Heat Size Calculator
Enter the mass, the material, the start and end temperatures and your heater's power and efficiency. The calculator gives the heat energy needed and how long the heater takes.
- Heat energy
- kWh
- Heating time
This ignores heat lost to the room and the container.
Heat = mass × specific heat × temperature rise. Heating 10 litres of water from 15 °C to 60 °C takes 10 × 4.186 × 45 = 1,884 kJ, about 0.52 kWh. A 3 kW heater does it in about 10.5 minutes.
Specific heat chart
Common materials. Search for a material.
| Material | Specific heat (kJ/kg·K) | Energy to warm 1 kg by 10 °C (kJ) |
|---|---|---|
| Water | 4.186 | 41.9 |
| Ice | 2.09 | 20.9 |
| Wood | 1.7 | 17 |
| Cooking oil | 2 | 20 |
| Air | 1.005 | 10 |
| Aluminium | 0.897 | 9 |
| Concrete | 0.88 | 8.8 |
| Steel | 0.49 | 4.9 |
| Copper | 0.385 | 3.9 |
kJ/kg·K.
Water and heat
Heated from 20 to 100 °C. Slide the litres.
The thermometer shows the rise.
How to work out heat energy
Mass to kWh. Step through it.
Mass
kg.
Insulate the container to cut heat losses.
Water, oil or metal
Materials. Compare them.
Highest.
Medium.
Low.
Water stores far more heat for its weight than metals do.
Before you heat
Heating checks. Tick each item.
Tick each item as you go.
Never leave heating oil unattended.
What you get
Heat sizing. Each tile explains one result.
kJ.
Electricity.
Minutes.
Imperial.
Specific heat.
Melting and boiling need extra latent heat on top of this.
Frequently Asked Questions
No question matches that search.
How much energy does it take to heat water?
About four point two kilojoules for each kilogram and degree Celsius.
How much energy to boil a litre of water?
About zero point zero nine kilowatt hours from twenty degrees.
What is specific heat?
The energy to warm one kilogram of a material by one degree.
How long does a kettle take?
About one and a half minutes per litre for a three kilowatt kettle.
How do I convert kilojoules to kilowatt hours?
Divide by three thousand six hundred.
Why does water take so long to heat?
It has a very high specific heat.
Does this include boiling?
No. Turning water to steam takes much more energy.
Know the energy before you heat
Work out the heat and time needed and pick a heater that gets the job done.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.