ᐅ A photovoltaic system that pays for itself and breaks even on a monthly basis

I exported the daily values (production, consumption from the grid, self-consumption, feed-in) from SE. This results in quite a long Excel sheet. I then ran a simplified query over it: "How much grid consumption could I cover per day with a storage capacity of 6 kWh instead of feeding it into the grid, for days x (1...365)?" In this calculation, it is generally assumed that the available capacity is around 90%, and the charging/storage losses relative to the feed-in amount are about 20% (realistic according to various storage studies; with charging forecasts and other optimization methods, you might achieve around 10%, but that would be quite optimistic). Even if the losses were zero, it would only slightly affect the calculation.

Well, simplified, it looks something like this, for example:
Very good day: July 27: 54 kWh production; 47.2 kWh feed-in; 12.2 kWh consumption; 6.8 kWh direct use; 4.8 kWh potential battery usage
Unfavorable day: October 5: 3.5 kWh production; 0.6 kWh feed-in; 15.4 kWh consumption; 2.9 kWh direct use; 0.2 kWh potential battery usage

Summed up, this shows how much I can store in a year—in my case, about 1000 kWh. On most days, this is around 1.5 to 3 kWh. The 200 kWh I subtract here represent losses from storage, charging, grid consumption, etc.

What is not taken into account, which makes things even less favorable for the battery:
- Storage charging also occurs based on forecasts in the morning and around midday, but there are many days when the sun shines more in the afternoon or when direct consumption happens mainly in the morning
- The battery inevitably draws power from the grid if there isn’t continuous bright sunshine all day

That’s why the 20% losses figure roughly covers these factors.

Today I also did a price comparison with a total of 8 offers, including 6 local suppliers and 2 from the internet. Converted to 9.99 kWp, the net price ranges from €1,120/kWp to €1,462/kWp, all without storage and including installation.

I don’t understand why the Greens haven’t come up with any sensible proposals, such as incentives for photovoltaic systems in new buildings. These left-wing populists only seem to think of bans and exploitative measures. There appears to be no interest in solid support or practical solutions.

Tassimat schrieb:

The lifespan of a system is often stated as 20-25 years. Is that really accurate? Do individual cells or modules fail, or is a complete replacement necessary later on?

Inverters have a shorter lifespan, so you should roughly expect to replace them once within 20 years. This is usually factored into the financial calculations.

As for the modules themselves... they have a significantly longer durability, and there is now plenty of experience to back this up. Almost all solid modules will still perform at over 90% capacity after 20 years, meaning a replacement after 20 years would generally be economically unreasonable (assuming there are no feed-in tariffs anymore). The modules will certainly continue to operate for several more years or even decades.

Lumpi_LE schrieb:

I exported the daily data (production, consumption from the grid, self-consumption, feed-in) from SE. This results in quite a long Excel sheet. Here, I ran a simplified query over it: "How much grid consumption could I have covered with a storage capacity of 6 kWh per day x (1...365) instead of feeding it into the grid." It is generally assumed beforehand that the available capacity is around 90%, and the charging/storage losses in relation to the feed-in (!) are about 20% (realistic according to various storage studies; with forecasting and other improvements, you might achieve 10%, but that’s already very optimistic). Even if these losses were zero, the calculation would be affected only slightly.

Anyway, simplified, it looks like this, for example:
Very good day: July 27: 54 kWh yield; 47.2 kWh feed-in; 12.2 kWh consumption; 6.8 kWh direct consumption; 4.8 kWh possible battery consumption
Unfavorable day: October 5: 3.5 kWh yield; 0.6 kWh feed-in; 15.4 kWh consumption; 2.9 kWh direct consumption; 0.2 kWh possible battery consumption
When added up, it shows how much I can store in a year, which is 1000 kWh for me. On most days, that’s around 1.5–3 kWh. The 200 kWh I subtract are losses from storage charging, grid consumption, etc.

Not considered are factors that make battery use less favorable:
- Storage charging also occurs during forecast periods in the morning and around noon, but there are many days when the sun shines more in the afternoon or when there is mostly direct consumption in the morning.
- The battery inevitably draws power from the grid if it’s not bright blue sky all day long.

But that’s why the 80% losses roughly cover these aspects.

Thanks, I got it. €8,000 for a 6 kW battery is obviously steep! That certainly disrupts any calculation together with the other factors.

I searched again for a technical article (bought or not ) on this. The system performance of the BYD battery is said to be 89.2% after all losses. That would mean a theoretically usable 5.71 kWh per day (on the good days).

Yep, and you first need to have those days. I just counted and there were only 18 days when I could have drawn more than 4 kWh from a battery... For me, a 3 kWh battery would probably be enough, but the cost is hardly less, so it’s definitely not worth it.
A DIY solution could be an option; there are guides for a 2 kWh battery for €2000, but even that isn’t really worthwhile.