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About this tool
Size a water heater in litres for bucket baths or showers, with the mixing maths, reheat time and energy per heating cycle.
This selector sizes a storage water heater from the mixing balance rather than a rule of thumb: hot water drawn equals bath volume multiplied by (bath temperature minus mains temperature) divided by (tank temperature minus mains temperature). A 20 litre bucket at 40 °C from a 60 °C tank with 20 °C mains therefore draws only 10 litres, which is why a 15 litre geyser suits one bucket bath and 25 litres suits two in a row. Reheat time and energy come from Q = m x c x dT with water's specific heat of 4,186 J/kg·K.
Open Geyser Capacity Selector on AltFTool — it loads instantly in your browser.
Enter the values you already know.
Fine-tune the options to match your scenario.
Read the result and use it in your planning or reporting.
Accounts for the cold water you actually add, so the tank is not oversized.
Shows the minutes on your element rating, so you know when to switch it on.
Drop the mains temperature to a winter figure and watch the required size and time rise.
15 litres covers one bucket bath comfortably, and 25 litres covers two people bathing back to back. A single 8-minute shower at 8 litres a minute draws about 32 litres of 60 °C water, which needs a 35 litre tank or an instant heater.
About one and a third at typical settings. A 15 litre tank delivers roughly 13.5 litres of usable hot water, and a 20 litre bucket at 40 °C from a 60 °C tank with 20 °C mains needs 10 litres, so the second bath runs short unless you wait for a reheat.
A 15 litre tank rising 40 °C on a 2,000 W element takes about 22 minutes; 25 litres over the same rise takes around 37 minutes. The maths is litres x 4,186 J/kg·K x temperature rise, divided by the element wattage.
60 °C is the usual compromise: hot enough to suppress bacteria and to stretch the stored water further when mixed, but not so hot that scalding and limescale become problems. Lowering the thermostat cuts standing loss but also cuts how many baths one tank gives, because you mix in less cold.
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