Diodes and Shading. 9
Sizing a Solar Charge Controller. 10
Solar Panel Efficiency. 10
Maintenance. 12
Ongoing Support. 12
Example: Model Exo-SP-F4-200, the Voc is 24.1V, therefore 5 panels in series is 120.5V and would require a licensed electrician to complete an installation.
The wattage of the solar panel is calculated by Max Power Voltage (Vmp) x Max Power Current (Imp), i.e. 10.2A x 19.8V = 202W.
When no power is being drawn from the solar panel, the Voc will be present.
To charge a 12V battery bank, dependent on the charge controller, approximately 7V is required between the absorption voltage requirement of the battery and the solar panel Voc. I.e. a calcium 12V battery that requires 14.8V absorption voltage, will need a panel with at least 21.8Voc. Most solar panels are approx. 23Voc. When calculating the array current, use the short circuit current (Isc).
The diagram to the right shows a simple photovoltaic (PV) / solar array connected to a 12V battery.
Never install a solar panel in a permanently shaded location, this can damage the bypass diode and cause hot spots.
If a solar isolation switch is used, it should be sized to handle the full short circuit current of the array, plus ~20% to avoid nuisance tripping. I.e. if an array is rated to 30Asc, then the circuit breaker should be at least 36A, the closest match will be a 40A circuit breaker.
When more than one solar panel is used, each solar panel can be connected to an individual solar charge controller, this will generally lead to the best performance but at the highest cost and complexity.
An alternative is to wire the panels in either series or parallel or a combination of both.
This type of installation, most common for off-grid 12V systems, each solar panel positive is connected together, and each negative connected together. In this case, the array voltage will remain the same as a single solar panel, however the array current will increase.
If a solar panel were to fail by an internal fault, such as an internal bypass diode short circuit, the fault current of the array would all flow through the failed diode. There are many examples of this causing fires, string fusing has been designed to minimise the risk.
In the example (above) of three solar panels, if the left panel were to fail from a shorted bypass diode, the middle and right solar panels would each pass 10A into the left solar panel. Therefore, 20A would pass through the 15A fuse, and cause it to disconnect the failed solar panel from the array.
The fuses should be located close to 3 to 1 branch connector.
Fusing is not required when two or fewer solar panel are used because it is not possible for the fuse to reach the required tripping current.
Parallel arrays provide good tolerance to shade and keeps the voltage low, and thus safer.
In this type of installation, commonly used in 24V systems, one solar panel positive is connected to the next solar panel negative. In this case, the array current will remain the same as a single solar panel, however the array voltage will increase. Typically, 24V systems require an open circuit array voltage of at least 36.6V.
Each group of panels wired in series is called a “string”.
The advantages of series wiring are:
The disadvantages are:
This type of installation, commonly used in larger systems, two or more solar panels are connected in series to make a string, and two or more strings are paralleled together. In this case, the array current AND the array voltage will increase.
A note on MC4 solar connectors, these types of connectors are waterproof, affordable, high voltage rated, are pre-installed on most solar panels and are usually disconnectable. The current rating is typically limited to approx. 30A when 6mm2 or larger PV1-F solar cable is used, therefore they would not be suitable for four 10A solar panels wired in parallel without overloading the connector. Series/parallel wiring arrangements are a good way to overcome this while still being able to use these types of connectors
The advantages of this type of system wiring are:
The disadvantages are:
Panels can typically be wired in parallel when the same type of solar cell and voltage is used. I.e. two solar panels using P-type mono-PERC cells and both 24Voc can be paralleled, but if a P-type mono-PERC cell and n-type IBC cell are paralleled, differing coefficients of performance will cause a mismatch in voltages, causing the higher voltage panel to be “dragged down” to the lower voltage panel and increasing the risk of panel failure. Consult your distributor for verification before paralleling different types of solar panels.
At the time of writing, the Exotronic PERC series of solar panels are all designed to use the exact same cell cut into 36 pieces and therefore the voltage and performance will be nearly identical, and will thus provide good performance when paralleled.
Panels can only be wired in series when the cell type and current are the same, this is quite rare. Therefore typically only the same solar panel make and model can be wired in series.
Example: 2x 200W Exotronic Solar fixed solar panels can be wired in series, and 2x 30W Exotronic fixed solar panels can be wired in series, and each string can be wired in parallel. But the 30W and 200W panel cannot be wired in series.
The most practical wire for solar panels is PV1-F solar cable, this cable is most common in 4mm2 and 6mm2. A very rough rule of thumb is for arrays of less than 20A can use 4mm2, and 20A or larger should use 6mm2. If a larger size is required, it is recommended to run two runs from the array to the solar controller. There is no harm in using larger size cable except for practicality and cost considerations.
PV1-F cable is highly UV resistant, durable, high voltage rated, designed to fit MC4 connectors, highly stranded, and tinned copper. It’s properties mean the cable is well suited for most applications including marine, and it is also cost effective.
Solar panels are rated at Standard Test Conditions (STC), this means solar panels are placed on a bed of light rated at W/m2 at 25C and at sea level. This is why it is not typical to see the solar panel output in typical conditions, combined with the other losses (dirt on panel, cable/connection losses, sun orientation, panel temperature, solar controller losses, battery efficiency losses) the actual output that will be seen will likely be much lower.
Often a rule of thumb is you will see up to 75% of the STC rating of the solar panel at midday in summer. I.e. 150W from a 200W solar panel.
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In some rare cases you may see close to full output, i.e. the solar panel has been stored in a cool garage, and suddenly exposed to full midday summer sun. As the panel heats up, performance will drop to the more typical 75% output.
For the best performance, the solar panel should be perpendicular to the sun, however normally brackets to make the solar panel face the sun provide minimal performance improvement at a much higher complexity and cost.
Each solar panel will have one or more bypass diodes. Despite the marketing claims, the main purpose of bypass diodes is to protect the cells from overheating. When a cell is shaded it causes the cell to increase resistance, as current flows through the resistance, the cell heats up, and if the current is not bypassed around the cell, it will cause a hot spot and subsequent failure of the cell, and as each cell is wired in series, failure of the whole solar panel.
In 12V arrays, the bypass diodes, while necessary for the purpose of protection of the cells, provide minimal performance improvement.
When a solar panel is completely shaded, it can become a resistance, causing the power flow of other solar panels to flow through it in reverse thus causing power loss in the system.
Blocking diodes can be installed to prevent this. However, this event is rare, and the diodes will cause power loss at all times. Therefore, unless specifically required by the manufacturer, blocking diodes should be avoided. Most good quality solar controllers should have their own blocking diode installed, and therefore an additional blocking diode is not necessary for single panel installations. They are also not a replacement for string fusing.
The solar charge controller or solar regulator must be sized appropriately for the array. A too small charge controller can be damaged, and too large can be unnecessary.
If using a PWM controller, typically you must use a larger controller than required. You must also use a 30-36 cell (17 to 20Vmp) solar panel on a 12V battery or 60-72 cell (34 to 40Vmp) solar panel on a 24V battery. To size a PWM controller, a simple calculation is: Power of Array in Watts / Battery Bank Voltage x 0.8 for losses, i.e. 400W / 12V x 0.8 = 26.7A controller required.
If using an MPPT controller, you can often size the controller smaller to reduce costs, while still allowing maximum performance in winter. When using an MPPT, ideally use a 36 cell or more (19Vmp+ limited by the maximum input voltage rating of the PV input of the solar controller) solar panel on a 12V battery. To size an MPPT controller, a simple calculation is: Power of Array in Watts / Battery Bank Voltage x 0.8 for losses, i.e. 400W / 12V x 0.8 = 26.7A controller required. However you can often downsize to a 20 or 25A controller as it is often only in summer when the most power is available, that the controller will reach the maximum output, and therefore oversizing is not necessary.
MPPTs will have significantly improved performance when it is required the most, i.e. during the cooler months where there is more likely to be shading, and low light conditions.
Solar cell efficiency is effectively how much light is converted to power in terms of m2 of the solar cell.
Solar panel efficiency is the same measurement, but takes into account the entire panel, i.e. the space between the cells and the frame. Therefore solar panel panels can be more efficient simply by decreasing the space between the cells, and to the frame. This is why larger solar panels are more efficient then smaller panels; less borders/gaps to cell ratio.
Practically speaking, when useable area is limited, a 22% efficient 300W solar panel could take up most of the available space, limiting the room for future panels and increasing the complexity of wiring, whereas it could be possible to install 2x 200W modules plus a 160W solar panel on a single controller, greatly increasing the total power of the array and keeping the wiring relatively simple.
There are many different solar technologies, the most common is monocrystalline. While the highest cost, it is higher in efficiency compared to polycrystalline and amorphous, and therefore reduces the quantity of solar panels needed and simplifies the system. Amorphous, while having improved shade tolerance, is very low efficiency, therefore even when compared to a shaded monocrystalline module, the overall output can often be worse.
Cells get exponentially more expensive as the efficiency increases, the easiest way for unscrupulous retailers to increase profits is to overstate the rated output of the solar panel. In the example below, the solar panel is 18V and advertised to be 350W, therefore it should have an Imp of approximately 19.4A. The cells, based on the specifications and appearance are ¾ cut 156mm cells with 5 wire busbars. The highest output possible on cells of this nature is approximately 7.5A. Therefore it is not physically possible for this solar panel to exist. A true output of a panel this size will be approximately 120 to 130W based on several reasonable assumptions. As Sunpower uses interdigitated back contact cells, there are no busbars on the front, so the cells type stated below is also incorrect.
The highest cost and most efficient solar cells commercially available at the time of writing are approximately 24% efficient, and therefore the maximum efficiency for a very large module could approach 23% efficient.
To ensure your solar panels are performing optimally, be sure to keep on top of this simple maintenance checklist.
A solar cell is a device that converts light into electricity via the ‘photovoltaic effect’. They are also commonly called ‘photovoltaic cells’ after this phenomenon, and also to differentiate them from solar thermal devices. The photovoltaic effect is a process that occurs in some semiconducting materials, such as silicon. At the most basic level, the semiconductor absorbs a photon, exciting an electron which can then be extracted into an electrical circuit by built-in and applied electric fields.
Due to the increased desire for more renewable sources of energy in recent years, solar power has seen increasing popularity. In , the total global energy usage was approximately 595 EJ (exajoules, x) . Meanwhile, the harvestable annual solar energy that falls upon the Earth’s landmasses is estimated to be 50,000 EJ . The sun provides more than enough energy to satisfy global energy needs (almost 84 times over). Therefore, there is arguably a much greater potential for solar to fulfil our energy requirements than other renewable sources.
The main component of a solar cell is the semiconductor, as this is the part that converts light into electricity. Semiconductors can carry out this conversion due to the structure of their electron energy levels. Electron energy levels are generally categorised into two bands: the ‘valence band’ and the ‘conduction band’. The valence band contains the highest occupied electron energy levels, whilst the conduction band contains the lowest unoccupied electron energy levels. The energy difference between the top of the valence band and bottom of the conduction band is known as the ‘band gap’ (Eg). In a conductor, there is no band gap as the valence band is not filled completely - thus allowing the free movement of electrons through the material. Insulators have very large band gaps which require copious amounts of energy to cross - and as such, inhibits the movement of electrons from the valence band to the conduction band. Conversely, the band gap in semiconductors is relatively small, enabling some electrons to move to the conduction band by injecting small amounts of energy.
This small band gap is what enables some semiconductors to generate electricity using light. If a photon incident on the semiconductor has energy (Eγ) greater than the band gap, it will be absorbed - enabling an electron to transfer from the valence band into the conduction band. This process is known as ‘excitation’. With the electron now in the conduction band, an unoccupied state is left in the valence band. This is known as a ‘hole’, and behaves like a particle analogous to an electron in the conduction band (albeit with positive charge). Due to their opposite charge, the excited electron and hole are coulombically bound in a state known as an ‘exciton’. This exciton must be split (also known as ‘dissociation’) before the charge carriers can be collected and used. The energy required to do this is dependent on the dielectric constant (εr) of the material. This describes the level of screening between charges in a semiconducting material and affects the binding energy of the exciton.
In materials with high εr, excitons have low binding energies - enabling dissociation to occur thermally at ambient temperatures. Excitons in materials with low εr have high binding energies, preventing thermal dissociation - thus requiring a different method of dissociation. A common method is to get the exciton to an interface between materials with energy levels that have an offset greater than the exciton’s binding energy. This enables the electron (or hole) to transfer to the other material, and dissociate the exciton.
Once dissociated, the free charges diffuse to the electrodes of the cell (where they are collected) - this is assisted by built-in and applied electric fields. The built-in electric field of a device arises from the relative energy levels of the materials that make up the cell. However, the origin of the built-in field depends on the type of semiconductor being used. For inorganic semiconductors such as silicon, other materials are often added to the semiconductor (a process known as doping) to create regions of high (n-type) and low (p-type) electron density. When these regions are in contact, charges will build up on either side of the interface, creating an electric field directing from the n-type to the p-type region. In devices using organic semiconductors, the built-in field arises from the difference between the work functions of the electrodes of the device.
The size of the band gap is also very important, as this affects the energy that can be harvested by the solar cell. If Eγ > Eg, then the photon will be absorbed, and any energy in excess of Eg will be used to promote the electron to an energy level above the conduction band minimum. The electron will then relax down to the conduction band minimum, resulting in the loss of the excess energy. However, if Eγ < Eg, then the photon will not be absorbed, again resulting in lost energy. (Note, the wavelength of a photon decreases as its energy increases).
When considering the solar spectrum, it can therefore be seen that a too large Eg will result in a significant number of photons not being absorbed. On the other hand, a too low Eg means that a large number of photons will be absorbed, but a significant amount of energy will be lost due to the relaxation of electrons to the conduction band minimum. Due to this trade-off, it is possible to calculate the theoretical maximum efficiency of a standard photovoltaic device, as well as estimate the optimum band gap for a photovoltaic material. Shockley and Queisser determined the theoretic maximum efficiency to be approximately 33% in , which corresponds to a band gap of 1.34 eV (~930 nm).
The characterisation of a solar cell determines how well it performs under solar illumination. The solar spectrum is approximately that of a black body with a temperature of K. This peaks in the visible range and has a long infra-red tail. However, this spectrum is not used for characterisation as the light must pass through the Earth’s atmosphere (which absorbs a significant portion of the solar radiation) to reach the surface. Instead, the industry standard is AM1.5G (air mass 1.5 global), the average global solar spectrum after passing through 1.5 atmospheres. This has a power density of 100 mW.cm-2 and is equivalent to average solar irradiation at mid-latitudes (such as in Europe or the USA). To ensure reliability and control during testing of solar cells, a solar simulator can be used to generate consistent radiation.
The key characteristic of a solar cell is its ability to convert light into electricity. This is known as the power conversion efficiency (PCE) and is the ratio of incident light power to output electrical power. To determine the PCE, and other useful metrics, current-voltage (IV) measurements are performed. A series of voltages are applied to the solar cell while it is under illumination. The output current is measured at each voltage step, resulting in the characteristic 'IV curve' seen in many research papers. An example of this can be seen in the figure below, along with some important properties that can be determined from the IV measurement. It should be noted that generally, current density (J) is used instead of current when characterising solar cells, as the area of the cell will have an effect on the magnitude of the output current (the larger the cell, the more current).
The properties highlighted in the figure are:
The PCE can be calculated using the following equation:
Here, Pout (Pin) is the output (input) power of the cell, FF is the fill factor, and Jsc and Voc are the short-circuit current density and open-circuit voltage respectively.
The short-circuit current density is the photogenerated current density of the cell when there is no applied bias. In this case, only the built-in electric field within the cell is used to drive charge carriers to the electrodes. This metric is affected by:
The open-circuit voltage is the voltage at which the applied electric field cancels out the built-in electric field. This removes all driving force for the charge carriers, resulting in zero photocurrent generation. This metric is affected by:
The fill factor is the ratio of the actual power of the cell to what its power would be if there were no series resistance and infinite shunt resistance. This is ideally as close as possible to 1, and can be calculated using the following equation:
Here, JMP and VMP are the current density and voltage of the cell at maximum power respectively.
Approximate values of the series and shunt resistances can be calculated from the inverse of the gradient of a cell’s JV curve at the Voc and Jsc respectively.
A solar cell is a diode, and therefore the electrical behaviour of an ideal device can be modelled using the Shockley diode equation:
Here, Jph is the photogenerated current density, JD is the diode current density, J0 is the dark saturation current density (current density flowing through the diode under reverse bias in the dark), V is the voltage, and T is the temperature. The final 2 symbols, e and kB, are the elementary charge (1.6 x 10-19 C) and the Boltzmann constant (1.38 x 10-23 m2.kg.s-2.K-1) respectively. However, in reality, no device is ideal and so the equation must be modified to account for potential losses that may arise:
Here, n is the diode ideality factor and all other symbols have their previous meanings. Using this equation, a solar cell can be modelled using an equivalent circuit diagram, which is shown below:
The series resistance (Rs) accounts for resistances that arise from energetic barriers at interfaces and bulk resistances within layers. Ideally, this is minimised to prevent efficiency losses due to increased charge carrier recombination. This can be achieved by ensuring good energy level alignment of the materials used in the solar cell.
The shunt resistance (Rsh) accounts for the existence of alternate current pathways through a photovoltaic cell. Unlike the series resistance, this is ideally as high as possible to prevent current leakage through these alternate paths.
Solar Simulator
There are several types of solar cells, which are typically categorised into three generations. The first generation (known as conventional devices) are based upon crystalline silicon, a well-studied inorganic semiconductor. The second generation are the thin-film devices, which includes materials that can create efficient devices with thin films (nanometre to tens of micrometres range). The third generation are the emerging photovoltaics - technologies which are still undergoing research to reach commercialisation.
The first and second generations contain the most-studied photovoltaic materials: silicon, gallium arsenide, cadmium telluride, and copper indium gallium selenide. These materials are all inorganic semiconductors, and generally work in the most direct manner: a photon is absorbed - creating an exciton, which is thermally dissociated (inorganic semiconductors typically have high dielectric constants) and subsequently transported to the electrodes via an electric field.
As silicon is the most-studied material, it can achieve some of the highest performances (with a peak efficiency of 26.1%) and was the first material to reach the commercial market. As such, the majority of solar panels use silicon as the photoactive material. The band gap of silicon is 1.1 eV, enabling broad absorption of solar radiation. However, this is lower than the optimum band gap (1.34 eV), resulting in energy losses when absorbing high energy photons. In addition, the band gap is indirect - reducing the absorption efficiency and thus requiring relatively thick layers to efficiently harvest sunlight. As with all inorganic materials, silicon has a high dielectric constant of 11.7 - allowing for the thermal separation of charge-carriers after generation.
Gallium arsenide (GaAs) boasts the highest performance of any photovoltaic material, reaching 29.1%. This is because GaAs has a direct and more favourable band gap of 1.43 eV - resulting in improved absorption with thinner layers and reduced energy loss. Additionally, GaAs has superior electron-transport properties to silicon. However, it is very expensive to produce as it requires high material purity, which generally limits it to space-based applications (such as satellites and rovers).
Cadmium telluride (CdTe) is a high-efficiency thin-film photovoltaic technology which has achieved an efficiency of 22.1%. CdTe has a similar band gap to GaAs at 1.44 eV, giving it the same advantages as seen in GaAs - good absorption in thin films and low photon energy losses. This material also boasts the possibility to be flexible, very low costs, and it has produced commercial solar panels that are cheaper than silicon with much shorter energy payback times (although with lower efficiency). Despite these advantages, there are some issues - cadmium is highly toxic and tellurium is very rare, making the long-term viability of this technology uncertain for now.
Copper indium gallium selenide (CIGS) has achieved similar performances to CdTe devices, with a peak of 23.4%. The compound has the chemical formula CuInxGa(1-x)Se2 where x can take a value between 0 and 1. This tunability of the chemical structure enables the band gap of the material to be varied between 1.0 eV (x = 1, pure copper indium selenide) and 1.7 eV (x = 0, pure copper gallium selenide). However, like GaAs cells, CIGS are expensive to fabricate and result in solar panels that cannot compete with the current commercial technologies. Furthermore, like tellurium, indium is very rare, limiting the long-term potential of this technology.
The third generation of photovoltaics - also known as the emerging photovoltaic technologies - includes dye-sensitised, organic, and perovskite solar cells. These materials can degrade when exposed to the environment, negatively affecting the performance and longevity of the solar cell. Glove boxes provide an inert atmosphere to prevent degradation.
Dye-sensitised solar cells (DSSCs) use organic dyes to absorb light. These dyes are coated onto an oxide scaffold (typically titanium oxide) which are immersed in a liquid electrolyte. The dyes absorb the light, and the excited electron is transferred to the oxide scaffold, whilst the hole is transferred to the electrolyte. The charge carriers can then be collected at the electrodes. These cells are less efficient than inorganic devices, but have the potential to be much cheaper, produced via roll-to-roll printing, semi-flexible, and semi-transparent. However, issues still exist with use of a liquid electrolyte due to temperature stability (as it can potentially freeze or expand), the use of expensive materials, and volatile organic compounds.
Organic solar cells (OSCs) use organic semiconducting polymers or small molecules as the photoactive materials. To date, efficiencies of 18.2% have been achieved by this technology. These cells work similarly to inorganic devices. However, organic semiconductors generally have low dielectric constants, meaning that the generated exciton cannot be thermally dissociated. Instead, the exciton must be transported to an interface with a material that has an energy level offset greater than the binding energy of the photon. Here, the electron (or hole) can transfer to the other material and split the exciton, allowing the charge carriers to be collected (as shown earlier in the general theory section). As excitons can typically only diffuse approximately 10 nm before the electron and hole recombine, this limits the thickness, structure, and ultimately - the performance of an organic photovoltaic cell. Despite this, these devices hold some significant advantages over inorganic devices, including: low cost of materials, lightweight, strong and tuneable absorption characteristics, flexibility, and the potential to be fabricated using roll-to-roll printing techniques. Currently, organic materials suffer from stability issues arising from photochemical degradation.
Perovskite solar cells (PSCs) use perovskite materials (materials with the crystal structure ABX3) as their light-absorbing layer. Perovskites were introduced to the field relatively recently, with the first use in a photovoltaic device reported in (where it was the dye in a DSSC achieving 2.2%). However, is considered the birth of the field, due to the publication of a landmark paper in which an efficiency of 10.9% was achieved. Since then the peak efficiency has risen to 25.5%, making PSCs the fastest-improving solar technology. These materials have remarkable properties, including strong tuneable absorption characteristics and ambipolar charge transport. They can also be processed from solution in ambient conditions.
There are still issues with stability and the use of toxic materials (such as lead) preventing the technology from being commercialised, but the field is still relatively young and very active. For more detailed information about perovskites, see our perovskite guide.
The table below shows the best research cell efficiencies for a variety of photovoltaic technologies (values courtesy of the National Renewable Energy Laboratory, Golden, CO).
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Solar Cell Type Highest Efficiency (Last updated 10/06/) Monocrystalline silicon (mono-Si) 26.1% Polycrystalline silicon (multi-Si) 23.3% Silicon Heterostructures (HIT) 27.1% Amorphous silicon (a-Si) 14.0% Monocrystalline gallium arsenide (GaAs) 29.1% Cadmium telluride (CdTe) 22.6% Copper indium gallium selenide (CIGS) 23.6% Dye-sensitised (DSSC) 13.0% Organic (OSC / OPV) 19.2% Perovskite (PSC) 26.1%