A common benchmark is a roughly 50% reduction in annual electricity costs from a well-sized system — but the honest detail behind that average is that the saving is hugely seasonal. In the bright months (roughly April to September) the savings are massive, often covering most or all of your daytime electricity. In the depths of winter the reduction is almost nothing, because generation is so poor in our short, dull days. The 50%-ish figure is what you get when you average those extremes over a full year.
How close you get to that depends on your usage pattern and self-consumption. The more of your generation you actually use (rather than export cheaply), the better the bill saving — which is why smaller, high-self-consumption systems can punch above their weight, and why load-shifting (running appliances, the immersion and EV charging when the sun's out or on cheap night rates) matters so much. Pairing solar with a battery, an EV on a night rate, and a hot water diverter or immersion timer all push the saving higher.
Set expectations realistically: solar will make a clear dent in your annual bill and start saving from day one, but it won't zero your winter bills and the payback is measured in years, not months. The honest framing is that the main justification for solar PV is that meaningful annual saving plus the lower carbon footprint — not a fantasy of free electricity year-round. Run your own usage data to estimate your specific saving rather than assuming the headline average applies exactly to you.