What Is the Unit of Sunlight? How Solar Radiation Is Measured with Pyranometers and Precision Methods
By Alternate Energy Hawaii · Last updated: October 29, 2026
Quick answer: The unit of sunlight depends on what you measure. The strength of sunlight is measured in watts per square meter (W/m²), up to about 1,000 W/m² at sea level on a clear day. The total sunlight over a day is measured in kilowatt-hours per square meter (kWh/m²), which solar companies call peak sun hours. The instrument that measures it is a pyranometer.
If you're an Oʻahu homeowner comparing solar quotes, you don't need to own a pyranometer. But you should know what the numbers mean, because the sunlight estimate behind your quote drives the system size, the price and the payback. This guide covers the units, the instruments, how precise measurement works and what it all means for a home in Hawaii. Alternate Energy Hawaii has designed and installed solar on Oʻahu since 1993, and we use these numbers on every system we design.
Quick Answers
- Unit of sunlight (strength): watts per square meter, W/m².
- Unit of sunlight (total over time): kilowatt-hours per square meter, kWh/m², or peak sun hours.
- SI units: W/m² for strength, joules per square meter (J/m²) for total energy.
- Instrument used to measure solar irradiance: a pyranometer (answer d), not a pyrometer, hydrometer or refractometer.
- Direct sunlight only: measured with a pyrheliometer.
- Brightness to the eye: measured in lux, which is not used to size solar panels.
What Is the Unit of Sunlight?
There's no single unit of sunlight because there are three different questions you can ask. How strong is the sun right now? How much solar energy arrived over a day or a year? And how long did the sun shine? Each question has its own unit.
Irradiance: W/m² (how strong the sun is right now)
Irradiance is solar power landing on a surface, measured in watts per square meter. Sandia National Laboratories describes it as "an instantaneous measurement of solar power over some area" (Sandia PVPMC). At the top of Earth's atmosphere the sun delivers about 1,361 W/m², a value NASA's SORCE and TSIS-1 satellite missions helped establish (NASA Goddard). By the time sunlight reaches the ground at sea level on a clear day, the maximum is around 1,000 W/m² (solar irradiance). That 1,000 W/m² figure is why solar panels are rated and tested at it.
Irradiation or insolation: kWh/m² (how much sun arrived over time)
Insolation, also called irradiation, is irradiance added up over time. Sandia defines it as "the cumulative energy measured over some area for a defined period of time," usually in kWh/m² per day, month or year (Sandia PVPMC). The U.S. Department of Energy notes that radiation data for photovoltaic (PV) systems are often given in kWh/m² (DOE). This is the number that best predicts how much electricity your panels will make.
Peak sun hours: the unit on your solar quote
A peak sun hour is one hour of sunlight at 1,000 W/m², which equals 1 kWh/m². So a roof that receives 5 kWh/m² in a day gets 5 peak sun hours, even though the sun was up for 12. Published estimates for Hawaii range from about 4 to 7.5 peak sun hours a day depending on location (SolarReviews).
Sunshine hours: how long the sun shone
Weather services also report sunshine duration, which is a measure of time, not energy. The World Meteorological Organization defines it as the total time in a day when direct solar irradiance is at or above 120 W/m² (WMO). A day can log 10 sunshine hours but only 5 peak sun hours, because much of that sunshine is weaker morning and afternoon light. For sizing solar panels, peak sun hours are the number that counts.
The SI unit of sunlight
In the International System of Units (SI), sunlight's strength is measured in W/m² and the total energy it delivers in joules per square meter (J/m²). Because a joule is small, weather data often uses megajoules per square meter (MJ/m²). One kWh/m² equals 3.6 MJ/m².
Other units you may run into
Solar water heating data is usually listed in British thermal units per square foot (Btu/ft²) (DOE). Older records use the langley, which is one calorie per square centimeter or 41,840 J/m² (solar irradiance).
What about lux?
Lux measures how bright light looks to the human eye, not how much energy it carries. Direct sunlight is about 32,000 to 100,000 lux. There is no single conversion factor between lux and W/m², because it depends on the light's spectrum (lux). A light meter on your phone can tell you if a spot is sunny, but it can't tell you how much power a panel will produce there.
| Unit | What it measures | Where you'll see it |
|---|---|---|
| W/m² (watts per square meter) | Irradiance: how strong sunlight is at one moment | Pyranometer readings, solar panel test ratings |
| kWh/m² (per day, month or year) | Irradiation or insolation: total solar energy over time | Solar maps, PV production estimates |
| Peak sun hours | Daily kWh/m² restated as hours at 1,000 W/m² | Residential solar proposals |
| Sunshine hours | Time direct sunlight is at or above 120 W/m² | Weather and climate records |
| J/m² or MJ/m² | Total solar energy, in SI units (1 kWh/m² = 3.6 MJ/m²) | Weather and climate data |
| Btu/ft² | Total solar energy, in imperial units | Solar water heating and space heating |
| Lux (lx) | Brightness as the human eye sees it | Lighting, photography, plant growth |
| Langley | 1 calorie per cm² (41,840 J/m²) | Older meteorological records |
Types of Solar Radiation
Sunlight reaches your roof in more than one way, and each type is measured differently (DOE):
- Direct (beam) radiation: sunlight that comes straight from the sun without being scattered. Measured as direct normal irradiance (DNI).
- Diffuse radiation: sunlight scattered by clouds, humidity and particles in the air. On Oʻahu's partly cloudy trade-wind days, a good share of what reaches panels is diffuse. Measured as diffuse horizontal irradiance (DHI).
- Reflected radiation: light bouncing off nearby surfaces such as light roofs, concrete or water.
- Global radiation: direct plus diffuse light on a flat surface, measured as global horizontal irradiance (GHI). This is the number most solar maps and pyranometers report.
Solar designers then convert these into plane-of-array (POA) irradiance, which is the sunlight hitting your panels at your roof's actual tilt and direction.
The Instrument Used to Measure Solar Irradiance Is a Pyranometer
This question shows up often on quizzes and exams, usually worded like this:
Question: The instrument used to measure solar irradiance is a ________. Select one: a. hydrometer b. refractometer c. pyrometer d. pyranometer
Answer: d. pyranometer.
A pyranometer is built to measure "the solar radiation flux density (W/m2) from the hemisphere above" a flat surface (pyranometer). That's exactly what solar irradiance is. The other choices are easy to mix up:
- a. Hydrometer: measures the density of a liquid.
- b. Refractometer: measures how much light bends when it passes through a liquid.
- c. Pyrometer: measures the temperature of a hot surface from a distance, such as a furnace (pyrometer). Similar name, different job.
- d. Pyranometer: measures solar irradiance in W/m². The correct answer.
How a pyranometer works
There are two main types (pyranometer):
- Thermopile pyranometer: a black sensor under a glass dome heats up in sunlight, and the temperature difference creates a small voltage. It responds evenly to sunlight from about 300 to 2,800 nanometers, which covers nearly the whole solar spectrum. This is the type used for high-precision measurement.
- Silicon (photodiode or PV reference cell) pyranometer: a small silicon cell produces current in proportion to sunlight. It covers a narrower band, roughly 350 to 1,150 nanometers for PV reference cells, but it responds the way a real solar panel does. That makes it handy for checking panel performance.
Other Instruments Used to Measure Solar Radiation
Research stations such as those run by NOAA use a set of instruments together to split sunlight into its parts (NOAA Global Monitoring Laboratory):
| Instrument | What it measures | Used for |
|---|---|---|
| Pyranometer | Global irradiance (direct + diffuse) on a flat surface, from the whole sky above | Solar site assessment, PV monitoring, weather stations |
| Shaded pyranometer | Diffuse irradiance only (a disk or band blocks the sun's direct beam) | Splitting global light into its parts |
| Pyrheliometer | Direct normal irradiance from the sun's disk only, on a sun tracker | Research stations, concentrating solar, utility-scale studies |
| PV reference cell | Irradiance as a silicon solar cell responds to it | Checking how real panels are performing |
| Multifilter rotating shadowband radiometer | Global, direct and diffuse irradiance at up to seven wavelengths | Atmospheric research |
| Pyrgeometer | Long-wave (infrared) radiation from the sky and ground, not sunlight | Climate and weather science |
| Sunshine recorder | How long the sun shines, not how strong it is | Historical weather records |
Solar Precision: How Precise Solar Radiation Measurement Works
Precision solar radiation measurement is about more than buying a good sensor. Accuracy depends on three things: the instrument's class, how it is calibrated, and how it's installed and cared for.
Pyranometer classes under ISO 9060:2018
The international standard ISO 9060:2018 classifies instruments that measure hemispherical (global) and direct solar radiation across a spectral range of about 0.3 to 3 micrometers, based on the results of indoor or outdoor performance tests (ISO). Pyranometers fall into three classes, A, B and C, with Class A the best performing (pyranometer):
| ISO 9060:2018 class | What it means |
|---|---|
| Class A | Best-performing pyranometers, used for research and high-accuracy solar monitoring |
| Class B | Mid-range accuracy |
| Class C | Entry-level accuracy |
| "Spectrally flat" / "fast response" | Extra labels for sensors that respond evenly across sunlight's spectrum, or react quickly to changes |
Calibration traceable to the World Radiometric Reference
Precise readings need a sensor calibrated against a known reference. The worldwide reference for solar measurement is the World Radiometric Reference (WRR), represented by the World Standard Group of instruments maintained by the World Radiation Center at PMOD/WRC in Davos, Switzerland (PMOD/WRC). Class A pyranometers are calibrated under standards such as ISO 9846, ISO 9847 and ASTM G167, Class B and C sensors usually under ASTM E824 and ISO 9847, and PV reference cells under IEC 60904-2 and IEC 60904-4 (pyranometer). Sensors drift over time, so they're checked and recalibrated on a schedule.
Installation and upkeep
- Level and unshaded: a pyranometer measuring global radiation must sit level, with nothing above the horizon blocking it.
- Right plane: to estimate panel output, a second sensor is often mounted at the same tilt and direction as the panels.
- Clean dome: dust, pollen, bird droppings and, near the coast, salt spray all block light. In Hawaii's salt air, regular cleaning matters.
- Logged over time: readings are recorded every few seconds or minutes and averaged. One sunny afternoon tells you very little.
- Long records: a full year or more captures seasonal swings, such as winter storms and summer trade-wind clouds.
How Solar Radiation Is Measured, Step by Step
- Choose what to measure. Global radiation for most solar projects, plus direct and diffuse for research or tracking systems.
- Pick the instrument and class. A pyranometer for global light, a shaded pyranometer for diffuse, a pyrheliometer on a tracker for direct, or a reference cell for panel performance.
- Mount it correctly. Level for GHI, or at panel tilt for plane-of-array readings, away from shade and reflections.
- Log data continuously. A data logger records W/m² at regular intervals.
- Convert to energy. Readings are added up into kWh/m² per day, month and year, and into peak sun hours.
- Compare with long-term data. Short ground records are checked against years of satellite-based data for the same spot.
- Apply it to the design. The numbers feed system sizing, panel layout and the production estimate on a quote.
Ground Sensors vs Satellite Data: How Homes Get Measured
Pyranometers on weather stations and large solar farms give the most direct readings. For a single home, installers usually rely on satellite-based solar data that models years of cloud cover for each location. Hawaii's statewide solar resource map, for example, uses a satellite radiation model developed by the State University of New York at Albany with the National Renewable Energy Laboratory (NREL), and reports average daily GHI in Wh/m²/day (State of Hawaii).
Satellite data gives a solid long-term average for your area. What it can't see is your roof: the tree next door, a neighbor's second story, a vent stack, or which way each roof plane faces. That's why a proper solar quote combines regional sunlight data with an on-site look at tilt, direction and shade. On Oʻahu that matters, because sunshine changes a lot between sunny leeward neighborhoods and cloudier windward and upland ones.
What Solar Radiation Numbers Mean for Your Solar System
Here's a simple example of how sunlight units turn into electricity. These are round illustration numbers, not a quote:
- A roof plane gets 5 peak sun hours a day.
- A 6 kW solar array is rated at 1,000 W/m².
- 6 kW × 5 peak sun hours = 30 kWh a day before losses.
- Heat, wiring, inverter losses and dirt reduce that, so the real figure is lower.
Change the sunlight from 5 to 4 peak sun hours and that same array produces 20% less. That's why the sunlight number behind a quote matters as much as the panel brand. It affects how big the system needs to be, how much it costs and how long it takes to pay off. Read our guide to calculating solar ROI for the rest of that math.
The same data matters after installation. A solar monitoring system compares what your panels produce with the sunlight available, so you can spot shading, dirt or equipment problems early, and our solar maintenance team can follow up when production drops. If you're curious about a solar tracker, direct-beam sunlight is what decides how much extra a tracker can gain.
Questions to Ask About the Sunlight Estimate on Your Solar Quote
- What sunlight data did you use for my address, and from what source?
- How many peak sun hours, or kWh/m² per year, does each roof plane get?
- How did you account for shade from trees, buildings and nearby roofs?
- What losses are included in the production estimate?
- How will I monitor actual production against the estimate once the system is on?
A good installer will answer each one plainly. Our guide to designing an Oahu solar panel system explains how these numbers fit into the rest of the design, and what to expect when you go solar walks through the process. For home systems, see residential solar in Hawaii.
How Sunlight Data Is Used Across Solar Products
- Photovoltaic solar panels: kWh/m² and peak sun hours size the array and estimate yearly production.
- Battery storage: daily sunlight sets how much surplus energy is available to charge a battery, such as a Tesla Powerwall, for evenings and outages.
- Solar water heating: sized with the same solar data, often in Btu/ft².
New to the terms? Our solar energy glossary covers the rest, and how photovoltaic technology works in Hawaii explains what happens once sunlight hits the panel.
Frequently Asked Questions
What is the unit of sunlight?
Sunlight's strength is measured in watts per square meter (W/m²), which is called irradiance. The total sunlight received over time is measured in kilowatt-hours per square meter (kWh/m²), usually per day. Solar quotes turn that daily total into peak sun hours, where one peak sun hour equals 1,000 W/m² for one hour.
What is the SI unit of sunlight?
In SI units, the strength of sunlight is measured in watts per square meter (W/m²) and the total solar energy received is measured in joules per square meter (J/m²), often shown as megajoules per square meter (MJ/m²). Solar companies usually use kWh/m², because that converts directly into the kilowatt-hours a solar system produces.
The instrument used to measure solar irradiance is a ________. Select one: a. hydrometer b. refractometer c. pyrometer d. pyranometer
The answer is d. pyranometer. A pyranometer measures solar irradiance, the sunlight falling on a flat surface, in W/m². A pyrometer measures the temperature of a hot object from a distance, a hydrometer measures the density of a liquid, and a refractometer measures how light bends in a liquid.
Is a pyrometer the same as a pyranometer?
No. A pyrometer measures the temperature of a hot object from a distance. A pyranometer measures sunlight. The names are similar, which is why the two are often mixed up on quizzes.
What is solar precision in solar radiation measurement?
Precision solar radiation measurement depends on the instrument's class under ISO 9060:2018 (Class A is the best performing), calibration traceable to the World Radiometric Reference, and correct installation: level, unshaded and with a clean dome. Readings are logged continuously and compared with long-term data.
What is the difference between peak sun hours and sunshine hours?
Peak sun hours measure energy: one peak sun hour is 1 kWh/m², the same as one hour of sunlight at 1,000 W/m². Sunshine hours measure time: the World Meteorological Organization counts sunshine duration as the time direct sunlight is at or above 120 W/m². Solar systems are sized with peak sun hours.
How many peak sun hours does Hawaii get?
Published estimates for Hawaii range from about 4 to 7.5 peak sun hours per day, depending on location. Sunny leeward areas sit near the top of that range and cloudier windward and upland areas near the bottom. Your own roof's tilt, direction and shading decide the number that matters for your system.
What is the difference between irradiance and insolation?
Irradiance is how strong sunlight is at one moment, in W/m². Insolation, also called irradiation, is the total solar energy collected over a period of time, in kWh/m². Think of irradiance as speed and insolation as distance traveled.
Can lux be converted to W/m² for sunlight?
Not exactly. Lux measures brightness as the human eye sees it, while W/m² measures all the energy in sunlight. There is no single conversion factor between the two because it depends on the light's spectrum. Use W/m² or kWh/m² for anything to do with solar panels.
Find Out How Much Sun Your Roof Really Gets
Alternate Energy Hawaii has designed and installed solar on Oʻahu since 1993, with our own employee installers and Hawaii contractor license C-26041. We'll look at your roof's direction, tilt and shading and show you the production estimate behind your quote, along with Hawaii solar tax credits and incentives that may apply. Call (808) 842-5853 or request a free solar estimate.
