And remember, that current solar panels will generate about the same energy during their life as used initially to produce it.
Ignorant rubbish. Solar cells have a 25 year guaranteed life to 80% output. Thats a hell of lot of energy
Next year Poland will introduce a Feed In Tariff (FIT). This will pay you when you feed back excess electricity, the rate isn't determined yet but last time I checked its was 80% of last years average price per KWHr
You can get an idea how much the cost will be with bulk panels costing around 0.8-1.0 USD per watt for instance. On top of this you will need a a Grid-tie inverter (or inverter and off-grid batteries or combination). Batteries are expensive and wasteful.
To qualify for a Feed in Tariff you will probably need a professional installation by a qualified provided. For this you will HAVE to get a quote from a Polish installer. In the UK, the prices are high, a rip-off to be sure.
The amount of electricity produced will depend on a lot of things, not least the location. This gives a ball park figure oksolar.com/abctech/solar-radiation.htm In Poland average per day over 365 days is 2-3hours.
Optimally placed (point south) and angled (30degrees?) means a 1kw solar installation would produce 1Kwhr * 2.5 hours * 365 days = 912.5KWHr per year. In Southern Poland the figure is more like 1200-1400 per year.
So if that installation cost 3K pln for cells, 2k pln for inverters and 3 k pln for installation= 8k PLN, you can calculate how much return you get by dividing the FIT payment of 912.5 KWHr by 8K PLN.
A 2.5KW or 4KW is more probable than 1KW.