ᐅ Photovoltaic system for water heating and grid feed-in

Hello everyone,

What do you think about the idea of using a photovoltaic system for domestic hot water heating, with self-consumption and feeding any surplus electricity into the grid — topic: Refusol photovoltaic heater?

Is it possible to achieve higher profitability compared to a solar thermal system? Has anyone built something like this?

And would this count as renewable energy? The current plan is a 365mm (14.4 inches) monolithic wall construction with Poroton T9 bricks and a gas condensing boiler, without mechanical ventilation with heat recovery.

Thanks for your feedback
Micha

I would definitely go for a heat pump water heater for the hot water supply (there are models available for around 700 euros with an 80L (21 gallons) tank from Ariston) instead of a tankless water heater. I think it's definitely better than solar thermal systems, since in summer the water overheats unnecessarily, and in winter the water doesn’t get warm enough. Feeding excess energy back into the grid doesn’t make much sense because the feed-in tariff is much lower than the market electricity price. It’s better to use the electricity yourself wherever possible (again, consistently—for example, we still have a split air conditioning system for heating and cooling, as well as an electric vehicle). That way, you still have enough surplus energy to feed back into the grid, but you maximize self-consumption first.

Hello Elina,

thank you for your opinion.
I had a solar installer visit yesterday. He confirmed that solar thermal systems are generally not economically viable. He also advised against my idea because, for example, heating water with gas is just too cheap. If he were building new, he said he would only use tankless water heaters, which also save money on installation.

Otherwise, he suggested covering the entire south side of the pitched roof with photovoltaic panels. In our case, that would be 42 modules, providing around 10 kW. Assuming a 30% self-consumption rate and selling the rest, this would generate income of about €800 plus savings from self-consumption of around €800 annually. A system of this size, including installation and an SMA inverter, would cost about €15,000 net. So the system would pay for itself after about 10 years, assuming the price per kWh stays the same; if it rises, the payback would be even sooner. You would also be prepared for battery operation in the future.

Now, 1-2 questions arise.

What do you think about an internal roof photovoltaic system? Are there any factors to consider compared to a rooftop system?
What do you think of the idea in general? Would this meet the requirements of the Renewable Energy Heat Act, provided I don’t install a domestic hot water heat pump? I understand from R.B. that the extra costs for a domestic hot water heat pump do not pay off.

Oh, and assuming I go ahead with installing such a system, it might also be worth considering installing an air-source heat pump. The question remains how well it would be supported by the photovoltaic system during winter months, especially if the electric heating element kicks in...

Thanks for your help.

bortel schrieb:

Otherwise, he suggests covering the entire south side of the pitched roof with photovoltaic panels. In our case, that would be 42 modules, producing about 10 kW. Assuming 30% self-consumption and selling the rest, this would generate around €800 in sales revenue plus about €800 saved annually from the self-consumed electricity. The system of this size, including installation and an SMA inverter, would cost €15,000 net. This means it would pay for itself in about 10 years, assuming the price per kWh stays the same—if it rises, the payback would be even sooner. You would also be prepared for battery operation at some point.

Now, a couple of questions come up.

What do you think about an integrated roof photovoltaic system? Are there any special considerations compared to a standard rooftop installation?

Just from memory:
10 kWp — as far as I know, if you exceed this system size, there is a cap on feed-in tariffs (payment for selling electricity). So make sure you stay below that limit.
I found 30% self-consumption quite ambitious. We have a 5.5 kWp system on our roof, and our self-consumption is about 20-22%. We make sure to run power-hungry appliances like the washing machine, dryer, and dishwasher when the system is producing electricity, especially these days. Often, when I come home in the evening, our total consumption is 0-1 kWh (I always note down the meter readings in the morning for the heat pump and household electricity, and in the evening for production and feed-in). The main consumption happens during the times when there’s no production (at night).
In winter, production can drop close to zero in the worst case. Last winter, we fed in less than 40 kWh for an entire month.

Also, consider — 30% of 10 kWp is about 3,000 kWh. What is your annual electricity consumption?
Ours has never been that high (we have two young children in the house), but we might not be the best example here. Friends often don’t understand how we calculate our consumption figures.
But assuming you have 5,000 kWh annual usage, covering 60% with photovoltaics is quite an ambitious target.

You can optimize your self-consumption by using batteries or, for example, an electric car like @Elina does (but the vehicle has to be parked at home to charge). Some battery manufacturers are currently entering the market and could potentially revolutionize the currently high price levels.

Integrated roof installation:
There were several reasons we chose rooftop (surface-mounted) installation instead of integrated.
- Panels generate a lot of heat when exposed to sunlight. We didn’t want that heat inside the “built-in space” (even though the roof isn’t insulated).
- If a panel gets damaged, you might have an open roof for some time until it’s replaced.
- A note from the solar installers: it’s possible that the standard size of solar panels available now won’t exist in a few years. Then you might have to fill a “gap” with different-sized panels.

The advantage of integrated installation is that it can partially replace the roof covering.

I’m just an enthusiast who has read a lot and spoken with solar experts as well as staff from the Energy Agency of Rhineland-Palatinate. There are certainly other factors (positive and negative) that might also come into play here.

In-roof systems are significantly more expensive and are somewhat less efficient (due to heat) – that’s why we decided against them. However, the solar installer should be able to calculate this in detail...

But why are 30% of a 10 kW system 3,000 kWh?

Did I make a typo? I actually meant 3 kWh.