Firstly, you calculate the energy stored in a thermal store or buffer tank in kWh, not kW.

kW = power/output rate
kWh = stored energy/capacity

The simple formula is

Stored energy in kWh = tank volume in litres × temperature difference × 0.001163

Where:

Tank volume = litres of water in the buffer tank / buffer vessel
Temperature difference = usable temperature drop in °C
0.001163 = the approximate kWh stored by 1 litre of water per 1°C temperature rise/drop

For example, if you have a 500 litre thermal store/buffer tank/buffer vessel heated from 40°C to 80°C, the usable temperature difference is 40°C.

500 × 40 × 0.001163 = 23.26 kWh

So, that 500 litre thermal store / buffer tank / buffer vessel would hold approximately 23.3 kWh of usable heat energy between 80°C and 40°C.

Another example:

A 1,000 litre buffer tank heated from 45°C to 75°C has a 30°C usable temperature difference.

1,000 × 30 × 0.001163 = 34.89 kWh

Thus, it stores approximately 35 kWh of usable heat.

To work out how long the store will last, divide the stored kWh by the heating load.

For example, if the building needs 7 kW of heat per hour:

35 kWh ÷ 7 kW = 5 hours

So, a 1,000 litre buffer tank or vessel storing 35 kWh could theoretically supply a 7 kW heating load for around 5 hours.

However, in real systems, the usable energy may be lower because of heat losses, mixing inside the tank, flow temperature requirements, poor stratification and the minimum temperature needed by radiators, underfloor heating or domestic hot water.

A useful rule of thumb is the following:

Every 1,000 litres of water stores about 1.16 kWh for each °C of temperature difference.

So, the bigger the tank and the wider the usable temperature range, the more heat energy the thermal store or buffer tank can hold.