My electricity cost me £12,500. I haven't taken any from the grid since the beginning of April, but still have to pay 53.96p per day, partly off-set by the electricity I sell back to them at 5.5p per unit (rip off) - about 10 units a day average. In the depths of winter (4 months), there isn't enough sunshine to generate anything meaningful, so I still pay the 53.96p per day plus around £1 a day for electricity used above what's generated.
That means total cost per year is about £197 per year standing charge plus £120 electricity used minus £100 for electricity sold back, say net £200 compared with total cost of £1000 per year before. The saving is therefore £800 per year.
That's a 12 panel 5kw system with roughly 10kwh batteries and 5kw inverter/charger. If I did it again, I'd go for a 7kw inverter because the panels make more than 5kw, but the inverter limits it. The LiFePO4 batteries are 52v so could be used on an ebike project if I decide to upgrade.
So there's some real word data if anyone is considering doing the same. My roof is the perfect south facing, but it's quite shallow; however, the panels are exceptionally good at capturing the light and they're quite efficient (22%). Make sure that you get decent panels.
To make your own calculation more simple, I can tell you that I've generated 2448 units since the end of January, so it's going to be roughly 5000 units per year. That's now worth about £1500 if you use it all. I'm an extremely light user at around 2800 units per year because I live on my own. A family or high use couple would need bigger batteries - maybe 12kwh.