Water Heater (Geyser) Heating Time Calculator
By Santaji GadepatilinUpdated October 27, 2026
How long a geyser takes to heat water depends on the tank size, the temperature rise and the heater's power. This free water heater heating time calculator solves for whichever value you leave blank and shows the formula and steps.
How to use the water heater heating time calculator
Enter the tank volume, cold and target temperatures, the heater’s power and efficiency. Energy used and heating time are calculated.
Water heater heating time formula
time = m c ΔT ÷ (P × efficiency)
Example
25 L from 20 °C to 55 °C with a 2 kW heater at 90%:
- Energy = 25 × 4.186 × 35 ÷ 3,600 ÷ 0.9 = 1.13 kWh
- Time = 34 minutes
Heating times for common tanks
From 15 °C to 55 °C at 90% efficiency:
| Tank | 1.5 kW | 2 kW | 3 kW |
|---|---|---|---|
| 10 L | 21 min | 16 min | 10 min |
| 15 L | 31 min | 23 min | 16 min |
| 25 L | 52 min | 39 min | 26 min |
| 50 L | 103 min | 78 min | 52 min |
| 100 L | 207 min | 155 min | 103 min |
| 150 L | 310 min | 233 min | 155 min |
Running cost
Energy used × electricity price. Heating 50 L by 40 °C uses about 2.6 kWh; at 0.15 per kWh that is about 0.39 per full heat-up.
Standing losses
Stored hot water loses heat through the tank walls. Well-insulated tanks lose 1–2 kWh a day; older tanks can lose more. Lagging pipes and setting a sensible temperature reduce this.
Temperature settings
- Storage tanks: keep at 60 °C or above to control Legionella bacteria.
- At the tap: 50 °C or below reduces scald risk; thermostatic mixing valves help.
- Each 10 °C reduction in set temperature saves a noticeable amount of energy in standing losses.
Heat pumps and solar
Heat-pump water heaters use about a third of the electricity of a resistance heater for the same hot water. Solar water heaters can cover most needs in sunny climates.
Frequently asked questions
How long does a geyser take to heat water?
A 25-litre, 2 kW heater raising water by 35 °C takes about 34 minutes at 90% efficiency.
How much electricity does it use?
About 1.16 kWh per 1,000 litres per °C, divided by efficiency. The example above uses about 1.1 kWh.
What temperature should I set?
Around 50–60 °C balances comfort, energy use and protection against bacteria such as Legionella.
How long does a 25 litre geyser take to heat?
About 30–40 minutes with a 2 kW element, heating from 15 °C to 55 °C.
How much electricity does a water heater use?
About 0.05 kWh per litre for a 40 °C rise, before standing losses.