ᐅ Photovoltaic System for 120 sqm of Living Space – Should the Entire Roof Be Covered?

konibar schrieb:

Exactly,
I get different results:

Premises:
Roof pitch 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° maximum solar incidence angle,
meaning the north panels do not see the sun from about November to February (when electricity demand is highest) at all.
They only receive some diffuse radiation, typically less than 5% of nominal power.

Therefore, north-facing panels at a 35° roof pitch make no sense in Germany.

Or with a mounting frame?

PS: Nominal power is defined at a 90° incidence angle,
at approx. 70° solar elevation, and
1 kW radiation power/m²
(if I recall correctly)

This cannot be generalized so easily. It depends on which panels you want to use. If I trust the solar experts (and we have spoken with several), the modern panels from the last 2–4 years have made tremendous progress. You no longer need direct sunlight to generate acceptable power output. Of course, a south or west orientation produces more. However, if you have enough space and no shading, north-facing installation also works fine. (We have a very large roof where the north side receives sufficient light at 35°.) With northeast orientation, you naturally have to accept some losses.

For our system, it’s not necessarily about achieving the maximum possible yield all the time but about covering our demand for as long as possible throughout the day.

In our case, this means having a battery for the heat pump and a correspondingly large system, so that the adequately sized battery can approach full charge even in winter.

To give you an idea, I have attached a photo of our house (taken last May; judging by the sun position, it must have been around 10 a.m.). The west side tree shadows in winter will be interesting, but they will be calculated accordingly.

Evolith schrieb:

You no longer need direct sunlight to generate an acceptable amount of electricity. Of course, a south/west orientation produces more. But if you have enough space and no shading, the north side works fine as well. (We have a very large roof, so the north side receives enough light at a 35° angle)

That would be nice, but I wouldn’t bet on it. Of course, if you don’t mind the financial aspect, you can still do it—it won’t cause any harm.

Evolith schrieb:

For our system, it’s not about maximizing output all the time, but rather covering our demand throughout the day for as long as possible.

Well, in winter the sun only moves through the south and stays low in the sky. Given my earlier skepticism, having panels on the north side won’t really help much in winter.

Evolith schrieb:

In our case, that means a battery for the heat pump and a sufficiently large system so that the appropriately sized battery can approach full charge even in winter.

Note: it is important to have a sufficiently large battery inverter for this. Because if the sun comes out for just two hours in winter, you want to charge the battery from the photovoltaic system with as much power as possible, without losses.

KingJulien schrieb:

Are you sure? That would be unfortunate; then nothing would be correct.

I played around with the settings again on PVGIS.
Although you cannot set the panel tilt to a negative value (north-facing), you can rotate it around the vertical axis. That effectively leads to the same result:
Here are the values for a 35° panel tilt for south and north orientations:

The results are roughly as expected:
For a north-facing orientation, you only receive diffuse light (around November to February).
The energy yield then drops to about 8% of the summer value, exactly when electricity demand is highest. That is only about one quarter relative to south-facing orientation.

The proportion of diffuse light is difficult to predict, as it strongly depends on the panel design and environmental conditions.

konibar schrieb:
Although you can’t set the panel tilt to a negative value (north-facing), you can rotate it around the vertical axis. This amounts to the same result:

In other words, PVGIS works well and is user-friendly when you fill in the correct fields.

There is also a step-by-step guide for "ersuchmaschinen," even available in German.