Are 15 kilowatt photovoltaic systems only worthwhile with battery storage?

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Are 15 kilowatt photovoltaic systems only worthwhile with battery storage?
  • By deutschewhiskybrenner

If you look at the current climate protection and energy transition scenarios, you can get dizzy. Within the next 20 years, Germany will need around eight times the photovoltaic capacity currently installed. The larger the roof systems are, the less undeveloped area is required for them. It can therefore be said without any doubt that it makes sense to "fill up" roofs and to dimension the photovoltaic roof systems on them as large as possible. Until now, this could also be justified economically for the individual. With self-consumption and feed-in tariffs, such systems usually pay off. The Bonn market research institute EUPD Research has now pointed out that increasing costs for the installation of a photovoltaic system and at the same time falling feed-in tariffs for solar power are reducing profitability.

But what does that mean? In a study financed by a battery storage manufacturer, EUPD Research stated that only battery storage would ensure the profitability of such systems. However, the results of return calculations depend very heavily on assumptions about the parameters, which differ considerably from individual to individual. Our readers therefore sharply criticized this report, especially since EUPD Research did not publish all of the calculation parameters. The Bonn market researchers have now done so, so that their statement can now be better assessed.

Photovoltaic system without battery storage

First of all, the question of whether a relatively large photovoltaic system with 15 kilowatts of power for a single-family house without battery storage is still worthwhile.

The feed-in tariff is regulated by law and is fixed at around 7.4 cents per kilowatt hour for 20 years for systems that went into operation in July. Solar self-consumption is not fixed. EUPD Research assumes a household that heats with a heat pump and owns two electric cars that are mostly parked away from the house during the day. Then the household has perhaps 12,000 kilowatt hours of electricity consumption (typical values: 4,000 kilowatt hours household, 4,000 kilowatt hours heat pump, 4,000 kilowatt hours electric cars with a mileage of 22,000 kilometers). On the basis of analyzed load profiles, the market researchers assume that without battery storage, 30 percent or 4,300 kilowatt hours of the self-generated electricity is used directly in this case, so that the corresponding electricity costs are saved. According to their calculations, the return on the project is then 1.7 percent. This is the return not related to the equity, but to the investment costs, which includes the financing costs (see info box for parameters).

With these parameters, a 15-kilowatt system without storage is also worthwhile, even if the return is low. If you calculate with the same parameters in the return calculator of the Stiftung Warentest, the result is also positive. After that you achieve a return of 4 percent (invoice with sales tax, without income tax, without financing costs).

Parameters that EUPD uses for the calculation:

15 kWp PV system: 960 kWh/kWp solar radiation; 1300 Euro/ kWp (net including installation) system costs, 1.5% operating costs; 2.0% inflation; 100% debt, 2.0% debt interest; Plant degradation 0.25 percent/year

15 kWh storage: 600 euros/kWh (net including installation) system costs, 85% system efficiency, 1.0% operating costs, 2.0% inflation, 100% borrowed capital, 2.0% borrowed capital interest

Electricity price: 22.7 cents/kWh (net share of energy price); 1.0% electricity price increase per year

Household: heating with heat pump and two electric cars; with 12,000 kilowatt hours of electricity consumption per year, self-consumption share without storage 30 percent; Self-consumption share with storage 65 percent

A look at the details shows how individual parameters really affect the wallet in the end. According to the Stiftung Warentest calculator, the annual income is around 1,420 euros. One of the more uncertain parameters is the actual self-consumption, which you ultimately achieve over 20 years. Assuming that the share of self-consumption is 5 percentage points lower and is therefore 25 percent instead of 30 percent, the return is reduced from 4 to 3 percent, but the annual income is only reduced to 1300 euros.

As a homeowner, you have to consider whether you make your investment decision more or less dependent on a percentage point of return if one computer assumes a higher self-consumption rate than the other. The assessment of how electricity costs will develop is similarly uncertain.

Are 15 kilowatt photovoltaic systems worth it? only with battery storage?

Investment costs also have a major impact on returns. They can be read from the offers of the installation companies. The assumption that the photovoltaic system costs 1300 euros per kilowatt peak, i.e. 19,500 euros in total, comes from the market survey by EUPD Research. However, if you get the system for 1,200 euros per kilowatt peak, the yield increases from 4 to 5.2 percent according to the Stiftung Warentest calculator. In the end, however, compared to other expenses around the house, it is not about a lot of money here either, but about a comparatively few 100 euros more or less per year.

Photovoltaic system with battery storage

What does it look like if you also install battery storage? Based on its market surveys, EUPD Research assumes that a kilowatt hour of storage capacity currently costs an average of 600 euros. With 15 kilowatt hours, self-consumption increases to 65 percent according to the assumed load profiles. That would be 9,360 kilowatt hours of the 12,000 kilowatt hours of power consumption. According to EUPD Research, the yield increases from 1.7 percent to 2.6 percent. With similar parameters, it increases from 4 to 4.15 percent at Stiftung Warentest (some parameters, such as maintenance costs, cannot be entered in the same way in the input mask).

In both calculations, the battery storage increases the return compared to a system without battery storage. However, this only applies to the assumed parameters. If the self-consumption only increases to 60 instead of 65 percent, for example because one of the cars charges outside more often, according to Stiftung Warentest the return is only 3.5 percent and thus lower than with the system without battery storage. But here too, as in the case of the system without storage, the actual impact of these differences on the wallet of home builders is comparatively small.

In principle, the example should be interesting because it shows when battery storage can increase returns. In this case, it works because, based on the assumptions made, around 5000 kilowatt hours are rolled over the storage system per year ((65 percent minus 30 percent) times the annual production). That's 336 full cycles per year. EUPD Research also assumes that the memory and electronics will last 20 years, since the maximum number of cycles specified by manufacturers is not usually reached in 20 years, according to EUPD Managing Director Martin Ammon. However, it is unclear whether the systems really last that long. Most manufacturers give a 10-year guarantee.

Both assumptions are not impossible, but favor the viability of storage. Because the stored and withdrawn kilowatt hour of solar power is the cheaper, the greater the total number of cycles (annual full cycles times service life). In the pv magazine home storage overview they can be found for many systems for 300 annual cycles and once for 10 years, as guaranteed by most manufacturers, and once for the specified number of cycles.

In Germany, however, only 250 cycles per year are often assumed, since solar generation is lower in winter. However, with a 15-kilowatt system, the average yield on a December day (in this case around 16 kilowatt-hours) is enough to fill the storage tank once. Martin Ammon therefore calculates that an average of 0.5 to 0.7 cycles per day will also be achieved in winter. In summer, even more than one cycle per day is possible if only one electric car is charged during the day from the photovoltaic system and storage (so that it goes faster) and then the second electric car.


In the example that EUPD Research gives as a reason for the statement in the last press release, the storage system really generates a positive return, so that the photovoltaic system with storage system pays off faster than one Plant without storage. However, the result depends very much on the assumed costs and parameters and on what proportion of self-consumption is actually achieved. In this respect, a household with two electric cars that mainly charge in the evening is ideal for refinancing the battery storage. In many other examples and with a dimensioning that does not exactly match this load profile, the result is likely to be different and the battery storage reduces the return.

In the EUPD Research example, the photovoltaic system is in the black even without storage, so the headline of the original pv magazine report is incorrect. With reservations, it should generally be the case that a photovoltaic system without storage pays off if the proportion of self-consumption exceeds 25 or 30 percent. If you can get them cheaper than the mean value found by EUPD Research, even with less self-consumption. The gap that is opening up as a result of rising prices and falling feed-in tariffs and which market researchers are pointing out is nevertheless problematic. Especially for larger commercial roof systems, where only low self-consumption rates are achieved.

Basically, it is difficult to focus on such yield calculations for single-family houses. The differences in absolute euros are often small and parameters that are difficult to estimate or predict play a role. Who knows what driving and charging behavior you will have in 15 years, what solar power will then be worth and how long the solar system will last beyond the assumed 20 years.

If you are satisfied with the purchase as doing something good for the energy transition, however, you clearly have the science on your side. This not only applies to the photovoltaic system, but also to battery storage. In the latest energy transition scenario from Fraunhofer ISE, a total of 180 gigawatts of battery storage is economically sensible and necessary by 2045, and the researchers assume that 120 gigawatts of this will be built as home storage. That is around 60 times as many as are currently installed.

Battery storage background and product overview

The pv magazine market overview is freely accessible and contains over 100 properties on more than 400 home storage products, including information on interfaces, efficiency information-> pv magazine market overview Home storage for photovoltaic systems

The detailed article on the home storage market overview in the June issue of pv magazine classifies the results and gives tips on what to look out for. The layout overview table included in the magazine article allows easier navigation through the online database. It contains information on the size, emergency power options, wallbox control and possible storage electricity costs with the respective product and allows a simple comparison.

-> Magazine articles and clear product table

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