ᐅ KfW70 Photovoltaics Instead of Solar Thermal: How Much Collector Area Is Needed?

Hello,

I am currently planning a bungalow for my parents, which is already designed to meet KfW 70 standards. The plan includes a gas condensing boiler with hot water heating, combined with solar thermal collectors—2 panels with a total area of 5.5 sqm (59 sq ft). The house has a living area of 110 sqm (1184 sq ft).

I recently installed a photovoltaic system on my own house and was considering doing the same for my parents. My PV system installer then asked me why I would still install solar thermal. He suggested using the entire roof area for photovoltaic panels instead, since self-generated and self-consumed electricity can also be credited according to the Energy Saving Ordinance 2009 (EnEV), of course only for the proportion of electricity that is self-used.

So now my question is, how much collector area do I need at minimum to compensate for the lost solar thermal if the annual electricity consumption is 2800 kWh, considering not all electricity can be self-generated? According to paragraph 5 of the Energy Saving Ordinance, the calculation seems quite complex, involving monthly evaluations, etc.

Has anyone here had experience with this? The roof could fit at least an 8 kWp system.

I would really like to implement this because, first, I would like to avoid having a 300 L (79 gallon) hot water storage tank in the utility room, and second, I am not entirely convinced by the cost-benefit ratio of solar thermal.

Of course, I will discuss this question with the photovoltaic installer, the structural engineer, and the energy certificate assessor, but before involving everyone, I thought I might gather some information or experiences here.

Nico

Hello,

Nico2xl schrieb:
... My photovoltaic system installer then asked me why I even install solar thermal systems. He suggested using the entire roof area for photovoltaics instead. Since self-generated and consumed electricity is also credited under the Energy Saving Ordinance 2009.

He is not wrong in this. To maximize self-consumption, photovoltaics are usually combined with a heat pump.

Nico2xl schrieb:
... Not everything is self-generated, of course. According to paragraph 5 of the Energy Saving Ordinance, the calculation is apparently quite complex, with monthly evaluations, etc.

Only consumption that occurs shortly after the "generation" is credited. For this, appropriate software is used, which the certifier will certainly have.

Best regards

Especially since photovoltaic systems have become much more affordable recently (our company supplies them for agricultural buildings, etc.) compared to solar thermal modules, which have only seen slight price decreases. Additionally, more can be achieved with solar electricity in summer than with solar heating.

We don’t have either system ourselves, but a neighbor in our street has photovoltaic panels on the roof and uses the solar electricity to heat domestic hot water as an alternative to the gas boiler. It apparently works very well. During the day, he uses as much of the electricity himself as possible (the washing machine and dishwasher are programmed for daytime) and feeds the surplus back into the grid.

That was our idea at one point, but no one could really determine the right size for the photovoltaic system. And if the solar setup is eventually outweighed by the desire to sell electricity, it doesn’t make much sense either. Besides, I’m not very keen on building an electromagnetic barrier over my or my child’s bed. At the neighbor’s bungalow, there is more distance from the panels than there would be in a single-family house. I’m a bit cautious since I had health problems when my office was next to a railway line with high-voltage lines, and no one knew the cause. After changing offices, I got better =)

What about you—have you considered small-scale wind energy for home use? Would that be an option as well?

€uro schrieb:
Hello,
He is not wrong here. To maximize self-consumption, photovoltaic systems are usually combined with a heat pump.
Only the consumption that occurs close in time to the "generation" is credited. For this purpose, appropriate software is used, which the certifier will certainly have.
Best regards.

Could you possibly explain this in more detail? From the paragraph, I understand that only the electricity used for heat generation / heat recovery / heating system / domestic hot water can be taken into account? So, if there are two meters in the basement – one for the heat pump (with a heat pump tariff) and one for household electricity, with the photovoltaic system connected to the household electricity meter (because the electricity purchase price is higher there than at the heat pump meter) – then the self-consumption of the photovoltaic electricity cannot be credited? Or am I missing something?

Wastl schrieb:
I understand from the paragraph that only the electricity used for heat generation / heat recovery / heating system / domestic hot water can be considered?

One should differentiate between proof of eligibility (accountability) and actual usage.
In the latter case, it makes quite a bit of sense to align consumption not only for heating but also for "normal" household use (e.g., washing machine, dishwasher) with the periods of maximum output from a photovoltaic system.
Typical heat pump tariffs (peak, off-peak, cut-off) are no longer always reasonable today. Any price advantage that still exists is often offset by unfavorable side effects.
There are considerable regional differences here, which do not allow for generalization.
In any case, it is beneficial to consume as much of one’s own “harvest” as possible.
This can also be taken into account when sizing heating and domestic hot water systems accordingly.

It should not be forgotten that heat pump tariffs are special tariffs and can be unilaterally canceled by the provider at any time! In many regions, price increases here have been significantly higher than for standard household electricity. In quite a few cases, prices have now reached the same levels as for normal household electricity. This quickly raises the question of whether the additional meter is still worthwhile. Remove it?

Best regards.