Switching from gas to solar / photovoltaic with / without heat pump

Solar thermal energy and photovoltaics produce rather little in winter. Short days, low sun, often nasty gray weather. Simply not the conditions for energy from the sun. It looks different in summer, but the problem with solar thermal energy is the surplus. In summer, you could heat an entire swimming pool with it, but you only need a few liters for showering. The rest just sits uselessly in the pipes. You can at least feed photovoltaic surpluses into the grid and get a little something for it.

Thank you for the clarification. Then wouldn’t a storage system be sensible? If I understood correctly, I get electricity from the photovoltaic system and hot water from the heat pump (heat pump). The heat pump in turn is powered by my photovoltaic system. I feed surplus electricity into the grid (or store it). If for whatever reason my photovoltaic system does not produce electricity, I draw it from the normal power grid (fallback solution). Is that summarized correctly? What is the lifespan of a photovoltaic system and a heat pump? How large do they need to be defined? What acquisition and maintenance costs should I expect annually?

Thank you for clarifying. Wouldn't a storage system be useful then?

In theory yes, in practice it depends. Such a storage system costs money and you have charging losses. In addition, you also lose the 6 cent feed-in tariff. For it to be worthwhile, the costs of the storage system must be correspondingly low or the grid electricity correspondingly expensive. For an unsubsidized storage system, calculations start from 40-60 c/kWh. But it must also be said that the calculations are quite dependent on the assumptions made: charging losses, loss of storage capacity over time, corresponding lifespan of the storage system, basic consumption of your household, typical weather (pure sun or sun-cloud mix, snow in winter, Lake Constance fog). On the topic of sizing, costs and maintenance, surely others can tell you more than I can. We adopted our sizing from the general contractor’s standard without giving it too much thought, even though now I am considering possibly expanding the photovoltaic system. Basically, I would start with the heating demand, then size the heat pump and then the photovoltaic system and possibly storage.

What is the durability of a photovoltaic system and heat pump? How large do they need to be defined? What acquisition and maintenance costs should I expect annually?

A photovoltaic system should last 30 years; the inverter usually breaks once during this time. No maintenance is required for roof pitches over 15°. A heat pump that is not oversized should last 15 to 20 years. In principle, all parts can be replaced... just like a car, eventually it becomes uneconomical. The photovoltaic system is sized as large as all the roofs on the property combined. The exception is north-facing roofs steeper than 25°. Then photovoltaic systems (without storage) cost about €1300 per installed kilowatt (peak). If the market normalizes again, €1000. The heat pump is sized based on the heating load of the house (have it calculated!) without surcharges for anything. If you still have an emergency chimney or an air conditioning system, then rather a bit smaller. The unit itself costs roughly €1000 per kW of capacity, very approximately. How much then comes for installation and additional piping depends too much on the circumstances. Heat pump maintenance costs: As long as you have a warranty, there are probably prescribed maintenance fees of €100 to €200 per year. But basically, nothing much is done. After that, you can have it checked every two or three years. You can also keep the outdoor unit clean yourself.

Then the calculation with the 45 cents is correct. However, most batteries are purchased for around 1000,- €/kWh.
However, with the small photovoltaic system, you probably won’t get enough cycles to end up in the black. It should be about 200 full charge-discharge cycles if you want to recover the costs within 15 years.

I have almost a full year with 6.6 kWp and 7.7 kWh and am currently at 230 cycles. Occasionally, I could have also run the battery a bit emptier with the heat pump. But I prefer to use the heat pump only with photovoltaic surplus and battery for household electricity since I have a heat pump tariff that is 11 ct cheaper. With an electric car, you can drain a battery even better (if not charging at the photovoltaic system during the day). However, the V2H concept stands in the way of that.

Assuming a battery lifetime of 15 years, that would be about 3500 cycles (6000 are guaranteed). If I calculate only with 7 kWh (minimum SOC can still be minimally set at 5%), that would be 3500 * 7 = 24,500 kWh, minus about 6% losses (mainly occurring at DC->AC and less if you have a higher base load), I would be at 23,000 kWh or approx. 3000 kWh per kWh of capacity.

At a gross procurement price of 30 ct, currently 6.63 ct feed-in tariff (which I miss out on -> with 6% battery losses effectively more like 7.05 ct), no (!) price increase and small business regulation (KUR) after 6 years (so about 1/3 -> on average 1.6 ct VAT on self-consumption with net 25 ct), I save about 21.35 ct with each kWh used over the battery. Accordingly, 1 kWh battery should cost at most 640 € to be cost-neutral under these assumptions. Of course, there is still greater planning uncertainty here since the battery could also last 20 years or only 12.

But for every additional cent of price increase (gross), the battery saves me about 0.95 ct (with partial VAT on self-consumption, while larger price increases tend to fall into the KUR period), which would offset about €28 extra per cent. That means for a 10 ct higher electricity price, the battery could already cost 280 € more per kWh (920 €).

Do I still have major errors here?

Ordinary numbers.

How do you come up with the

approx. 6% losses

?

Did you determine that, or does the system indicate it? Usually, it is over 15%...

Decent numbers.

How do you come up with the

?

Did you determine that, or does the system specify it? Usually, it's over 15%...

AC? For me, it's a DC connection. I have known these ballpark figures used for years.

Since I avoid using the multimeter on the unscrewed inverter ;), I calculated it from the log data. There you have the currents and voltages as well as power going to and coming from the battery in 5-minute time resolution. That way, you can see how much ended up in the storage and how much was drawn out again. After almost a year, with over 1600 kWh from the battery, that’s also over 100 kWh loss.

And I also watched the live values updated every second at various times (you could also log that with Python) to understand how much power the inverter draws at what times. Even at a 160 W base load, the inverter uses around 25 W for the power electronics (that would already be your 15% during those night hours), so it helps the battery efficiency to set the threshold at which the battery becomes active higher, since this inverter base load at 160 or 350 W household consumption is relatively fixed. But this only makes sense for the overall system if it should not last through the night. I have already recommended this to Torsten from "Weissnichs Welt." I use it myself, for example on rainy days, to efficiently consume the battery's energy. If, for example, some photovoltaic power is available anyway, there is no additional inverter consumption for the battery but it’s already covered by photovoltaics.

You also have to be careful with the smart meter data in the log, because household consumption is reported several hundred watts too low at high photovoltaic power -> that corresponds exactly to the inverter conversion losses. But that can be easily corrected and leads to plausible constant consumption values throughout the day during vacation periods.