Biogas as a replacement for solar thermal energy in new construction

We are building a new house this year with approximately 170 m² of living space. Natural gas is planned as the energy source. According to the Renewable Energy Heat Act, at least 15% of renewable energy must be covered by solar thermal energy.

170 m² * 0.04 m² (solarfläche) / m² (living space) = 6.8 m² solar surface = at least 3 panels.

Now I have read that by using biogas (at least 30%) I can replace solar thermal energy.

Is that correct? Has anyone already gained experience with this?

How large should the hot water storage tank be for a 4-person household (2 adults, 2 children)?

Thank you.

You mean district heating from a biogas plant? Do you have district heating laid in front of the property?

Alternatively, heat pumps can also be installed.

or simply more insulation

Hello, thank you for the answers. By biogas I meant certified gas with at least 30% biogas, which also costs more (probably about 1/3 more). The law mentions something about cogeneration.

More insulation does not seem to be that easy, says the architect.

We have now decided on a gas condensing boiler and a 1000-liter energy buffer, fed by the boiler and the solar system (4 panels with a total of 10m2). Hot water is stored in a hot water buffer with a capacity of 160 liters.