Off-grid solar system (also called photovoltaic island system) is a subtype of photovoltaic system. Power is generated independently and there is no connection to the public power grid. It is often stored temporarily so that the energy can be used throughout the day. In addition to the island system, a distinction is made in photovoltaics between a power system and a zero power system.
The right solar system for you should not only depend on your power requirements, but other factors as well. All important decision factors and tips for buying an off-grid solar system can be found here.
The normal feeding system
A power system is a photovoltaic system that feeds the excess electricity generated to the public grid. The solar energy from this system first covers your electricity needs, and you can earn something with what is left. According to the Renewable Energy Sources Law (EEG), feeding to the public network is remunerated.
Depending on the power rating of your system, you can feed the electricity you no longer need into the public grid for 8.20 cents per kilowatt hour (kWh) (as of August 2022). It then gets electricity from the public grid when there is a heavy cloud cover or at night when the sun is no longer shining.
The Zero Feed System
The difference between a zero power system and a normal one is that excess self-generated electricity cannot be injected into the public power grid. The photovoltaic system is set up so that it cannot supply electricity to the grid, but the system is expanded to include a solar energy storage unit.
This is a specially developed lithium-ion solar battery that temporarily stores the total solar energy of the day. It can then be used at a later date for personal needs. If you have a zero feed system, you won’t receive any feed fees, but you will save yourself a lot of red tape. The injection of self-produced electricity requires a commercial registration.
Solar cells produce the most electricity when the sun shines during the day. However, because most households only consume most of their electricity at night, households with a simple power system without storage often lack electricity and have to purchase additional external electricity. Thanks to the additional electricity storage, the zero power system helps to save costs and additional self-sufficiency.
While electricity storage is optional for power systems, zero power systems require it to be mandatory. Because without injection, you produce a large amount of surplus electricity during the day, which you have to store. If the storage runs empty, you can also use electricity from the public grid with zero power systems as with the power system.
The off-grid solar system
The autonomous solar system is the type of photovoltaic system that allows the greatest self-sufficiency. Like zero power systems, the standalone solar system has an energy storage unit through which the electricity produced during the day can be used at night.
However, the big difference with the autonomous solar system is the lack of connection between the house and the public electricity network. This means there are no costs for purchasing external electricity. At the same time, however, the home is completely dependent on the hours of sunshine and self-produced solar energy.
The autonomous solar system is a common model for caravans because complete independence from the electricity grid must be guaranteed when traveling or camping. Mini PV systems are often used here, integrated into the roof or into the ground.
Building an off-grid solar system
The structure of an independent solar system is a bit more complicated than the structure of a normal power system. Like the power system, the autonomous system also has numerous solar modules that produce electricity and an inverter that converts the direct current generated into alternating current. This step is necessary to use solar energy at home.
However, a solar battery that stores the total solar energy is essential for the independent solar system. The charge controller is also very important. This controls the charging and discharging of the solar battery to prevent overcharging of the battery.
When does an independent solar system make sense?
Is it worth taking the risk of not being connected to the public electricity grid? And in which cases does it definitely make sense to choose the independent solar system?
The first logical situation to use an off-grid system is when connection to a public power grid is simply not possible. This is often the case in mountain huts, boats or remote regions in developing and emerging countries. These areas would often live without electricity if there was no possibility of an off-grid solar power generating system.
Other than that, it makes a lot of sense to use an off-grid solar system if the houses are not occupied all year (for example, only in the summer months) or have very low energy consumption. This category mainly includes holiday homes and garden houses. The installation of an autonomous solar system here is worth more than in a single-family home with high electricity consumption throughout the year.
With our electricity consumption calculator, you can quickly determine your electricity consumption and costs.
Are isolated systems subject to registration?
According to the EEG, PV systems must be reported, so the location and output of the system must be reported. The island system is the exception and can be installed with little or no bureaucratic effort.
What factors should you consider before purchasing a standalone solar system?
Before you invest in a standalone solar system, you should pay close attention to a few key figures. In addition to efficiency and peak system performance (measured in peak kilowatts), your storage system’s key numbers must also exactly match your needs.
The most important criteria are storage capacity, maximum depth of discharge or usable capacity, efficiency and useful life. Added to this are considerations about the region’s climate, such as the average hours of sunshine and the regularity of the sun.
Make sure that the maximum output of the system is significantly higher than its actual consumption. In this way, you can be sure that you always produce enough electricity, even if the sun does not shine for a while. The general rule of thumb for off-grid solar system is a maximum output that is at least 1.2 to 1.5 times the amount of your electricity requirement.
If you have little space, you should use higher efficiency solar modules. Monocrystalline solar modules have the highest efficiency at over 20%, closely followed by polycrystalline modules (approx. 17%). However, thin-film modules are only around 7% efficient, so they are very inefficient by comparison. If you go for thin-film modules, you would have to install three times as many PV modules as the monocrystalline variant to achieve the same output.
Which battery to use for a standalone solar system?
When selecting a battery for an off-grid system, the following factors should be considered.
- Sufficient storage capacity
- High efficiency
- When choosing storage, you need to make sure that the battery has enough storage capacity to store the energy from the PV array. At the same time, it must be large enough to provide you with enough power for a long time without running out. Here it is not only the available storage capacity that is important, but above all the “usable storage capacity” depends on the maximum discharge depth. To protect the life and functionality of the batteries, they are never completely discharged, typically only 60-90%.
- Be sure to check your storage efficiency. While lead-acid batteries are around 70-85% efficient, lithium storage is typically around 90-98%. Due to their higher efficiency, lithium storage systems are often the best option for off-grid solar systems. If the efficiency is too low and too much energy is lost in the storage process, you will need to compensate with higher output from the system. And that can be an expensive affair.
- To sustainably store energy from your off-grid system, your energy storage unit must have a long lifespan. Lead storage life is around 5-10 years while lithium storage can be expected to last around 10-15 years. However, since PV systems have a lifespan of 20 to 35 years, you must purchase multiple storage units during this time. The lifespan of the individual storage units therefore decides whether you only need two or even as many as four. So don’t just look at the price of memory, but also include lifespan in your calculation.
The region where you install your standalone solar system also significantly influences your purchasing decision. The fewer hours of sun you get in a year, the bigger and more powerful your system should be. In this case, the efficiency of the stand-alone PV system must also be as high as possible to compensate for the missing hours of sunshine.
Likewise, the regularity of the sun plays a role in the choice of storage. The more erratically the sun shines on you, the bigger your memory should be. With this, you can easily spend a few days without sun.
Since the standalone solar system is not connected to the public grid, you never have the option of getting external electricity. Therefore, you should first educate yourself thoroughly and seek expert advice before making your decision.
The autonomous solar system is only perfectly suitable in a few cases. For example, if your electricity usage is low, you only use electricity in the summer months, or your system is installed in a remote area.
If completely self-sufficient power supply is not that important to you, then a simple power system with storage is the best option in most cases. Because it’s cheap and flexible at the same time, you can earn money by feeding it into the grid, and an optional storage system ensures that you use a lot of electricity yourself. Plus, you get a 19% sales tax rebate and can draw electricity from the public grid at any time.
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