Photovoltaic system: Costs, saving potential? - Experiences?

What exactly do you mean by that? So I have received the full value-added tax for the purchase (over 3,000€) back from the tax office. For the feed-in, I regularly file the advance VAT return via Elster. What I have not yet declared is the feed-in remuneration. Taxes are unfortunately not really my thing. I still have to go to the tax advisor soon and clarify quite a few things there.

Feed-in and self-consumption must be included in the preliminary notifications and in the annual report. Additionally, self-consumption must be included as Anlage EÜR in the income tax return. Both annual declarations were to be submitted by July, unless a tax advisor does this for you. Your statement, however, that YOU submit the declarations so far speaks against that.

The income tax return has always been done by the income tax assistance association for us so far. Due to the photovoltaic system, however, the association is no longer allowed to prepare our tax return. Therefore, we still have to consult a tax advisor for the 2020 tax return. As I said, I have submitted the preliminary notification via Elster so far. I had an employee from the tax office briefly explain to me what I need to enter there. Although I have already received very different statements regarding the billing. The employee from the tax office said I should use 20 cents/kWh as the basis for the tax calculation for own consumption. A friend of mine also does his tax return through a tax advisor, and he was apparently told that there is no fixed regulation for the calculation. Therefore, one could simply use 9 cents/kWh as the calculation basis for both own consumption and feed-in. Of course, that would immediately halve the tax burden.

What exactly do you mean by that? So I got the entire VAT for the acquisition (over €3,000) refunded by the tax office.

This VAT is only refunded to "entrepreneurs" with the intent to make a profit. I do not see that given when waiving the feed-in tariff!

I would have to look into that again.
For example, I still received a statement regarding the feed-in tariff from the network operator at the end of the year. However, the payment recipient listed there is the company Senec. I am still officially the recipient of the statement.
In addition, I still receive compensation from Senec for the feed-in that exceeds the annual allowance.

A friend of mine also does his tax return through a tax consultant, and he was told that there is no fixed guideline for the calculation. Therefore, one could simply use 9 cents/kWh as the calculation basis for both personal consumption and feed-in.

No, that is not correct. There is no fixed value to be applied, but there is a prescribed calculation basis. For VAT, it is the procurement costs for your supply from the grid operator (net). Basic fees and premiums must be taken into account. This usually results in somewhere around 20 cents, so some tax offices accept a flat rate of 20 cents. Since no calculation method is shown in the declaration, it may well be that the tax office does not notice anything with your friend for the small amounts, but it remains a classic tax evasion/tax fraud. For income tax, the value to be applied is the forgone feed-in tariff. Income, profits, depreciation, and costs must then be offset in the EÜR annex. If this results in a profit, it is taxed as income at the regular equity tax rate. If it results in a loss, it reduces the taxable income. However, caution is required here, because as rightly points out, the annex must generate profits over 20 years prospectively; otherwise, the entrepreneurial status and all benefits (including the VAT already repaid) are lost.