If your goal is purely hot water rather than running the house, you need far fewer panels than a full system — but the approach matters. A handful of PV panels feeding a diverter into your immersion can cover most of your hot water in the bright half of the year. As a rough benchmark, it takes around four PV panels to produce as much hot water heating as a single dedicated solar thermal panel, so a small PV array plus a diverter is a sensible hot-water-focused setup.
To size it, work out how much hot water you actually use. A common planning figure is around 45 litres of hot water per person per day, heated to about 60°C (to guard against Legionella) and mixed down. That tells you the energy your tank needs, which in turn tells you whether a few panels plus surplus will cover it or whether you'll be topping up from another source. Households with low hot-water demand can get a lot of their needs from a modest array in summer.
Be realistic about winter, though: in the darkest months there's little surplus to divert, so a hot-water-only solar setup will still need a backup — typically an immersion timer running on a cheap night rate, or your existing gas/oil boiler. If hot water is genuinely all you want and you have the budget, dedicated solar thermal is more efficient per panel; if you want flexibility to use the electricity elsewhere too, a small PV array with a diverter is the more versatile choice.