Last edited: August 2018
All solar power systems make use of solar panels which transfer the energy of photons (sunlight) into electrons (electricity).
Solar Panel Specifications
- Poly or Mono - It used to be more important whether a solar panel was made using polycrystalline solar cells or monocrystalline solar cells. This terminology refers to a difference in production process. These days the differences have become very small. We would only advice taking monocrystalline solar modules in case aesthetics are important or in case the available (roof)space is limited.
- Watt or Wattpeak (Wp) - The amount of wattpeak is the amount of power a solar module produces under standard test conditions. The most commonly used solar modules (especially on-grid) are solar modules with 60-cells or 72-cells. These solar modules generally have a rating of respectively 260-290Wp and 310-350Wp. In smaller off-grid projects solar panels between 5 and 200W are often used.
- Maximum Power Voltage (Vmp) - In case the inverter or charge controller is using MPPT (Maximum Power Point Tracking), the voltage of the solar module is less important. However, this is often not the case for small off-grid systems. Therefore, solar modules with an nominal voltage of around 18V are used for charging 12V batteries whereas 36V solar modules (or two 18V solar modules in series) are used for charging 24V batteries.
- Module Efficiency (%) - Unless there is a lack of available space, it is normally not advisable to opt for highly efficient (and costly) solar modules (>18%). Currently normal solar modules are 16-18% efficient.
Solar Panel Brands & Quality
These days almost 90% of all solar panels produced in the world originate from Eastern Asia with the vast majority coming from China. Though most factories use highly automated production lines nowadays, there are still some factories with low quality solar modules.
In order to choose the right module for your project one of the most important aspects is the lifetime of your project. If you only need the system to work for the coming 5 years then probably any solar module should be good enough. If it should last for the whole duration of the expected lifetime of 25 years, then it is smart to go with a Tier-1 solar module manufacturer.
Which manufacturers are tier-1 solar module manufacturers is determined by Bloomberg New Energy Finance (BNEF). In case the solar modules from a particular manufacturer have been used in multiple large scale “bankable” projects, the manufacturer could become part of the tier-1 list. BNEF Tier 1 brands with a particularly strong brand presence in Africa are Suntech and Jinko Solar.
Please note that there are plenty of stories of manufacturers “cheating” by putting a label of a higher wattage onto a solar module with a lower actual output. These modules are often sent to Africa as there are not any suitable testing facilities to avoid this practise. This would be another reason to choose to buy from a Tier-1 supplier.
When buying from shops on the streets, a very basic test to check whether the label matches the output is measuring the panel size. Normally you would expect roughly 160Wp for each square meter of surface area. If a solar panel measures 60cm * 100cm and has a label for 120Wp you are probably not buying a 120Wp solar module.
Solar Charge Controllers
Which charge controller to use very much depends on the overall system design. Solar Charge Controllers are only used in DC-coupled off-grid systems.
Solar Charge Controller Specifications
- MPPT or PWM - Whether to use a Maximum Power Point Tracking solar charge controllers or to use Pulse Width Modulation solar charge controllers mainly depends on the system size. The cheaper PWM charge controllers are mainly recommended for small systems with maximum a few solar modules. They require a solar module which matches the voltage of the battery. Read more about the difference here.
- Voltage - The voltage depends on the system design. Most charge controllers have options for being integrated in systems with multiple system voltages (12, 24 and 48V). The actual charging voltage is dynamic and depends on the settings of the charge controller.
- Rated Charge Current - Depending on the voltage, the rated charge current determines at which rate the batteries can be charged. The optimal rate depends on the specifications of the connected batteries and solar panels.
Solar Charge Controllers Brands & Quality
Getting a good quality solar charge controller normally pays off in the long run. A solar charge controller can do little harm to the solar panels, however, a bad quality charge controllers (or the wrong settings) can greatly damage what is normally the most expensive part of a solar power system: the batteries.
Morningstar is an American company which specialized in solar charge controllers and have been optimizing their robust products for the past 25 years. Victron Energy also has the complete range with the additional benefit that it is possible to use only one monitoring solution when also using an inverter. The German company Steca also has a good reputation, however, does not seem to be used that much in Africa.
In most off-grid solar power systems the battery is the most expensive component. Choosing the right one (and combining it into a solid system design) will determine whether the system is going to last only 6 months or up to 15 years. Currently, the most common battery type is still based on lead-acid technology, however, it will not take too long before li-ion will start taking market share.
Lead-acid batteries (deep cycle)
Lead-acid batteries have a normal lifespan, depending upon application and type, of between 2 and 10 years. The most common types are:
- Flooded Lead-acid batteries - Flooded batteries generally need to be filled up with battery water (demineralized water) once in a while. When well maintained, the lifespan of these batteries are slightly longer than AGM and Gel batteries.
- AGM Lead-acid batteries - AGM, standing for Absorbed Glass Mat, are batteries which are most commonly used for small off-grid solar power installations. They hardly require any maintenance which makes them good in applications where there are no technicians or caretakers around.
- Gel Lead-acid batteries - Gel batteries are generally the most expensive lead-acid battery type. They also do not require any maintenance and perform slightly better with higher ambient temperatures.
For a good description of the characteristics of lead-acid batteries and the differences between the technologies read this article on batterystuff.com.
Li-ion batteries have a normal lifespan, depending upon application and type, of between 7 and 15 years. Until a few years ago it still made more economic sense to buy Lead-acid batteries and replace them every 3-5 years than buying Li-Ion. However, due to the rapid cost decline of Li-Ion batteries, it is now more economical to go with Li-Ion batteries when the batteries are expected to be used for a long time. The upfront investment is still higher, however, the total cost of ownership or cost/kWh stored is lower.
There are 5 dominant Li-Ion technologies which all have their own benefits/drawbacks. The most common Li-Ion battery types are NMC and LFP.
|LFP / LiFePO4||lithium-iron-phosphate|||||||||||||
|NMC / INR||nickel-manganese-cobalt|||||||||||||
|LMO / IMR||lithium-manganese-oxide|||||||||||||
|NCA / NCR||nickel-cobalt-aluminum|||||||||||||
|LCO / LCR||lithium-cobalt-oxide|||||||||||||
For a more extensive outline of Li-Ion battery technology read this article on batteryuniversity.com.
When considering the system design the following parameters are of importance:
- Voltage - Normally the system voltage is determined from the system design. 12V, 24V and 48V are most common. To reach these voltages one can choose either 2V or 12V (and sometimes 6V) battery cells connected in series, parallel or both. As a rule of thumb it is good not to connect more than 4 battery (strings) in parallel.
- Amp Hour (Ah) - The Amp Hour rating of a battery tells you how much energy it can store. It is useful to multiply with the voltage in order to compare different batteries. A 12V battery of 200Ah has the same capacity as a 6V battery of 400ah (both 12V * 200Ah = 2400Wh or 2,4kWh). Please note that the actual storage capacity depends on many factors including DoD, discharge rate and temperature.
- Depth of Discharge (DoD) - The level to which a battery is discharged is the Depth of Discharge. As a general rule, it is not recommended to discharge lead-acid batteries more than 50% to avoid damaging the batteries and decreasing the lifespan. This means that the above mentioned 2,4 kWh battery actually only has a usable capacity of 1,2 kWh. Li-Ion batteries can normally be discharged much more, generally up to 80 or 100%.
- Number of cycles - The “number of cycles” or “cycle life” is the amount of “cycles” a battery lasts. One cycle is one discharge of the battery to a specific DoD. Batteries are able to perform many more cycles to a low DoD (for example 20%) compared with a high DoD (80%). Though the cycle life is sometimes stated on the datasheet, be wary of trusting the stated cycle life as there is an incentive for the manufacturer to inflate this number. At 50% DoD the cycle life for Lead-acid batteries can vary from 300 - 3000 cycles (1 - 8 years), Li-Ion batteries are generally able to last for 2000 - 8000 cycles (6 - 20 years) at 80 or 100% DoD. Aside from DoD the cycle life also heavily depends on ambient temperature and the charging pattern.
When looking at datasheet of a battery one can see many more characteristics. Finding the right battery type and quantity for your project can best be done by someone with a lot of experience in doing so.
Battery Brands & Quality
Since lead-acid batteries have been around since the 19th century there are many manufacturers and applications. Many of them have a line specialized for deep-cycle / renewable energy storage applications which generally require somewhat thicker lead plates.
Surrette/Rolls, Deka and Trojan Batteries (all from the USA) batteries have been used in many African countries for years, especially for the smaller 12V battery banks. Larger battery banks made up of 2V cells are more often made to specification by BAE or Hoppecke (both from Germany). The Kenyan brand Chloride Exide has a particularly strong presence in East Africa.
As Li-Ion batteries are not used that much as of yet, there is no clear market leader. At an increasing rate they are integrated into the solar home systems, however, these systems are sold under one brand name (instead of multiple individual components).
Which inverter to use also very much depends on the overall system design.
DC-Coupled vs. AC-Coupled
Smaller DC-coupled systems normally do not use an inverter as all the connected appliances would run on DC-power. Larger systems DC-coupled systems which power AC-appliances do need an inverter. In case it is also necessary to be able to charge the batteries when the sun is not shining, a inverter/charger is necessary. An inverter/charger can for example be connected to a generator or the grid.
AC-Coupled battery based systems use at least one PV-inverter and one inverter/charger. Read a further explanation between the differences between AC-Coupled and DC-Coupled systems here <link to the process>. When the system is not combined with a battery, and is used for direct self-consumption or grid feed-in, this only requires the PV-inverter.
- Voltage - Unlike charge controllers, inverters and inverters/chargers have one specific system voltage (12, 24 or 48V)
- Nominal output - The nominal output determines how many and which appliances you can power with the inverter. With a 2000W inverter you can power one 2000W hair dryer or four hundred 5W LEDbulbs. Normally inverters can provide a higher output for a limited time period.
- Charging current - An inverter/charger can also charge the batteries using AC-Power from a generator or the grid. How fast it can charge the batteries will depend on the charging current.
For PV-Inverters the most important specification is the nominal output.
- Nominal output - The nominal output (in watt or kW), determines how much DC-power can be converted into AC-power. Normally it is recommended that the size of the PV-inverter is slightly lower than the size of the solar array. For 6000Wp of solar modules, a 5000W PV-inverter is normally a good fit. This is due to the fact that solar modules rarely produce more than 80% of their rated output (only on a very cold, sunny day they might).
Inverters Brands & Quality
As with most consumer electronics, inverters do not have an infinite lifetime. Just like the charge controllers, getting a good quality inverter (and associated settings) is important to protect your batteries from discharging them too much as well as wrong charging voltage/current.
For AC-coupled battery based systems SMA Solar Technology have been the clear market leaders with a extensive product portfolio and a good reputation for quality. Another company with a more limited product portfolio but also a good reputation is Schneider Electric. When there is no battery integrated in the system, SMA is still a good option, however, there are also good value brands like Huawei, SolarEdge and Ginlong Solis.
For DC-coupled systems, Victron Energy has a wide range of inverters and inverter/chargers and dealers can be found in most African countries. Likewise, Magnum and Outback inverters are often available, however, their product range is more limited. There are a lot of Chinese/Indian brands of inverters, however, they still have to gain a good reputation for quality in most African countries.
Balance of System
The “balance of system” is used as the term for all additional components which ensure the system works and is protected properly. In the system design it is important to think about:
The solar mounting system
Specialized solar mounting systems, often made of steel or aluminium, ensure the solar panels stay in position. When possible, it advised to go for a roofmount as this is cheaper and ensures the solar panels are better protected from theft. Whether a roofmount is possible, however, depends on the orientation, pitch and structural quality of the roof.
The benefit of a groundmount is that the solar panels can be placed in the optimal orientation and pitch. A groundmount is usually fixated to the ground by pouring concrete.
When solar modules were still more expensive, mounting structures which could follow the sun were popular as this would increase output. These days, however, it is normally more economical to just add some additional solar modules to get to the same output.
Cables and conduit
A good solar installer ensures that proper cables and connectors are used. This for example means UV-resistant solar-cable to connect the solar modules or armoured cable when putting cable underground. Especially in low-voltage systems with high current it is important to use cables with a high cross-section/diameter.
Since breakers do add costs to the system, they are often left out of a design. They are essential, however, to ensure the system is protected in case of a for example a short circuit, and that you can ensure to be able to work on a system safely.
When the costs of a solar power system exceeds roughly $5000, or when it is critical that the system remains functional at all times, it is recommended to add remote monitoring. Particularly the systems from SMA and Victron Energy are well equipped for remote monitoring.
Grounding & Lightning protection
As can be seen from this map, the area around the equator in Africa is one of the most active areas in the world with regards to lightning strikes. Aside from proper grounding, it is important to protect your equipment from lightning strikes. This can be done to various protection levels but always starts with a proper risk analysis.
Due to the less predictable nature of solar energy it is often required to build in some redundancy in off-grid systems as it is not often accepted not to have any power during a period of high cloud cover. This redundancy can be achieved by increasing the amount of batteries / solar panels. However, especially for larger systems, often it is still more cost effective to include a generator as part of the energy mix.
Due to the fact that in many countries in Africa the grid has been unreliable and does not extend to all areas, there are plenty of suppliers of generators. In deciding which one to use it is important to check the functionality of the control panel of the generator and whether this is compatible with the inverter you are using (to ensure it is possible to automatically start and stop the generator.