Heat Density Converter
Print pageAll Equivalents Reference Table
| Unit | Equivalent Value |
|---|
Surface Heat Density (also termed Surface Heat Energy Density or Solar Radiant Exposure, symbolized by qA or EA) measures the total thermal energy or electromagnetic radiation deposited per unit surface area (qA = Q ÷ A). Across solar photovoltaic (PV) yield forecasting, solar thermal collector sizing, meteorology global horizontal irradiance (GHI) tracking, building facade solar heat gain calculations, and industrial furnace refractory linings, heat density is categorized across three major scientific unit systems: International System of Units (SI metric fundamental: Joules per Square Meter / J/m2, Kilojoules per Square Meter / kJ/m2, Megajoules per Square Meter / MJ/m2), Imperial / US Customary standards (Btu per Square Foot / Btu/ft2), and meteorological / solar radiation standards (Langleys / Ly, Calories per Square Centimeter / cal/cm2).
Our free online Heat Density Converter provides instant, high-precision conversions across all SI metric, Imperial solar heating, and meteorological solar insolation units:
- Remarkable Meteorological-CGS Identity:
1 Langley (Ly) = 1.0 cal(th)/cm2 = 41,840.0 J/m2 = 41.84 kJ/m2 = 0.04184 MJ/m2 = 3.68478 Btu/ft2. - Btu (IT) per Square Foot to J/m2: Multiply Btu/ft2 by
11,356.5267(1 Btu/ft2 = 11,356.53 J/m2 = 11.3565 kJ/m2 = 0.27143 Langleys). - Joules per Square Meter to Btu/ft2: Multiply J/m2 by
0.0000880551(10,000 J/m2 = 0.88055 Btu/ft2 = 0.23901 Langleys). - Megajoules per Square Meter to Btu/ft2: Multiply MJ/m2 by
88.0551(1 MJ/m2 = 88.0551 Btu/ft2 = 23.9006 Langleys = 23.9006 cal/cm2). - Langleys to Btu (IT) per Square Foot: Multiply Langleys by
3.68478(1 Ly = 3.68478 Btu/ft2 = 41,840 J/m2). - Btu (th) [Thermochemical] per Square Foot to J/m2: Multiply by
11,348.9318(1 Btu(th)/ft2 = 11,348.93 J/m2 = 0.27125 Langleys).
Master Surface Heat Density & Solar Insolation Conversion Table
The table below displays exact mathematical conversion relationships, SI J/m2 multipliers, and imperial Btu/ft2 equivalents relative to 1 Joule per Square Meter (1 J/m2):
| Heat Density Unit Name | Symbol | Exact Value in J/m2 | Btu/ft2 & Langley Equivalent | Domain & Technical Application Standard |
|---|---|---|---|---|
| 1 Joule per Square Meter | J/m2 |
1.0 J/m2 (Base SI Unit) |
0.0000880551 Btu/ft2 (0.0000239006 Ly / 0.001 kJ/m2) |
SI Fundamental Surface Heat Density Standard |
| 1 Langley | Ly, cal/cm2 |
41,840.0 J/m2 (41.84 kJ/m2) |
3.68478 Btu/ft2 (1.0 cal(th)/cm2 / 0.04184 MJ/m2) |
Meteorology Solar Irradiation & Pyranometer Standard |
| 1 Calorie (th) per Square Centimeter | cal(th)/cm2 |
41,840.0 J/m2 (41.84 kJ/m2) |
3.68478 Btu/ft2 (1.0 Ly) |
CGS Physics Laboratory Solar Energy Benchmark |
| 1 Btu (IT) per Square Foot | Btu/ft2 |
11,356.53 J/m2 (11.3565 kJ/m2) |
1.0 Btu/ft2 (0.271428 Ly / 0.0113565 MJ/m2) |
US Imperial HVAC, Roofing & Solar Thermal Rating |
| 1 Btu (th) per Square Foot | Btu(th)/ft2 |
11,348.93 J/m2 (11.3489 kJ/m2) |
0.999331 Btu/ft2 (0.271246 Ly) | Thermochemical HVAC Solar Radiation Standard |
| 1 Megajoule per Square Meter | MJ/m2 |
1,000,000.0 J/m2 |
88.0551 Btu/ft2 (23.9006 Ly) | Global Commercial Solar PV Daily Irradiation Metric |
| 1 Kilojoule per Square Meter | kJ/m2 |
1,000.0 J/m2 |
0.088055 Btu/ft2 (0.023901 Ly) | Sub-Hourly Solar Irradiance Measurement |
Step-by-Step Solar PV Panel Daily Energy Yield Calculation Example
To convert a rooftop solar array receiving 18.5 Megajoules per Square Meter (18.5 MJ/m2/day) of daily solar radiation into Imperial Btu/ft2 and meteorological Langleys (Ly):
Step 1 (Imperial Conversion): qA = 18.5 × 88.0551 = 1,629.02 Btu/ft2/day
Step 2 (Meteorological Langley Conversion): qA = 18.5 × 23.9006 = 442.16 Langleys/day
Step 3 (Peak Sun Hours kWh/m2 Equivalent): Solar Energy = 18.5 MJ/m2 ÷ 3.6 = 5.139 kWh/m2 (5.14 Peak Sun Hours)
Thus, a daily solar insolation of 18.5 MJ/m2 equals 1,629.02 Btu/ft2 or 442.16 Langleys, providing 5.14 Peak Sun Hours of usable solar energy.
Real-World Solar Insolation & Heat Density Benchmarks
Below is a comparative reference chart showing daily radiant energy exposure values across Peak Sun Hours, desert climates, temperate seasons, and winter conditions:
| Solar Environment / Climate Condition | Heat Density in SI (MJ/m2 / J/m2) | Imperial & Langley Equivalent (Btu/ft2 / Ly) | Solar Engineering & Climate Context |
|---|---|---|---|
| 1 Peak Sun Hour (1000 W/m2 for 1 Hr) | 3.60 MJ/m2 (3,600,000 J/m2) | 317.00 Btu/ft2 (86.04 Langleys) | Standard solar panel testing benchmark (STC 1 kWh/m2) |
| Sahara Desert Daily Solar Irradiation (Summer) | 25.00 MJ/m2/day | 2,201.38 Btu/ft2/day (597.52 Langleys/day) | Extreme equatorial solar radiation (~6.94 Peak Sun Hours) |
| Temperate Region Spring Daily Irradiation | 15.00 MJ/m2/day | 1,320.83 Btu/ft2/day (358.51 Langleys/day) | Moderate latitude average sunny day (~4.17 Peak Sun Hours) |
| High-Latitude Winter Overcast Day Irradiation | 5.00 MJ/m2/day | 440.28 Btu/ft2/day (119.50 Langleys/day) | Low winter solar angle & cloud cover (~1.39 Peak Sun Hours) |
| Industrial Boiler Firebox Wall Absorbed Heat (1 Hr) | 500.0 MJ/m2 | 44,027.5 Btu/ft2 (11,950.3 Langleys) | High thermal flux absorption in power station steam boilers |
History & Physics: 1947 Samuel Pierpont Langley Honor & Solar Pyranometry
Samuel Pierpont Langley & the Bolometer Invention
In 1878, American astronomer and physicist Samuel Pierpont Langley (1834–1906), third secretary of the Smithsonian Institution, invented the bolometer—an extraordinarily sensitive instrument capable of measuring minute fractions of infrared thermal radiation. Langley’s bolometer allowed astronomers to map the solar spectrum thermal energy distribution for the first time.
1947 Adoption of the Langley (Ly) Solar Irradiance Unit
In 1947, the American Meteorological Society (AMS) officially established the unit Langley (symbol: Ly) in honor of Samuel Pierpont Langley, defining 1 Langley ≡ 1 calorie per square centimeter = 41,840 J/m2. The Langley became the universal global unit for expressing solar radiation received at Earth’s surface (Global Horizontal Irradiance, GHI) across pyranometer networks and agricultural evapotranspiration models.
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Frequently Asked Questions (FAQ)
What is a Langley in solar energy measurement?
A Langley (Ly) is a meteorological unit of heat density and solar radiation exposure defined as 1 calorie per square centimeter: 1 Langley = 41,840 J/m2 = 41.84 kJ/m2 = 3.68478 Btu/ft2.
How do you convert J/m2 to Btu/ft2?
To convert surface heat density from J/m2 to Btu/ft2, multiply by 0.0000880551 (or divide by 11,356.53). For example, 1,000,000 J/m2 (1 MJ/m2) × 0.0000880551 = 88.055 Btu/ft2.
How do you convert MJ/m2 to Langleys?
To convert Megajoules per square meter (MJ/m2) to Langleys, multiply MJ/m2 by 23.9006. For example, a daily solar radiation of 20 MJ/m2 × 23.9006 = 478.01 Langleys.
What is the relationship between Peak Sun Hours and MJ/m2?
One Peak Sun Hour (1 kWh/m2) equals 3.60 Megajoules per square meter (3.60 MJ/m2) or 86.04 Langleys. Dividing daily solar radiation in MJ/m2 by 3.6 yields the daily Peak Sun Hours.