Using Q equals mass times specific heat times temperature rise, then finding heat-up time.
This example shows how much energy it takes to heat a cylinder of water and how long an immersion heater takes to do it. The numbers are chosen for the worked example.
The formula. Q = m x c x change in T, where m is mass, c is specific heat capacity and change in T is the temperature rise. For water, 1 litre is about 1 kg and c is about 4.2 kJ per kg per °C.
The situation. A 120 litre cylinder (so about 120 kg of water) is heated from 10 °C to 60 °C. The temperature rise is 60 - 10 = 50 °C.
Step 1 — energy required. Q = 120 x 4.2 x 50 = 25,200 kilojoules (kJ), which is 25.2 megajoules.
Step 2 — convert to kWh (optional). 1 kWh is 3600 kJ, so 25,200 / 3600 = 7.0 kWh.
Step 3 — heat-up time. A 3 kW immersion heater delivers 3 kJ every second. Time = energy / power = 25,200 kJ / 3 kJ per second = 8400 seconds, which is 140 minutes, or 2 hours 20 minutes.
Takeaway. Energy scales with both the amount of water and the temperature rise, and time is just energy divided by power. This is why a larger cylinder or a colder start takes noticeably longer to recover. Real systems also lose some heat, so allow a margin, and follow current Part L guidance for efficiency.
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