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Heating water takes 4.186 kJ per litre per degree, so 160 litres raised 35 degrees needs 6.5 kWh a day. A solar thermal system typically supplies 50-70% of that across a year — most of it in summer — which sets both the saving and the long payback.
Water heating energy
kWh = litres x 4.186 x temperature rise / 3600; saving = kWh x solar fraction x 365 x price
Winter irradiance is a fraction of summer and demand is highest then. A system sized for winter would massively overheat in July.
PV plus a diverter to the immersion heater is often cheaper per kilowatt-hour and more flexible, which is why solar thermal installations have declined.