I have had quite a bill from the local energy supplier for many years. Admittedly due to some extensive usage of electrical equipment such as computers in various forms. The AC is also quite power hungry but the days when its service is needed is limited to around 25 days (I did not count, but could serve as a ballpark figure in Canton Zurich…) a typical year.
Pay and forget would kind of solve the problem I guess but with the increased solar panel efficiency this is an option for us who has some roof/balcony or similar to host some panels.
I obviously did a bit of reading and feeding back to the grid would obviously be quite interesting to reduce the electricity bill but it has two drawbacks. More than 600 W requires some bureaucracy which I would be happy to live without and it does not give the same sense of independence/backup compared to the island approach since it requires the grid to be online (which it typically does but nevertheless).
So I settled for the hybrid island approach meaning the panels charge batteries via an intelligent hybrid inverter which also serves as the AC power source for the devices that should be at least partially solar powered. If the panels don’t deliver enough of energy to maintain a certain battery capacity, the batteries are charged via the AC grid.
Regarding my actual solution… I currently have 4 monocrystalline panels (Penta+ ASM6610M-series) with a theoretical capacity of 305 W each, in series-parallel (meaning two pairs of serially connected panels which are connected in parallel). (If you slept during those lessons in school and don’t work in that area, let’s say serial increases the voltage and parallel the current.) These panels are located on our winter garden roof and the hybrid converter is in a room in the cellar. Since the hybrid converter is kind of loud, imagine a medium noisy vacuum cleaner, it should really be in a room where you don’t need to conduct a lot of serious business on a regular basis.
My hybrid converter is a very common 24 V model from Voltronic Power (ODM, sold under a number of brands) rated for 2.4 kW which offers a USB interface (I use a simple client from https://github.com/nrm21/skymax to poll the device for values). It should be possible also to set values (e.g. change from PV charging to bypass mode) but I currently only use it to get values and expose them on a website for my personal pleasure. (I did play with a container feeding a MQTT message broker to expose the current state in a dashboard but my raw output in a table with some homegrown graphs turned out even more useful at the moment. I might revisit that later.)
Battery wise I was choosing among AGM, Gel or Lithium. I settled for the middle road, 4 times 12 V 140 Ah Gel batteries connected in a matching series-parallel connection, which are supposed to be good for 5-7 years. Hopefully we have cheaper Lithium batteries or some other great option when it is time to replace them.
To help the batteries to keep healthy I am using an equalizer to balance the battery load depending on its current state and a battery pulser desulfator to extend the lifetime.
On top of this I have a DC switch (40 A) for the panels, a DC fuse for the batteries (100 A), 16 A AC switch and a Residual current circuit breaker (30 mA) in case some connected AC device (or human) misbehaves.
In theory this system should be able to deliver up to 6 kWh per day but what I have seen during the short time I have had it connected is more like 3 kWh per day. Since a kilowatt hour costs 15.8 rappen on average (considering the “hoch- and niedertarif” hours and rates) I would not consider my setup very cost effective. In about 26 years we should have the costs back (assuming my time is free…) but the batteries won’t last that long.
Financially I think we will have to let the “backup feature” bear some of the costs, redundancy and safety comes at a cost, but I have to admit that the driver for this project was not money but rather the fun of producing ones own electricity…