KFW70 photovoltaic instead of solar thermal, how much collector area?

Hello,

I am currently planning a bungalow for my parents, or rather it has already been planned to meet Kfw70 requirements. A gas condensing boiler with hot water heating through solar thermal energy with 2 collectors totaling 5.5 sqm is planned there. The house has a living area of 110 sqm.
Now I just installed a photovoltaic system on my own house and was thinking about doing the same for my parents. My photovoltaic system installer then asked me why I would still install solar thermal energy at all. He suggested I rather use the entire roof area for the photovoltaic system. Because self-generated and self-consumed electricity is credited according to the Energy Saving Ordinance 2009. Of course, only the self-consumption share of the generated electricity.
Now the question arises for me: how much collector area do I need at minimum to compensate for the missing solar thermal energy with an annual electricity consumption of 2800 kWh? Of which, of course, not everything is self-generated. According to paragraph 5 of the Energy Saving Ordinance, the calculation is apparently quite complicated with monthly evaluations etc.
Has anyone of you already had experience with this? At least an 8 kWp system would fit on the roof.
I would really like to implement this because, first, I would like to save the 300 L hot water tank in the utility room, and second, I am not really convinced by the "cost-benefit factor" of solar thermal energy.

Of course, I will ask this question to the photovoltaic system installer + structural engineer/energy certificate calculator, but before making a big fuss, I thought I might get some preliminary information or experiences.

Nico

Hello,

... My photovoltaic system installer then asked me why I still install solar thermal at all. He suggested I should rather use the entire roof area for the photovoltaic system. Because self-generated and consumed electricity is also taken into account according to the Energy Saving Ordinance 2009.

He is not wrong in this. To keep the self-consumption rate as high as possible, photovoltaic systems are usually combined with a heat pump.

... Not everything is self-generated, after all. According to paragraph 5 of the Energy Saving Ordinance, the calculation is apparently quite complicated with monthly evaluations, etc.

Only the consumption that takes place shortly after the "harvest" is credited. For this purpose, appropriate software is used, which the certifier surely has.

Best regards

Especially since photovoltaic systems have become significantly cheaper recently (our company sells them for agricultural buildings, etc.) compared to solar thermal modules, which have only become slightly cheaper. Moreover, more can be achieved with solar electricity in summer than with solar heat.

We ourselves have neither, but a neighbor on our street has photovoltaic panels on the roof and uses the solar electricity to heat domestic water as an alternative to the gas boiler. It apparently works wonderfully; he uses as much of the remaining electricity himself during the day (the washing machine and dishwasher are programmed for daytime use) as possible, and the rest is fed into the grid.

That was also our idea at one time, but no one could really determine the proper sizing of the photovoltaic system, and if the solar investment is going to be exceeded anyway due to sales enthusiasm, then it makes no sense, especially since I am not very keen on erecting an electromagnetic wall over my or my child's bed. At the neighbor’s bungalow, there is more distance than in a single-family house. But I'm also a bit of a burned child since I was in poor health when my office was next to the railway line (high voltage), and nobody knew the cause; after changing offices, it was fine again =)

How about you regarding wind energy for home use? Would that also be an option?

Hello,
He is not wrong here. To keep the self-consumption share as high as possible, photovoltaic is usually combined with a heat pump.
Only the consumption that takes place shortly after the "harvest" is credited. For this purpose, appropriate software is used, which the certifier certainly has.
Best regards.

Could you possibly explain this a bit more precisely? I understand from the paragraph that only the electricity used for heat generation / heat recovery / heating system / domestic hot water can be considered? That means if you have 2 meters in the basement – one for the heat pump (with heat pump tariff) and one for household electricity, and the photovoltaic electricity is connected to the household electricity meter (since the purchase price there is more expensive than at the heat pump meter), then the self-consumption of the electricity from the photovoltaic system cannot be credited?! Or am I overlooking something?

I understand from the paragraph that only the electricity used for heat generation / heat recovery / heating system / domestic hot water can be taken into account?

One should differentiate between proof (eligibility) and actual use. In the latter case, it certainly makes sense to align consumption, not only heating etc., but also "normal" household consumption (e.g., washing machine, dishwasher..) with the times of maximum output of a photovoltaic system. Typical heat pump tariffs (HT, NT, shutdown) are no longer always sensible today. The, if still existing at all, price advantage is often offset by unfavorable side effects. There are fairly strong regional differences here that do not allow for generalization. It is advantageous in any case to consume as much of your own "harvest" as possible. This can also be taken into account, for example, when sizing heating and hot water systems.

Not to forget. Heat pump tariffs are special tariffs and can be unilaterally terminated by the provider at any time! In many regions, price increases here were significantly higher than those for normal household electricity. In not a few cases, they have meanwhile reached the same order of magnitude as normal household electricity. This quickly raises the question of the usefulness of the additional meter. Remove it?

Best regards