Install photovoltaic power storage yes or no?

Calculations with the storage work with different assumptions. If you only calculate using sunshine hours, it looks like the left side of the illustration. If you also include cloudy days, it can look like the right side. From this results an assumption about the cycles of the electricity storage. This assumption determines the calculated price of one kW from the electricity storage. So you can calculate it either favorably or unfavorably.

(Source: HTW Berlin)

I don’t understand the picture.

- What does the cloudiness have to do with the profitability of a storage system? I cannot influence the weather; with or without storage, production is given.
- Why does battery power last until the next morning after a cloudy day but not after a sunny day? According to the picture, less electricity is consumed with clouds (in the evening both batteries are at 80%).
- Why is there photovoltaic power available at 7:30 pm on a cloudy day but not on a sunny day?
- Why is the battery only charged up to 80%?
- Why is the feed-in limitation not considered?
- Today’s systems work with forecast calculations for storage control; why is this not depicted?
- What is actually the "result" of the pictures? I see a full battery cycle per day in both, and then?

Maybe you could provide a sample calculation of how you arrive at the price per kWh from the storage. Thanks.

We have a storage system ourselves. Until the end of November, it was fully charged every day. However, thanks to the heat pump, we cannot get through the night without external power despite the battery. But all that is also due to the mismatch between photovoltaic generator and storage capacity (28.5 kWp to 10.2 kWh). Whether the storage is worthwhile is largely decided by its lifespan. You can really only calculate it at the end. Generally, it is said that one can expect profitability at a price of <450-500€/kWh.

The image shows that a storage system during changeable weather not only charges during the day but also discharges. This affects the assumption of cycles per year.

Simplified calculation method surcharge battery electricity over service life and cycle assumption: Usable storage capacity x charge cycles per year = battery electricity p.a. e.g. 8kWh x 200 cycles = 1600 kWh
calculated service life x annual production = total production. e.g. 10 x 1600 kWh = 16000 kWh
purchase price : production = cost per kWh e.g. €4000 : 16000 kWh = €0.25 per kWh
Now one can calculate with 15 years, then it comes out to €0.167 per kWh.
Another assumes 10 years and only 150 cycles, suddenly the kWh costs €0.33 per kWh.
Another calculates the manufacturer-specified cycles and no service life, ending up somewhere else again.
Some assume a number of cycles corresponding only to the number of sunny days, resulting in correspondingly higher costs.

Any example calculation can be dissected arbitrarily based on the basic assumptions – and in every direction.

Ok, then we mean the same. With a "calculation" that consists practically only of unknown variables, which are estimated freely, anything and nothing can come out, depending on the personal desired result.

Our storage system went into operation on 7.10.2020. As of now, we have charged 577.82 kWh into the battery and discharged 505.15 kWh. So roughly 100€ savings on external purchase.

Hi, so about €600 savings per year, around €6000 over 10 years. What does the storage cost in return? Is it worth it? Olli

We paid about €4350 for the storage (after deducting the subsidy). That should be sufficient. There is a 10-year warranty on the storage, so I assume a longer lifespan. Even then, it rarely fails completely, it just loses capacity.

Of course, it is clear that this is neither economically nor ecologically a return miracle. It is simply nice not to need any external electricity from March to October, to decouple somewhat from electricity price developments/taxes, and to have a little influence on the electricity mix. After all, we produce easily three times as much electricity as we consume annually.

... We have a storage system ourselves. Until the end of November, it used to be fully charged every day. However, thanks to the heat pump, we still don't get through the night without external power despite the battery. All this is also due to the mismatch between the photovoltaic generator and the storage capacity (28.5kWp to 10.2kWh). ...

So you have about 8kWh in the battery, with most batteries having a discharge limit somewhere around 20%. That leaves (only) about 6kWh usable. Is it really the case that your heat pump drew about 6-7kWh overnight (until the sun supplies power again)? Is that a realistic value? Seems a bit much to me ...