Which solar battery is suitable for your personal needs? Store the electricity or feed it into the grid? What types of solar batteries are there, and how do they differ?
If you own a solar powered generator system, there are two options: you can either feed the surplus green electricity into the public grid or store it. In recent years, the feed-in tariff has steadily decreased. In many cases, it is, therefore, worthwhile storing the electricity generated so that it can be reused at a later time.
Some even go one step further and establish a stand-alone solar system to be self-sufficient with electricity. Stand-alone systems have no connection to the public power grid but have solar panel batteries with a large storage capacity. This enables the highest degree of independence.
Although there are many different types of power storage, only a few are suitable for solar power storage. These batteries, specially designed for solar power storage, can help you get the maximum yield from your solar array. Strictly speaking, these memories are not batteries, but rechargeable batteries, because only these can be recharged. But because the battery is often used as an umbrella term, we use solar and solar batteries as synonyms here.
Use a photovoltaic system with or without a solar battery
During the day, when the solar radiation is greatest, your PV system produces the most electricity. However, since nobody is at home at this time, little of this electricity can be used. During the day, electricity is usually only required for the refrigerator and freezer. The excess electricity is then fed into the public grid for a small fee. The feed-in tariff is currently just over 6 cents/kWh (as of July 2022).
In the evening, on the other hand, most people are at home and use various electrical devices. The stove, microwave and dishwasher are used for cooking. After that, you might use the television and lights throughout the house and charge cell phones and laptops. Washing and drying laundry also consumes a lot of electricity. The electricity consumption is significantly higher in the evening than during the day.
If you have already fed all the excess, self-generated electricity into the public grid, you will no longer have any electricity for your consumption in the evening. This means you now have to obtain electricity from the public grid, which currently costs around 37 cents/kWh (as of July 2022). If you integrate a solar battery into your solar system, you can therefore save significantly on costs. You store the excess energy you generate during the day and use it 100% – and it’s free of charge. This means that less or even no external electricity has to be purchased in the evening.
The advantages and disadvantages of a solar battery
- Higher benefit of the PV system – High acquisition costs (often 5,000 – 9,000 $)
- Reduction in electricity costs due to lower external consumption – Solar batteries can be harmful to the environment if they are not disposed of properly
- Solar power can be used at any time of the day – Regular replacement is necessary
- Greater independence from electricity prices – Only worthwhile if there is a large amount of excess electricity
- You achieve a higher degree of self-sufficiency
- If you choose the right storage system, self-consumption is more profitable than feeding it into the grid
It is important to note, however, that solar batteries are not suitable for every household. A smaller tank with a storage volume of 6 kWh costs around $7,000 – 9,000. Aside from the fact that this is a large investment, the memory only pays for itself if its storage capacity is used to the full, it does not require constant maintenance and repair, and at the same time, it has a long service life.
Because solar systems today have a service life of 20 to 35 years, but solar batteries only last 10 to 15 years. Depending on the type, the power storage often has to be replaced. Don’t underestimate the cost of additional memory, and don’t just look at the price before you buy it, but rather the service life!
In addition to storage capacity, quality and service life, other factors are also decisive. To make the right choice when purchasing, we have listed all the important criteria for you in the next section.
What criteria do you have to consider when buying a solar battery?
When choosing a solar battery, it is important to consider the following aspects. Because cheaper is not always better, but in most cases means that you have to compromise on one of these aspects. In the long term, this is reflected in additional costs. These are the criteria:
- Storage capacity
- Maximum depth of discharge
- Service life and number of charging cycles
- Battery docking
The storage capacity of the solar battery
The storage size of the battery indicates the maximum amount of energy it can store. This is measured in the unit kWh. Homes usually need a solar battery with a storage volume of between 3 and 10 kWh to store the solar power produced daily.
The price of the solar battery depends very much on the capacity. The larger the capacity, the more expensive the battery. These rules of thumb will help you choose the right capacity:
This power is measured in kilowatt-peak (kW) units. In short, kWp is the maximum energy output the plant can produce. The larger the PV system and the higher the efficiency of the individual solar cells, the higher their performance.
If you consume 4,000 kWh per year, we recommend purchasing a 5,000 Wp (5 kWp) system. The sun does not always shine, and especially in winter, only about half of the maximum output is produced. The actual electricity yield is, therefore, always slightly lower than the maximum output of the PV system.
Don’t you know your electricity consumption? No problem. Our power consumption calculator will help you!
With an annual consumption of 4,000 kWh, a solar battery with approx. 4 kWh storage capacity is recommended. Storing all of the day’s energy isn’t worth it because some energy is already being used during the day for the freezer, refrigerator, and numerous devices on standby. Also, larger capacity memory is much more expensive.
If you don’t regularly use 100% of your battery’s capacity, your investment will be less profitable. It’s better to spit some power into the grid now and then than to have spare capacity.
The efficiency of a solar battery
The service life of a lithium ion solar battery always depends on its quality. Apart from that, however, also from the technology used. The service life of lead storage is around 5 – 10 years, while lithium storage can be expected to last around 10 – 15 years. The maximum achievable charging cycles also depend on the technology.
A charge cycle is each complete discharge of the solar battery down to the maximum depth of discharge. Incidentally, not only are the complete discharges counted but the accumulated discharges as one cycle. This means that a battery with an 80% maximum depth of discharge also has a complete charge cycle behind it if it has been discharged four times by 20%. Even if it has been fully recharged in the meantime, this counts as one charge cycle.
The more load cycles are run through, the more the maximum storage capacity decreases and the higher the susceptibility to problems becomes. Therefore, the service life of a solar battery also depends on the number of maximum possible charging cycles.
A solar battery goes through an average of 200 charging cycles per year. A service life of 2,000 full charging cycles corresponds to the usage of approx. 10 years.
On the other hand, several 12 volt batteries must be purchased because PV systems are 20 to 35 years old. It is, therefore, definitely worthwhile to use storage devices with a longer service life. Even if the purchase price is higher, they are often more profitable in the long run since only two storage tanks are required here instead of three or four.
Photovoltaic systems produce direct current, which is first converted to usable alternating current by a pure sine wave inverter. There are now two different options for storing the electricity produced:
- Option 1: A DC storage (Direct Current = direct current) is connected in front of the inverter and stores the direct current that comes directly from the solar panels.
- Option 2: An AC storage (alternating current = alternating current) is connected behind the inverter and stores the electricity that the inverter has already converted.
With DC storage, the direct current flows unchanged into the storage and is only converted into alternating current by the inverter when used. With the second option, on the other hand, the alternating current that has already been converted must first be converted back into a direct current to store it. Before it can be used, its inverter converts the electricity into an alternating current.
Thus, DC systems save two conversions compared to AC systems. Because some energy is lost with each conversion, this usually results in a higher energy yield in DC storage.
Types of solar batteries
Every solar battery works on the same principle. Electricity is converted into chemical energy. This concept is reversed when it comes to discharging. Both processes release heat, and part of the energy is lost. Solar batteries differ in filling. Currently, two different types of batteries are mainly used: lead-acid and lithium-ion batteries.
The older type of battery on the market that has become the standard over the years is the lead-acid battery. This is also used in cars, among other things. These battery advantages are a low acquisition cost and a long series of historical empirical values. This type of solar battery is ideal for small to medium loads. This inexpensive battery type performs worse than the lithium version in terms of service life, efficiency and depth of discharge.
Further development of the lead-acid battery is the lead-gel battery. These are more robust and do not need to be serviced as often. Therefore, lead-gel batteries are also a little more expensive.
Although lead-acid batteries often compare poorly to lithium batteries, they have been tested and rated as reliable and safe for decades. Because lithium batteries have only been used more and more for a few years, there are only a few long-term reports here. So efficiency and lifetime claims are fairly accurate for lead-acid batteries, while they can vary slightly for lithium batteries.
Lithium Ion Battery
A 12v lithium battery is a right choice if you are interested in a maintenance-free solar battery with high performance and durability. Lithium batteries have become increasingly popular as a newer technology, especially in recent years. Of the various lithium-ion batteries, the lithium iron phosphate battery is particularly suitable for a solar system.
The increased use is because these batteries are generally more powerful, and lithium prices have fallen sharply. Nowadays, these are used in smartphones, laptops and electric cars. The lithium solar battery scores 5,000 – 7,000 full charging cycles (vs 2,000 to 4,000 cycles for lead batteries) and a long service life of approx. 10 – 15 years (with lead, it is approx. 5 – 10 years).
In addition, lithium storage with 90 to 98% efficiency is far more efficient than lead-acid batteries (70 to 85%). In addition, solar batteries with lithium have a usable capacity of 80 – 100%, whereas the lead alternatives can only be discharged up to 60 – 80%, depending on the battery.
Despite the higher initial cost, lithium-ion batteries are definitely on the rise and are growing in popularity yearly. In 2015, 90% of the funded storage systems were lithium batteries.
Which solar battery do I need?
Now you might be wondering which of the three battery types to choose. It is important to note that the solar battery must match your solar system. If the amount of excess electricity is not very large at your place, you often drive better, even without a solar battery. Then feed your electricity into the grid, receive payment and save on the acquisition costs of a more expensive battery.
The decisive factor here is the performance of your PV system and the amount of excess electricity.
Be sure to pay attention to the criteria for buying a solar battery (found in the section above), use the rules of thumb and consult a competent solar installer. Suppose you are going to use your PV system for a long time. In that case, it is best to use the lithium-ion battery family and lithium-iron-phosphate batteries, which have a significantly longer service life. Lead batteries, on the other hand, are only recommended for comparatively high performance and service life at a low price.
Since solar systems, including storage, are very important for the energy transition, more and more PV funding programs are at the federal and state levels.
For a solar battery to be worthwhile for you, it must be cheap and have a long service life. But other factors such as storage capacity, usable capacity, efficiency and depth of discharge are also important. Very important: Don’t underestimate the costs of additional storage if your PV system should have a much longer service life.
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