PV system for the whole roof or better to undersize?

To receive the KfW funding, the developer has dimensioned the photovoltaic system to 6KWp. However, according to the electrician, up to 10 KWp fits, and I am offered the option to expand the photovoltaic system for an additional cost.

But I have done the following calculation, and it makes little sense to me:

-The additional cost would look like this: 4KWp * 1000€/KWp = 4000€
-4KWp would produce 4000KWh/year. Of that, self-consumption, based on values I read in this forum: 4000KWh * 20% = 800KWh.
-800KWh * 0.3€/KWh (according to Check24) = 240€ / year

This would mean a depreciation of 4000 / 240 = 16 years.

And these were all very optimistic numbers. Did I forget something? I actually don’t find it very sensible as an investment. That’s why I wonder why there is this mantra "you should ALWAYS fill the roof completely." I think it is optimal if the photovoltaic system is slightly undersized, as this leads to the most energy being used for self-consumption.

Well .. the question of what you yourself consume also depends on your further plans.

What about an electric car and a wallbox? Maybe not today, but in the next 5 years? Heat pump and with increasing occupancy also an increase in the winter yield of the photovoltaic system and thus better support for the heat pump.

Have I forgotten something?

With the revenue from feeding 3,600 kWh into the grid = €360 per year

There is surely a heat pump? At 10kWp, it would almost (almost) consume the entire yield from Nov-Feb on its own. This would mean that 15% self-consumption has already occurred during that period. If an additional 10% comes in the remaining months, you should break even with the system in 10 years.

To receive the KfW funding, the developer has sized the photovoltaic system at 6KWp. However, according to the electrician, up to 10 KWp would fit, and I am offered to expand the photovoltaic system at an additional cost.

But I have done the following calculation, and it makes little sense to me:

- The additional cost would be: 4KWp * 1000€/KWp = 4000€
- 4KWp would produce 4000KWh/year. Of this, self-consumption, based on values I read in this forum: 4000KWh * 20% = 800KWh.
- 800KWh * 0.3€/KWh (according to Check24) = 240€ / year

That would mean I amortize this in 4000 / 240 = 16 years.

And these were all very optimistic numbers. Did I forget something? I actually don’t find it a very sensible investment. That’s why I wonder why there is this mantra "you must ALWAYS maximize the roof". I think it is optimal if the photovoltaic system is slightly undersized, as this way most energy goes to self-consumption.

Basically,

a) Usually, you don’t calculate with 20% but with 30%. 35% is rather the maximum, and you really have to actively use the photovoltaic system for that. b) Otherwise, you forgot the annual electricity price increase. Just take the 10 or 20 year average. c) The yield is missing.

With fair photovoltaic offers, you get an amortization of the photovoltaic system in about 10-12 years, if nothing breaks. After that, you still have the same amount of time during which the system generates electricity for you for free.

Someone is really trying to line their pockets. 4kW corresponds to roughly 9-10 additional modules. Slightly larger inverter and more mounting material. I think €1000 extra cost for material. Racking, electrician, etc. are already there and should cost little to nothing extra. €3000 margin is a decent extra income for the solar installer.