ᐅ Photovoltaic System for 120 sqm of Living Space – Should the Entire Roof Be Covered?
Created on: 25 Oct 2021 23:00
K
Kalibri
Hello everyone,
I am currently looking into expanding our planned photovoltaic system.
The standard total capacity is 3.75 kWp. I have been considering upgrading it to 5 kWp.
Now, browsing through the forum, I see many examples with 10 kWp and more.
Of course, it makes sense to cover the roof as much as possible, but the price is an important factor for us.
There are two of us living in a 120 sqm (1292 sq ft) home.
What are your experiences with achieving the best possible balance between cost and performance?
I am currently looking into expanding our planned photovoltaic system.
The standard total capacity is 3.75 kWp. I have been considering upgrading it to 5 kWp.
Now, browsing through the forum, I see many examples with 10 kWp and more.
Of course, it makes sense to cover the roof as much as possible, but the price is an important factor for us.
There are two of us living in a 120 sqm (1292 sq ft) home.
What are your experiences with achieving the best possible balance between cost and performance?
KingJulien schrieb:
This is the calculation from two independent solar installers. One would personally recommend orienting panels only to the south.
If someone else does the calculation, they will come up with a different result.
It’s not about the absolute numbers, but about their ratio.
The outcome remains the same. If I cover my entire roof, the payback period is only slightly longer.
PS: My calculation shows a longer payback period. But the main point stays the same.
PPS: I do include ongoing costs in my calculations, naturally. Okay. In my rough calculations, it always results in a loss for me, with or without subsidies, so I’m trying to understand why it pays off for others.
H
hampshire26 Oct 2021 15:42Rumbi441 schrieb:
I always end up with a loss in my rough calculations. That is so unusual that I am curious about your calculation method. How do you calculate it? What assumptions do you make?
KingJulien26 Oct 2021 15:47With a conservative calculation on my part, it’s still a tight situation. There are better investment options out there. But for me, it’s enough that I won’t lose money within 20 years. Everything beyond that is a bonus.
However, future electricity prices and self-sufficiency can certainly influence the calculation in the desired direction. In the end, it’s only a rough estimate for each individual.
I also hope to achieve a decent level of self-sufficiency through heat pump optimization and an electric car.
You can also consider a time frame of 25 or 30 years. Or assume that direct marketing will soon become more profitable than feed-in tariffs. If you want to 😉
PS: Have you already tried the calculator from test?
PPS: The biggest factor for profitability is the initial investment cost!
However, future electricity prices and self-sufficiency can certainly influence the calculation in the desired direction. In the end, it’s only a rough estimate for each individual.
I also hope to achieve a decent level of self-sufficiency through heat pump optimization and an electric car.
You can also consider a time frame of 25 or 30 years. Or assume that direct marketing will soon become more profitable than feed-in tariffs. If you want to 😉
PS: Have you already tried the calculator from test?
PPS: The biggest factor for profitability is the initial investment cost!
KingJulien schrieb:
This is the quote from two independent solar installers. One would personally only install south-facing panels.
Let someone else do the calculations and they come up with something different.Exactly,
I get a different result:
Assumptions:
Roof pitch 35° (degrees), panels mounted flat. (Ecliptic angle 23° (degrees))
For example, in Frankfurt (latitude approximately 50°N), the south-facing panels would see a maximum solar incidence angle of 52° (degrees) at midday in winter:
(90° - latitude) + roof pitch - ecliptic angle.
For the north-facing panels it would be:
(90° - latitude) - roof pitch - ecliptic angle = -18° (degrees) maximum solar incidence angle,
meaning the north-facing panels do not receive any direct sunlight from about November to February (when electricity demand is highest).
They only receive some diffuse radiation, typically less than 5% of nominal output.
Therefore, north-facing panels on a 35° roof pitch do not make sense in Germany.
Or with a mounting rack?
ps.: Nominal output is defined at a 90° solar incidence angle,
at approximately 70° solar altitude and
1 kW irradiation per square meter
(if I recall correctly).
KingJulien26 Oct 2021 18:44@konibar I’m not sure if you meant me. But since you quoted me, I’ll respond.
konibar schrieb:I never claimed otherwise.
That’s why north-facing panels don’t make sense at a 35° roof pitch in Germany.
konibar schrieb:Nice calculation if it has to be complicated. Or just use PVGIS for a quick estimate. Anyone can do that in 10 minutes.
Assumptions:
Roof pitch (DN) 35°, panels mounted flat. (Ecliptic angle 23°)
Then, for example in Frankfurt (latitude approx. 50°N), the south-facing panels would see a maximum solar incidence angle of 52° at midday in winter:
(90° - latitude) + roof pitch - ecliptic angle.
For the north-facing panels, it would be
(90° - latitude) - roof pitch - ecliptic angle = -18° max incidence angle,
meaning the north-facing panels wouldn’t see the sun at all from about November to February (when electricity demand is highest).
They only receive some diffuse radiation, typically less than 5% of nominal power.
KingJulien schrieb:
Nice calculation, if it needs to be complicated. Or just use PVGIS for an initial estimate. Well, it’s actually a fairly simple equation, but:
PVGIS doesn’t help.
I tried it but found that the tilt of the north side (relative to the direction of irradiation!!!) would have to be entered as a negative value. Unfortunately, PVGIS doesn’t accept that.
If you don’t take this into account, it inevitably leads to incorrect results.