ᐅ Solar panel system for the entire roof or is it better to undersize it?

To qualify for the KfW subsidy, the developer has sized the photovoltaic system at 6 kWp. However, according to the electrician, it is possible to install up to 10 kWp, and I have been offered to expand the system for an additional cost.

But I have made the following calculation, and it doesn’t make much sense to me:

- The additional cost would be: 4 kWp * €1000/kWp = €4000
- 4 kWp would produce about 4000 kWh/year. Based on values I have read in this forum, the self-consumption would be: 4000 kWh * 20% = 800 kWh.
- 800 kWh * €0.30/kWh (according to Check24) = €240 per year

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

And these are all very optimistic figures. Have I missed anything? I actually don’t find this a very sensible investment. That’s why I wonder why the mantra is always “you should ALWAYS max out your roof.” I think it is most optimal if the photovoltaic system is slightly undersized, so that the most energy can be used for self-consumption.

Well, the question of your own energy consumption also depends on your future plans.

What about an electric car and a charging station? Maybe not today, but within the next 5 years? A heat pump and, with increased occupancy, likewise an increase in the winter output of the photovoltaic system, which in turn provides better support for the heat pump.

Alec_Trevelyan schrieb:

Did I forget something?

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

There is surely a heat pump, right? At 10 kWp, it would almost (or completely) cover its own energy consumption from November to February alone. This would already account for 15% self-consumption during that period. If an additional 10% is added in the other months, you should reach break-even with the system within 10 years.

Alec_Trevelyan schrieb:

To qualify for the KfW funding, the developer sized the photovoltaic system at 6 kWp (kilowatt peak). However, according to the electrician, up to 10 kWp would fit, and I was offered the option to expand the photovoltaic system for an additional cost.

But I made the following calculation, and it doesn’t make much sense to me:

- The extra cost would be: 4 kWp * €1000/kWp = €4000
- 4 kWp would produce 4000 kWh/year. From that, self-consumption—based on figures I’ve seen on this forum—is: 4000 kWh * 20% = 800 kWh.
- 800 kWh * €0.30/kWh (according to Check24) = €240/year

This means a payback time of 4000 / 240 = 16 years.

And these were all very optimistic numbers. Am I missing something? I don’t really find it a sensible investment. That’s why I wonder why there’s this mantra that “you should ALWAYS fill the roof completely.” I think it’s optimal if the photovoltaic system is slightly undersized, so that most of the energy produced is used for self-consumption.

In general,

a) Usually, you don’t calculate with 20% but rather 30%. 35% is more like the maximum, and that requires actively using the photovoltaic system.
b) You also forgot to consider the annual increase in electricity prices. Just use a 10 or 20-year average.
c) The yield is missing.

With fair photovoltaic offers, you can expect a payback period of about 10 to 12 years if nothing breaks. After that, you have roughly the same period during which the system generates free electricity for you.

Someone is really trying to make a big profit here. 4kW corresponds to roughly 9-10 additional modules. The inverter is slightly larger, and more mounting hardware is needed. I estimate around €1000 (about $1100) extra material costs. Scaffolding, electrician, etc., are already in place and should cost little to nothing extra.
A €3000 (about $3300) margin is quite a decent bonus for the solar installer.