ᐅ The Right Storage Solution for a Heat Pump with Photovoltaic System?

Hello everyone,
we are about to build our new single-family home and I would like to ask for your help...

The question is about the right storage system: whether to use a stratified storage tank with instantaneous water heating for domestic hot water, or a buffer tank/domestic hot water tank.
Also, the question about the correct sizing of the photovoltaic system.

Here are the framework conditions:
New build with 190 sqm (2050 sq ft) living area and a basement
KFW 55 (1 liter house)
Decentralized ventilation system (97% heat recovery)
Heating: Novelan LAD 9 HID 1
Combined with a photovoltaic system
No additional heat sources such as wood stove or solar thermal

My heating installer has offered me a 600 liter (160 gallons) stratified storage tank Novelan MFS 600 S.

Now I wonder if for summer operation a separate domestic hot water tank with buffer storage would be better? Since then only the domestic hot water tank would need to be heated. How large would you size these?

Would you still choose a heat pump tariff, or do you think it no longer makes sense?

Thank you very much. Christian

600 liters (158 gallons) is far too large. A underfloor heating system doesn’t require a storage tank; for domestic hot water, 80 liters (21 gallons) per person is sufficient for showering. For domestic hot water, you need 45-50°C (113-122°F), while underfloor heating only requires 30-35°C (86-95°F).
An oversized storage tank means unnecessary heating and excessive heat loss.
The photovoltaic system should be as large as possible. It will still hardly meet demand in winter, but will generate good income during the rest of the year.

until 6/16 Roadster 2.5 from 8/13
Model S P85+ Signature Red

A 600-liter (160-gallon) storage tank for the heat pump is totally unnecessary. The heating contractor should be advised to update their knowledge to the 21st century.

The size of the photovoltaic system depends on your budget, roof size/orientation/shading, and your goals (return on investment, environmental concerns, or other motivations). If self-consumption matters, which is usually the case nowadays, then your own electricity usage including the heat pump also plays a role. In general, the larger the system, the lower the cost per kWp. Practical sizes are just under 7 kWp or around 10 kWp.

You need to calculate the heat pump tariff yourself, as it varies individually depending on the provider. Based on forum discussions, it often does not make financial sense.

Alex85 schrieb:
600l storage tank for the heat pump,

In a household of four people with extensive showering by all four within one hour... In this case, a hot water storage tank might actually make sense.

toxicmolotow schrieb:
For a household of 4 people with extensive showering by all 4 within 1 hour... then it might actually make sense as a hot water storage tank.

Extensive... how long would you have to shower at 38°C (100°F) to use up 600 liters (160 gallons) of hot water, typically at 45°C (113°F)? Although hot water is also being produced during that time, if necessary with an electric heating element.
On the other hand, the buffer tank lowers the annual performance factor because of the required increase of the supply temperature.

I'm not talking about the heating buffer tank. In that case, less is better than more.

Modern rain showers easily use 20 liters per minute (about 5 gallons), with two of those in the house and running for 30 minutes, you’ll need 1,200 liters (320 gallons) at 38°C (100°F).

I can’t calculate it exactly, but the tank will be empty because an 8kW heat pump can’t keep up with that demand. And the electric heating element...

You complain about a poor COP, but still want to use a tankless water heater. Go figure.