Heat Energy Calculator: Q = m·c·ΔT and Water Heating Time
Choose the material, enter its mass and start and end temperature. You get the heat energy, and with a heater’s watts and efficiency, the time and electricity cost.
- Nothing you type is stored
- Free
- No signup
Heating time
11.6min
- Heat energy
- 1,255.8 kJ
- In kilocalories
- 300.1 In kilocalories
- Electricity used
- 0.388 units (kWh)
- Electricity cost
- ₹2.71
Ignores heat lost while standing; storage geysers use more over a day.
Show working
- Q = 15 × 4,186 × 20 = 12,55,800 J
- t = 12,55,800 ÷ (2,000 × 0.9) = 698 s
How to use
- Pick water, milk, oil, a metal or enter a specific heat.
- Enter mass (1 litre of water ≈ 1 kg) and start and end temperatures.
- Enter heater watts, efficiency and rate per unit to see time and cost.
Good to know
Q = m × c × ΔT, where m is mass in kg, c is specific heat in J/kg·K and ΔT the temperature rise. Water has c = 4,186 J/kg·K, so heating 15 litres from 25 °C to 45 °C needs 15 × 4,186 × 20 = 1,256 kJ (0.35 kWh).
Heating time = Q ÷ (heater watts × efficiency). A 2 kW geyser at 90% takes about 11.6 minutes for that job, using about 0.39 units. Storage geysers also lose heat while standing, so daily use is higher than one heating.
Specific heats used: water 4,186, milk about 3,930, cooking oil about 1,970, aluminium 897, iron/steel 450, copper 385, air 1,005 J/kg·K. It does not include boiling or melting (latent heat), which needs much more energy.
Questions
How long does a 15 L geyser take to heat?
From 25 °C to 45 °C with a 2 kW element at 90%: about 12 minutes. From 15 °C to 50 °C: about 20 minutes.
How many units to heat a bucket of water?
20 L from 20 °C to 45 °C needs 2.09 MJ = 0.58 kWh, about 0.65 units at 90% efficiency.
What is specific heat?
Energy to raise 1 kg of a material by 1 °C. Water’s is high, which is why it takes long to heat.
Does it include boiling?
No. Turning water to steam needs extra latent heat (about 2,260 kJ per kg).
Why enter efficiency?
Some heat escapes to the tank, pipes and air; immersion heaters are near 90–98% efficient at the element.