Solar power storage is batteries that are specially built for use with photovoltaic systems. They store the solar power generated during the day so that it can also be used at night or when the sky is very cloudy. The most common types of these power storage devices are lead-acid and lithium-ion – solar batteries that differ in several ways.
Why use a solar power storage system?
A PV system is a logical decision for anyone who takes climate change seriously. After all, those who produce their own solar power supply themselves with clean, sustainable energy. Solar energy is emission-free and emits neither particles nor CO2.
To illustrate, the “feed-in method” is a counter-example. If you don’t have a storage system, you can either use the electricity produced at the very moment of production or feed it into the grid and receive a feed-in tariff from the grid operator. The problem: Most electricity is produced during the day, but the least electricity is consumed because most people are only at home in the evening. Only then will the TV, oven, stove, washing machine, dryer, etc., actually be used. Therefore, most of the electricity produced during the day is fed into the grid for around 7 ct/kWh (as of April 2022).
In the evening, external electricity has to be purchased for around 36 ct/kWh (as of April 2022). For every kWh that you feed in and “buy back” in the evening, you pay 29 cents. Admittedly, 29 cents per kWh is better than 36 cents, but anyone with storage can use 100% of their electricity, and it’s free. This means that less or no external electricity has to be purchased.
The disadvantages of solar power storage
Solar power storage is anything but cheap. Intermediate storage with 6 kWh storage costs around $7,000 – $9,000. Raising such a sum as an initial investment is a major hurdle for many.
Another point is the lifespan of the memory. Although LiFePO4 batteries last 10 – 15 years today, solar systems can be used for up to 35 years. You would have to buy several batteries in order to be able to store your electricity over the entire service life of the system. Under certain circumstances, this can be less profitable than simply feeding the electricity into the grid without storage.
In addition, there is another often-mentioned objection in terms of environmental protection. The disposal of memory is not always environmentally friendly. In particular, cheap lead-acid batteries with a short service life have to be bought and disposed of several times during the lifetime of a solar system. This pollutes the environment more than you think.
Solar power storage: pros, cons, and conclusion
As you can see, buying a solar power storage unit requires a not-inconsiderable sum of money. Whether the purchase is worthwhile in individual cases depends on many different factors. Last but not least, the service life and performance of the PV system are important criteria.
If you want to buy a solar power storage system, you should definitely look at the criteria in the next section and take the tips to heart. If, on the other hand, you still want to wait for newer technologies, you are also well advised to do so. Because from year to year, the memory gets better and even cheaper.
If you also want to eliminate the costs of purchase, maintenance, repair, and insurance, you can also rent a solar system. To find out more, you can submit a free inquiry here or read about the pros and cons of both options here: Buying a solar system vs. renting it.
How much does a solar power storage unit cost?
Solar power storage units cost between $5,000 and $9,000. An average storage system with a storage volume of 6 kWh costs around $7,000 – $9,000. Storage costs account for a large part of the total costs of a photovoltaic system.
The costs in each case depend not only on the memory size but also on efficiency, service life, and other criteria.
Which criteria must be considered when buying solar power storage?
When choosing a solar power storage unit, you shouldn’t just consider the price. The following aspects are particularly important:
- Memory size
- Efficiency
- Lifespan
- Maximum depth of discharge
- Number of charging cycles
- Memory coupling
- The memory size
The storage size indicates the maximum amount of energy that can be stored. This is measured in the unit kWh. As a rule, private homes need a solar power storage unit with a storage volume of between 3 and 10 kWh to store the solar power produced daily.
In general, memory size and price are proportionally related. The more you want to store, the more expensive your required storage is. To choose the right storage for your household, you can use the following rules of thumb:
1. Your PV system should have at least 1 to 1.2 times the output of your annual electricity consumption.
This power is given in units of kilowatt peak (kWp). 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. So if you consume around 4,000 kWh per year, we recommend purchasing a 5 kWp (5,000 Wp) system.
If you don’t know your current power consumption, you can use our power consumption calculator to find out quickly.
2. Your power storage should have a storage capacity of about one kWh for every thousand kWh of annual consumption.
3. With an annual consumption of 4,000 kWh, a solar power storage unit with approx. 4 kWh storage capacity is recommended. 4 kWh x 365 days = 1,460 kWh stored energy per year. This corresponds to just over a third of the total annual consumption.
It’s not worth storing all of the day’s energy 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. To achieve maximum profitability, the storage capacity should always be fully utilized. Feeding some excess electricity into the grid now and then is better than having an oversized storage tank that often doesn’t charge fully and has cost a fortune.
The efficiency of a solar power storage
The efficiency indicates how efficient the energy transfer between solar power and chemical energy is. In other words, efficiency describes the relationship between usable energy in storage and the originally supplied solar power. This ratio is expressed as a percentage.
Lead batteries have an efficiency of around 70 to 85%, while lithium RV batteries usually have an efficiency of 90 to 98%. Lithium storage is, therefore, more efficient and loses less energy during conversion.
How long is the service life of a solar power storage system?
Of course, the service life of the solar battery always depends on its quality. Apart from that, the average service life of lead storage tanks is around 5 – 10 years. With lithium storage, you can expect about 10 – 15 years.
Because solar generators have a service life of 20 to 35 years, several solar power storage units have to be purchased during this time. 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.
Maximum depth of discharge of a solar power storage unit
The maximum depth of discharge of solar power storage is shown in two batteries.
The depth of discharge is the DoD value. 100% depth of discharge stands for empty storage and 0% for full storage. In practice, the maximum depth of discharge is used as an indication of the usable storage capacity since solar power storage usually cannot be completely discharged.
For example, with a maximum depth of discharge of 95%, 5% of the stored energy remains in storage to preserve storage life. Complete discharges put a lot of strain on the memory, which would drastically shorten the life of the memory.
Therefore, before making a purchase, pay close attention to the DoD value of the solar storage because this indicates what percentage of the energy can be used from the total storage volume.
The number of charging cycles
A charging cycle is every complete discharge of the solar power storage unit 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 a 99% maximum depth of discharge also has a complete charge cycle behind it if it has been discharged three times by 33%. Even if it has been fully recharged in the meantime, this counts as one charge cycle.
A solar battery’s lifespan is related to the number of years it has been in use and the number of charge cycles it has had. The more loading cycles are run through, the lower the maximum memory capacity and the higher the susceptibility to problems.
A solar power storage unit 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.
Memory coupling
Because solar systems produce direct current, which is first converted to usable alternating current by the 3000w power inverter, there are two different options for connecting the storage system.
Option 1: A DC storage (Direct Current = direct current) is coupled in front of the inverter and stores the direct current from the solar panels.
Option 2: An AC storage (alternating current = alternating current) is coupled behind the inverter and stores the electricity that the inverter has already converted.
With this option, the already converted alternating current must first be converted back into a direct current to be able to store it. Before it can be used, its inverter converts the electricity again into alternating current.
With DC storage, on the other hand, the electricity flows unchanged into the storage and is only converted into alternating current by the inverter when it is used. This saves one conversion compared to AC storage. This usually results in a higher energy yield because some energy is lost with each conversion.
When is a solar system worthwhile? How can I benefit from PV support programs? And how does solar energy work?
Solar power storage types: lead vs. lithium
Solar power storage, which you will find most often in the market, is lead-acid or lithium-ion batteries. This is because these types deliver the best memory size, efficiency, and service life results.
Although both types of storage are very similar at first glance, there are crucial differences between the long-established lead-acid batteries and the newer lithium technology:
While lead-acid storage has an efficiency of around 70 to 85%, lithium storage is far more efficient at 90 to 98%. In addition, solar power storage with a lead only has a usable capacity of 60 to 80%, whereas lithium alternatives can be discharged to 80 to 100%, depending on the battery. This means that less of the stored electricity in lead-acid batteries are usable.
Lithium storage systems are also superior to lead-acid batteries in terms of charging cycles and service life. Solar power storage devices made of lithium have approx. 5,000 – 7,000 full charging cycles and a service life of a good 20 years, while the latter, made of lead, only achieves 2,000 to 4,000 full charging cycles and usually lasts 10 to 15 years.
Although all of these points speak very clearly in favor of lithium storage, lead storage also has its advantages: they are significantly cheaper, and there is no risk of overheating. With some lithium storage devices, overcharging can lead to temperature increases that can only be avoided with an energy management system.
It is also important to emphasize here that lead-acid batteries have been used as backup power solutions for a long time, have stood the test of time, and are, therefore, the safe option. Although lithium storage has the edge in all categories (except price), these values are only based on projections and forecasts. Because the technology is so new, there is not enough experience from 20+ years of use. So it would help if you did not blindly rely on any information provided by the manufacturer.
Summary
For a off grid solar power system to be worthwhile for you, it must be cheap and have a long service life. Don’t underestimate the cost of additional storage if your PV system should have a much longer service life.
As described in the section on purchasing criteria, solar power storage mainly differs in price, capacity, lifespan, and efficiency. Inform yourself thoroughly, and follow our tips and the recommendations of competent fitters. Ask the installer to issue you a storage pass and place the battery in a rather cool place.