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

Zaba123 schrieb:

a) Usually, calculations are based on 30%, not 20%. However, 35% is more like the maximum, which requires actively using the photovoltaic system.

30% on average for a system. The first 2 kWp certainly around 80%, the last 2 kWp more like 10%.

With fair photovoltaic offers, the system pays for itself in about 10-12 years if nothing breaks. After that, you essentially have the same amount of time where the system generates electricity for free.

Even when you have received back what you initially invested, nothing is truly amortized yet. For that, you also need to factor in a discount rate. The phrase “if nothing breaks” says the rest.

To the original poster: I would decide this based on gut feeling; this is not an investment decision but a consumer choice. Considering effort and construction risks, an ETF is always the better option. But 4,000 euros is more like pocket money—you won’t ruin your retirement savings with it, or perhaps the exact opposite.

Feed-in tariff and self-consumption combined should amount to around 500€/year.

This results in a payback period of just 8 years!

Which system reliably achieves such a return?
An ETF can only be meant as a joke here.

Teryamy schrieb:

Just recovering what you initially invested does not mean you have broken even yet. You also need to consider the interest rate. The statement “if nothing breaks” covers the rest.

That may be theoretically correct, but you also don’t factor in that over 20 years, without photovoltaics, you lose, for example, x € monthly due to higher electricity grid consumption—amounts that could otherwise be invested in ETFs with a 7% return. For example, I would have to buy an additional 5000 kWh per year from the supplier without photovoltaics plus storage, since my average consumption over 5 years is 6800 kWh.

However, for me, it’s not about pros and cons or squeezing out the last penny. I simply enjoy my photovoltaics as well as e-mobility.

So, what does amortization actually mean in this context? The electricity consumption from the utility provider doesn’t amortize either.

My experience with only a small balcony solar system is that you start paying much closer attention to your own habits. Currently, not only do I not draw electricity from the utility company for the power I generate and don’t feed back into the grid, but I have also reviewed and adjusted my overall electricity use and habits accordingly. As a result, besides the 460 kWh of self-consumed electricity, I avoid using more than an additional 100 kWh altogether. It’s a small amount, since I live in a rental apartment, but still helpful overall. Why less consumption? Because I’ve been dealing with the various eco-modes of my devices and nowadays accept that some devices run longer in these modes, etc.

In 2004, my first year working from home, I consumed around 4,000 kWh.
Then I gradually reduced it to about 2,100 to 2,200 kWh.
For the billing period ending April 30, 2023, it was down to about 1,760 kWh. At that time, the balcony solar system had been running for 6 months.
For the current billing period, I just reported around 1,470 kWh.

I have not changed my end-user appliances themselves.

kbt09 schrieb:

And what exactly does amortization mean in this context? The electricity consumption from the grid operator doesn’t amortize either.

That’s nonsense. Of course, the amortization of photovoltaic systems can be calculated. It’s not about the electricity itself in general, but whether I need to buy it or generate it myself. And also about the feed-in tariff.

In this case, the result is around 10 years. Depending on whether you consider interest on borrowed capital or rising electricity prices or not. This is a very good return, especially given the high level of security.

kbt09 schrieb:

In my experience with just a small balcony solar unit, you also engage more intensively with your own habits.

Yes, the effect you described often occurs. But it has nothing to do with the question here. The question was whether to install 6 or 10 kWp. That has little impact on consumption. Sometimes even the opposite, because with 10 kWp you might actually become less economical.

But let’s just stick to the numbers above, instead of feelings and behaviors—that way we are on the safe side.

Over two years ago, we installed our system on the roof: 10 kWp. We didn’t maximize it but sized it according to our consumption (6,500 kWh, heat pump, lots of home office, 4 people). For the past six months, there’s been a Tesla parked outside, which is occasionally charged via a standard socket due to the lack of a wall box.

Conclusion: We should have fully utilized the roof space! We will do that eventually. In summer, we are almost constantly 100% self-sufficient. What an amazing feeling! Even when the electric vehicle socket is connected, we still feed energy back into the grid. However, in winter, the situation reverses drastically, and we generally only manage to cover our normal household needs (cooking and the heat pump’s additional heating requirements are the main consumers). We have nearly halved our electricity demand. That means only half the electricity costs, plus about €1,000 in feed-in compensation over two years.

So, fill your roof completely. 6 kWp is nothing.