Falling feed-in tariffs and rising electricity prices make the self-consumption of solar power more attractive. Because if you use more of your own electricity from the roof, you save money. Self-consumption can be increased by retrofitting existing PV systems with a power storage unit.
Retrofitting power storage in existing photovoltaic systems
Operators of photovoltaic systems can usually only use 20 to 30 percent of their solar power themselves. During the day when the sun is shining, consumption is usually not too high. The demand increases in the evening and at night, but the solar yield is low. This can be changed with battery storage. The power storage does not necessarily have to be purchased with the PV system, most batteries can also be retrofitted.
When is it worth thinking about retrofitting an electricity storage system?
Without energy management and solar battery storage, a household consumes less than a third of the PV electricity it produces itself. More than two-thirds of the electricity is fed into the grid and is now only poorly remunerated with the feed-in tariff. The state-guaranteed remuneration rate for newly registered solar systems is currently 8.6 cents / kWh.
The electricity that has to be bought from the grid costs 40 cents/kWh and more. Each stored kilowatt hour of electricity, therefore, saves over 30 cents. If the photovoltaic system has a size of 6 kWp, it produces around 6,000 kWh of electricity per year. About 1,500 of these have been used up to now. In theory, the entire residual electricity could be stored, but in practice, this is usually not economical. As a rule of thumb, self-consumption can be increased to 50 to 80 percent with an economically working storage system.
The example shows that the household without an electricity storage system still has electricity costs of 1,013 euros per year, while the household with a self-supply rate of 50% only has 542 euros. In this way, almost 500 euros could be saved per year. In case three, the memory is even slightly oversized, since more is stored than is used. Since the production costs for photovoltaic electricity are the same in all three cases, only the size of the storage system and its price decide whether retrofitting is worthwhile.
The balance tends to be worse for older systems, which receive even higher feed-in tariffs and can achieve significantly more income from the feed-in tariff. This reduces the savings potential through a power storage unit. Retrofitting is a sensible alternative for solar systems that have been phased out, so-called post-EEG systems, which can only achieve a very low market price of just under 4 cents per kilowatt hour. The limiting factor here is the size of the system: If it does not supply enough electricity for storage, it is not worth investing in the battery.
How big should the storage be?
The correct dimensioning of the electricity storage is – regardless of whether it is installed with the system or retrofitted – the central aspect when it comes to the question of the economical storage operation. The cost of battery storage is roughly estimated at 1,000 euros per kWh of storage capacity. If the storage tank is dimensioned too small, the self-consumption remains low and the savings potential is not exhausted. If the battery is too large, expensive storage capacity is bought that is not really needed. The aim of the dimensioning should not be complete self-sufficiency, i.e. a self-consumption rate of 100%, but only an optimal size that balances the degree of self-sufficiency and economy. As a rule of thumb, about one kilowatt-hour of storage capacity should be used for each kWp of the system. This increases the proportion of self-consumption to around 60%.
Not only the size of the PV system has an influence on the design of the electricity storage system, but also the consumption behavior. A household that consumes a lot of electricity, especially in the evening and at night, needs larger storage than a household that realizes its peak consumption during the day.
Is it worth enlarging the PV system?
Especially in the early days of photovoltaics, private users mainly built solar systems with a few kWp. The reasons for this were the high costs and the lower output of the home solar system kit. Retrofitting such an old system with an electricity storage device is generally not worthwhile, as there is not enough electricity available for storage. In view of the fact that the demand for electricity will increase in the next few years and the costs for grid electricity will almost certainly not decrease either, the expansion of the existing or a new, more powerful photovoltaic system is a sensible option. In principle, it is important to note that Extension or new construction lead to a reassessment of the system. This means that the older, higher feed-in tariff is only paid for the existing system. Entitlement to the high rate of reimbursement does not apply to new, additional, or exchanged modules. A change, therefore, needs to be carefully considered. Perhaps it makes more sense to wait until the EEG subsidy expires before thinking about a new installation and the purchase of an electricity storage system. If there is still enough space on the roof, there is no reason not to expand the system and retrofit it with battery storage.
Which storage systems are particularly suitable for retrofitting the PV system?
From a technical point of view, in principle, any photovoltaic system can be combined with any battery storage system. However, what works without any problems with a new installation is a bit more complicated with retrofitting. Here it depends on whether the storage is to be installed on the AC or DC side. AC-side means that the storage is connected behind the inverter (that is, the direct current from the PV system has already been converted into alternating current). Stores integrated on the DC side are located behind the inverter of the PV system, i.e. they are supplied with direct current by it.
Storage and inverters
A battery can only ever be charged with direct current. This is why it is always behind an inverter, i.e. where the AC current from the PV system has already been converted into DC current. In order for the stored electricity to be used, it has to be converted back into an alternating current. This double conversion in AC storage is associated with higher losses compared to DC storage.
If the storage system is integrated on the AC side, i.e. between the PV system and the inverter, this means that it must have its own internal inverter (also referred to as a battery inverter). This is not necessary with DC-side integration – the storage is located after the inverter of the system (the PV inverter) and can access it. But it must also be designed for the additional load. When retrofitting, this often means that the inverter has to be replaced, which incurs additional costs. Therefore, the PV system is easier to retrofit with AC storage and the AC-side connected products are often recommended, despite the higher conversion losses.
So-called hybrid inverters, which can temporarily store solar power with the help of an internal or external battery, offer an alternative solution. This means that PV and solar inverters are combined in one device. This also makes it easier to retrofit with memory installed on the DC side. A distinction is made between single- and three-phase hybrid inverters, with the latter being particularly useful if, for example, an electric car is to be charged via the PV system.
Is there a subsidy for retrofitting a solar power storage unit?
Electricity storage for home use has been around since the early 2010s. The solar battery has now arrived on the mass market – in 2018 sales for the home storage market were 1.3 billion euros. Since high-performance batteries make the use of volatile renewable energies such as photovoltaics more attractive, there are numerous funding programs for electricity storage. The nationwide funding program 275 of KfW was discontinued at the end of 2018, but funding via program 270 (renewable energies – standard) is still possible. The KfW loan is available for electricity storage that is purchased with a new photovoltaic system as well as for retrofitting.
The majority of the electricity storage subsidies were implemented by the federal states but also by municipalities, and many programs have now been discontinued. There were also regional differences: while in Saxony, for example, the retrofitting of existing photovoltaic systems with an electricity storage system was subsidized, it was excluded in Rhineland-Palatinate. The amount of funding also differed significantly in some cases. When funding battery storage, it should also be noted that the funding amounts are often exhausted quickly and the program is then put on hold or discontinued. In general, it can be said that there are only a few funding offers for battery storage today.
Are there alternatives to retrofitting the PV system with an electricity storage system?
The acquisition costs for a power storage unit are anything but low and it takes some time for the solar battery to pay for itself. It may therefore make more economic sense to increase self-consumption in a different way. For example, a wall box can be supplied with solar power, which can be used to charge an electric car. It is also conceivable to feed a heat pump to support the heating system or to heat water. The introduction of an energy management system involves the least financial effort. Consumers can be switched via the central monitoring and control unit when there is a surplus of solar power. Use cases are the operation of a pool pump or switching on an electric hot water tank (boiler).
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