Economic efficiency of a 10-kWp system vs. a 5-kWp system

Because in summer you simply can't burn through as much electricity as the system produces. The 20% is already somewhat okay. You would have to get an air conditioner, a pool, and a pool heater to make a difference there.

The lower part below the dashed line is our self-consumption. Over the year, we achieve 46% self-consumption. But we specifically wash and dry during the day and operate a pool.

Here is an updated invoice again:
- Investment costs (net) taken from a table from a solar system portal on the WWW, which stem from a statistical survey.
- The first 5 years are not yet considered as a small business, so that the investment costs can be calculated without VAT.
- Sales tax: 5 cents/kWh for self-consumption
- Calculation of electricity costs for self-consumption as follows:
currently 24 cents, due to electricity price increase of 3% per year, an average value of 36 cents is assumed for the next 20 years.
- Income tax according to income-surplus calculation (EÜR). A 5% depreciation of the investment costs over 20 years is assumed, which is deducted from the income. For simplicity, it is assumed that the income tax on self-consumption is the same as that on feed-in remuneration. A marginal tax rate of 42% is assumed.

5-kWp system
============

Investment costs: €7,600 net

Yield per year:
70%*3500 kWh*12.2 cents/kWh + 30%*3500 kWh*36 cents/kWh
=
€298.90 + €378
=
€676.90

Sales tax on self-consumption in the first 5 years, as long as one is not yet a small business, converted over 20 years
5/20*(30%*3500 kWh*5 cents/kWh)
= €13.13

Income tax according to EÜR:
0.42*(3500 kWh*12.2 cents/kWh - 0.05*€7,600)
=
0.42*(€427 - €380)
=
€19.74

Return = (€676.90 - €13.13 - €19.74) / €7,600 = 8.5%

10-kWp system
=============

Investment costs: €13,300 net
Yield per year:
80%*7000 kWh*12.2 cents/kWh + 20%*7000 kWh*36 cents/kWh
=
€683.20 + €504
= €1,187.20

Sales tax on self-consumption in the first 5 years, as long as one is not yet a small business, converted over 20 years
5/20*(20%*7000 kWh*5 cents/kWh)
= €17.50

Income tax according to EÜR:
0.42*(7000 kWh*12.2 cents/kWh - 0.05*€13,300)
=
0.42*(€854 - €665)
=
€79.38

Return = (€1,187.20 - €17.50 - €79.38) / €13,300 = 8.2%

Is this better now?

Ignore the last post.
>>>
Here is an updated calculation again:
- Investment costs (net) taken from a table on a solar system portal on the WWW, which is based on a statistical survey.
- For the first 5 years, one is not yet considered a small business owner, so the investment costs can be calculated without VAT.
- Value added tax: 5 cents/kWh for self-consumption
- Calculation of electricity costs for self-consumption as follows:
currently 24 cents, due to electricity price increase of 3% per year, an average value of 36 cents is assumed for the next 20 years.
- Income tax according to income-surplus calculation (EÜR). A 5% depreciation of the investment costs over 20 years is assumed, which is offset against the income. For own withdrawal, the difference between the saved electricity provider price and the lost feed-in tariff must be taxed, i.e. 36 cents/kWh - 12.2 cents/kWh = 23.8 cents/kWh. A marginal tax rate of 42% is assumed.

5-kWp system
============

Investment costs: €7600 net

Yield per year:
70% * 3500 kWh * 12.2 cents/kWh + 30% * 3500 kWh * 36 cents/kWh
=
€298.90 + €378
=
€676.90

VAT on self-consumption in the first 5 years, as long as one is not yet a small business owner, converted over 20 years
5/20 * (30% * 3500 kWh * 5 cents/kWh)
=
€13.13

Income tax according to EÜR:
0.42 * (0.70 * 3500 kWh * 12.2 cents/kWh + 0.30 * 3500 kWh * 23.8 cents/kWh - 0.05 * €7600)
=
0.42 * (€296.94 + €249.90 - €380)
=
€70.07

Return = (€676.90 - €13.13 - €70.07) / €7600 = 7.8%

10-kWp system
=============

Investment costs: €13300 net

Yield per year:
80% * 7000 kWh * 12.2 cents/kWh + 20% * 7000 kWh * 36 cents/kWh
=
€683.20 + €504
= €1187.20

VAT on self-consumption in the first 5 years, as long as one is not yet a small business owner, converted over 20 years
5/20 * (20% * 7000 kWh * 5 cents/kWh)
= €17.50

Income tax according to EÜR:
0.42 * (0.80 * 7000 kWh * 12.2 cents/kWh + 0.20 * 7000 kWh * 23.8 cents/kWh - 0.05 * €13300)
=
0.42 * (€683.20 + €333.20 - €665)
=
€147.59

Return = (€1187.20 - €17.50 - €147.59) / €13300 = 7.7%

Is it better now?

Stop it already :) It won't get better, even if you include more factors in the calculation.

Assumptions about electricity price developments, especially over 20 years, are simply unreliable. No one can know this. Likewise, self-consumption can only be estimated and can vary significantly up or down. But you are trying to create a calculation here that is exact to decimal places. That doesn't fit with these variable factors. So throw them overboard.

The conclusion remains: Larger systems are cheaper per kWp in terms of investment. Period, full stop. :)

Without calculating now, that is too theoretical an approach for me. The result is apparently calculated down to the cent, yet there are still so many unknown variables that in the end only an apparent accuracy is created.

My basic statement is: An installation is economical if the returns during the depreciation period are higher than the costs. Self-consumption then improves the balance as the cherry on top.

Not much more can be read from your result either. A large installation leads to a (however strong) lower self-consumption, which marginally affects the overall balance. However, the main part comes from the consideration of costs versus returns.

That is why a stated result as a return bothers me. In my view, this is not an investment in the financial sense, but an investment for the future and the prospect of "free" electricity or loan repayment contributions after refinancing. After all, we are not investors but outfitting our own home.