Hot Water Cylinder Size and Heating Power Calculator (Preliminary)

What size hot water cylinder do I need?

The cylinder must hold enough to cover the hot water used back to back in the busiest hour of the day. The calculation has three steps.

1. Daily demand. People times use per person: V_day = N × q.

2. Convert to storage temperature. A shower uses 40 °C water but the cylinder is kept at 60 °C. Because hot water is blended with cold, less is drawn from the cylinder: V_store = V_day × (t_use − t_cold) / (t_store − t_cold).

3. Peak period. The part of daily use drawn back to back (morning showers for example) sets the cylinder. Not all the stored water can be drawn at temperature, so divide by the usable fraction: V = V_store × share / usable fraction.

Example

A household of three uses 50 litres of 40 °C water per person per day. Mains water is at 10 °C and storage at 60 °C. Daily demand is 150 L and its equivalent at storage temperature is 150 × 30 / 50 = 90 L. If 60 percent of that is drawn in the morning, 54 L, and the usable fraction is 80 percent, the required volume is 67.5 L. Pick the next size up.

Heating power and heat up time

Heating one litre of water by one degree takes 1.163 Wh. So τ = 1.163 × V × ΔT / P (V in litres, P in watts, τ in hours). An 80 litre cylinder heats from 15 °C to 60 °C with a 2 kW element in 1.163 × 80 × 45 / 2000 = 2.1 hours and uses 4.2 kWh.

How many kW for an instantaneous heater?

Without storage the water must be heated as it flows and the power depends on flow rate: P = 1.163 × V̇ × ΔT (V̇ in litres per hour). Raising 10 litres a minute from 10 °C to 40 °C takes 20.9 kW. That is why electric instantaneous heaters draw high currents and gas combination boilers are sized larger for hot water than for heating.

Why store at 60 °C?

Legionella bacteria can multiply in lukewarm water. Storage systems are therefore commonly kept at 60 °C or above and blended with cold water at the point of use. A higher storage temperature also gives more usable water from the same cylinder.

Limits

Use per person is the most uncertain input and varies several times over with habits. For apartment blocks, hotels and similar buildings, simultaneous demand, circulation loss and recovery capacity are handled together: this tool only gives a first idea at single dwelling scale.