How to measure albedo on-site for solar projects with our free app
- Trygve Mongstad
- 1 day ago
- 6 min read
Measuring albedo on a rooftop is becoming increasingly important for bifacial and vertical solar installations, where reflected light can significantly influence energy yield. For vertical solar panels, high albedo can increase the solar energy yield by up to 40%. Yet for many projects, albedo is still treated as an assumed value rather than something that is actually measured on site.
Solar installers and consultants often ask the question:
“How do you measure albedo at a project site in practice?”
The challenge is that conventional albedo measurement equipment is expensive and impractical for normal site visits, and visual estimation by eye is highly unreliable. This gap is what led us to develop the Albedo App, a free, smartphone-based tool for on-site albedo estimation.
--> Try the Albedo App now – it’s completely free and easy to use.
Why Albedo Matters, and Why It’s Hard to Judge
Albedo describes how much incoming light a surface reflects, expressed on a scale from 0 (no reflection) to 1 (full reflection). For bifacial and vertical PV systems, this reflected light makes up a significant part of total energy yield.
At first glance, albedo may seem easy to estimate. Many surfaces look bright or dark, and it’s tempting to rely on visual judgment. However, the human eye is a very poor estimator of albedo.
Our visual system adapts extremely quickly to different lighting conditions. This makes us very good at seeing detail, but very bad at judging absolute rooftop reflectance. Two surfaces with significantly different albedo values can appear surprisingly similar, while lighting conditions, shadows, and surrounding colors further distort perception.
In practice, guessing albedo by eye is almost impossible, even for experienced professionals.
Real Roofs Are Even More Complex
Beyond human perception, real rooftops introduce additional uncertainty:
surfaces age and discolor
the type and albedo of the roofing membrane is normally uncertain or unknown
moisture levels change throughout the day
green roofs vary with vegetation type and season
dirt, dust, and wear can change the albedo
As a result, using a generic assumed albedo value often introduces significant uncertainty into energy yield simulations.
Conventional Albedo Measurement: Accurate, but Impractical
From a scientific perspective, albedo measurement is a solved problem. It can be performed using:
two paired pyranometers
upward- and downward-facing irradiance sensors
However, this equipment is expensive, time-consuming to deploy, and never available during normal site visits on a rooftop. There is also no working handheld device designed for quick on-site albedo measurements.
This leaves a gap between what would be useful and what is realistically feasible in day-to-day project work.
The Missing Simple Albedo Measurement Method
After encountering this challenge across many projects, we realized something important: Laboratory-grade precision is impossible, but visual guessing is clearly not good enough.
What was missing was a practical middle ground: a way to obtain a data-based albedo estimate directly on site, using tools solar installers and project developers already have.
Introducing the Albedo App for On-site Easy Measurements
To address this gap, we developed the Albedo App: An on-site albedo measurement application that runs directly in a smartphone browser.
The app is:
free to use
accessible on any modern smartphone
available without installation
open to everyone, not only Over Easy Solar customers
Instead of relying on human perception, the app uses the smartphone camera, which responds far more consistently to differences in reflected light intensity than the human eye. We’ve also developed an algorithm to self-calibrate and get a more exact estimate from two pictures taken on the site.
What You Need: Just a Smartphone and a Gray Card
The only external reference required is a gray card. A gray card is a flat surface with a known, neutral reflectance of 18%. It is widely used in photography to calibrate exposure and brightness, and it can be bought on Amazon or Ebay anywhere.
In the Albedo App, the gray card serves as a reference:
the camera captures both the surface and the gray card under identical lighting conditions
light levels are compared
the surface albedo is estimated relative to the known rooftop reflectance
Gray cards are inexpensive, lightweight, and easy to carry, making them well suited for on-site measurement.
Albedo estimate with a smart phone and a gray card
What you get is:
a repeatable, camera-based estimate
a significant improvement over just guessing the albedo
a more realistic input for energy yield simulations
In many projects, even modest changes in albedo can have a noticeable impact on simulated energy yield for vertical PV systems and give you significant bifacial gain.
Built for Real Project Workflows
The Albedo App is designed to be useful beyond the moment of measurement.
Users can:
store measurements in a database
access previous measurements later
download a PDF report for each measurement
The PDF report can be used as documentation and attached to:
project files
energy yield calculations
internal reviews
communication with colleagues, partners, and customers
This makes the app suitable not only for early feasibility studies, but also for structured project workflows.
Workflow of the Albedo App
How to Use the Albedo App
Using the Albedo App is designed to fit easily into a normal site visit.
Go to albedo.overeasy.solar
The app runs directly in your browser - no installation required.
Test the app - even without a gray card (optional)
You can try the app without a gray card to understand the workflow and see how the analysis works before acquiring a gray card.
Create an account (optional)
You can create an account to store all your measurements on-site and access them for documentation and energy yield estimates later. The location and any notes you want to add are saved with the measurement.
Place the gray card on the roof surface
When on the rooftop, place the gray card flat on the surface you want to measure. Make sure it is exposed to the same lighting conditions as the surrounding area.
Take the first photo (with the gray card)
Use the app to take a photo of the roof surface with the gray card visible in the frame.
Remove the gray card and take a second photo
Remove the gray card and take a second photo of the same roof surface, keeping the camera position and angle as similar as possible.
Click “Analyze”
The app compares the two images and estimates the surface albedo based on the known reflectance of the gray card.
Save or store your measurement
Download a PDF report directly to your phone (you will find it in your Downloads folder), and/or create a user account to store your measurements and access them later, for example when you are back in the office.
Best Practices for On-Site Albedo Measurement
To obtain the most reliable estimates, we recommend the following:
Best measurements in overcast conditions with good daylight
Diffuse light reduces glare and shadowing effects and generally produces the most stable results.
Be careful in direct sunlight
Avoid measuring directly toward or directly away from the sun.
Position the camera so sunlight comes in from the side, and take more tests with slightly different angles to obtain an average of measurements.
Surface texture matters
Smooth, homogeneous surfaces are easier to measure.
Rough or highly textured surfaces are more challenging, especially under direct sunlight where the texture can result in self-shading on the surface.
Choose large, unobstructed areas
Large, flat roof sections without equipment or shading objects give the most representative measurements.
Measure dry surfaces whenever possible
Wet surfaces appear darker (lower albedo) and are more prone to produce reflections and glare that distorts the measurement. Since most solar energy production happens under dry conditions, this is the most realistic condition.
Additional Recommendations
Avoid shadows or glare (reflected light) in the measurement area
Keep camera position and angle consistent between photos
Ensure the measured surface fills most of the image – preferably the entire photo
Ensure that the gray card completely covers the oval in the middle of the photo when capturing the gray card, but it should not extend outside of the indicator rectangle.
Take multiple measurements if conditions are uncertain
Note weather and surface conditions in the saved measurement for your project documentation
The Over Easy Approach: Open, Lightweight and Easy to Use
The Albedo App is free to use and does not require you to be a customer of Over Easy Solar. It was developed to solve a practical problem we encountered in real projects and to make albedo estimation more accessible to the industry - and where rooftop vertical solar installations are becoming increasingly popular.
By lowering the barrier to on-site albedo measurement, we hope this tool helps reduce uncertainty, improve transparency in yield assumptions, and support better-informed design decisions.
The Albedo App is available as a web application and can be accessed directly from your smartphone browser on iPhone or Android. All you need is your phone and a gray card.
And as always - contact us if you have any questions or suggestions!
