Heat Density Converter
Convert between joules per square meter, calories per square centimeter, langleys, and more heat (energy) density units — for solar irradiation and thermal energy science.
Quick Reference — 1 unit = ? Kilojoule per Square Meter (kJ/m²)
| Unit | Value |
|---|---|
| Joule per Square Meter (J/m²) | 0.001 |
| Kilojoule per Square Meter (kJ/m²) | 1 |
| Megajoule per Square Meter (MJ/m²) | 1000 |
| Joule per Square Centimeter (J/cm²) | 10 |
| Calorie per Square Centimeter (cal/cm²) | 41.84 |
| Langley (Ly) | 41.84 |
| Watt-hour per Square Meter (Wh/m²) | 3.6 |
| Kilowatt-hour per Square Meter (kWh/m²) | 3600 |
Formula
Multiply by (from factor / to factor) | Heat density = Power density × time
Frequently Asked Questions
What is "heat density," and how is it different from heat flux density?
Heat density (also called areal energy density or, in solar science, irradiation/insolation) measures total heat energy received or delivered per unit area — in joules per square meter (J/m²) — with no time component. Heat flux density measures the rate of that delivery (watts per square meter). Heat density equals heat flux density integrated over time: a constant flux of 1,000 W/m² sustained for 1 hour delivers a heat density of 3,600,000 J/m² (1 kWh/m²).
What is a langley, and where is it used?
The langley (Ly) is a traditional unit of solar irradiation, defined as exactly 1 thermochemical calorie per square centimetre (1 Ly = 4.184 J/cm² = 41,840 J/m²). It is named after the American astronomer Samuel Pierpont Langley and was widely used in meteorology and solar energy research through the 20th century to record daily or monthly solar radiation totals at weather stations. Modern science has largely shifted to SI units (MJ/m² or kWh/m²), but langleys still appear in older climate datasets and some agricultural/solar literature.
How much solar energy hits the ground in a day, in kWh/m²?
Typical daily solar irradiation (heat density) varies enormously by location and season: a sunny desert location might receive 7–8 kWh/m²/day; a temperate location on a clear summer day, 5–6 kWh/m²/day; an overcast winter day in a northern climate might be under 1 kWh/m²/day. This is the figure solar panel installers use to estimate energy yield: a 1 m² panel at 20% efficiency in a location averaging 5 kWh/m²/day would generate roughly 1 kWh of electricity per day.
How do you convert calories per square centimeter to joules per square meter?
1 cal/cm² = 41,840 J/m² (since 1 cal = 4.184 J and 1 cm² = 0.0001 m², so 4.184 J ÷ 0.0001 m² = 41,840 J/m²). This is numerically identical to 1 langley, since the langley was defined using the same calorie-per-square-centimetre basis. To convert cal/cm² to the more common MJ/m², multiply by 0.04184 (e.g. 500 cal/cm² = 20.92 MJ/m²).
How is heat density used outside of solar energy — e.g. in fire or explosion engineering?
Heat density (thermal energy received per unit area) is also used in fire engineering to assess radiant exposure — for example, the total heat dose a firefighter's protective clothing or a building façade receives during a fire, measured in kJ/m² or J/cm². It is also relevant in laser and welding applications (energy density delivered to a material's surface) and in agriculture, where cumulative solar radiation (often in MJ/m²/day) is used to model crop growth and irrigation needs.