Why Your Solar Cells Lose Power: A Guide to Unpacking Cell-to-Module (CTM) Losses

You’ve done the math. You’ve sourced high-efficiency solar cells, added up their individual power ratings, and calculated the theoretical output of your new module design. But when the final module comes off the line and goes under the flasher, the reading is disappointingly lower. A few percentage points have vanished somewhere between the individual cells and the laminated end product.

This frustrating gap is known as Cell-to-Module (CTM) power loss, and it’s one of the most critical—and often misunderstood—challenges in solar module manufacturing.

CTM loss is the difference between the potential power of the components and the actual power of the finished assembly. According to the International Technology Roadmap for Photovoltaics (ITRPV), the industry is on a mission to shrink this gap, with median CTM losses projected to drop from around 3% in 2021 to just 1.5% by 2031. But achieving this requires more than just better cells; it demands a systematic way to diagnose where the power is escaping.

The challenge is that CTM loss isn’t a single issue, but a complex cocktail of optical and electrical inefficiencies. Is your glass reflecting too much light? Is your encapsulant slightly opaque? Or is resistance in your solder joints silently bleeding away watts? Without a clear methodology, you’re just guessing.

This guide offers a systematic approach to deconstructing CTM losses, helping you isolate optical factors from electrical ones and create a clear path toward holistic module optimization.

The First Step: Establishing a Rock-Solid Baseline

Before you can diagnose what’s lost, you have to know precisely what you started with. You can’t improve what you don’t measure, and the foundation of any CTM analysis is an accurate, statistically significant baseline of your raw solar cells.

The process involves:

1. Sampling: Select a representative batch of the exact cells you plan to use in your module.
2. Characterization: Measure the current-voltage (I-V) curve of each individual cell under Standard Test Conditions (STC) using a precisely calibrated cell tester.
3. Summation: Add the maximum power (Pmax) values of all the cells that will go into one module.

This sum represents your theoretical maximum power—the module’s potential in a perfect, loss-free world. It’s the benchmark against which everything else is measured. The difference between this number and your final module’s power is your Total CTM Loss. Now, the real investigation begins: breaking that total loss into its core components.

Isolating Optical Losses: What Your Eyes Can’t See

Optical losses happen when photons that should reach the active surface of a solar cell are blocked, reflected, or absorbed before they can get there. Think of it as a series of filters dimming the light on its way to the silicon.

Key culprits include:

• Glass & Coatings: The front glass, even with an Anti-Reflective (AR) coating, will reflect a small percentage of light. The quality and application of this coating are critical.
• Encapsulant: Materials like EVA (ethylene vinyl acetate) or POE (polyolefin elastomer) are designed for high transparency, but no material is 100% perfect. Their refractive index and clarity directly impact how much light passes through.
• Shadowing: Ribbons, wires, and busbars used to connect the cells physically block a portion of the cell’s surface, creating shadows and reducing the active area available for light absorption.

So how do you measure the combined impact of these factors? The most effective method is to build a simple test coupon.

By laminating a single, pre-characterized solar cell with the same glass, encapsulant, and backsheet used in your final module, you can isolate the optical effects. When this single-cell coupon is measured under a flasher, the power drop from its original baseline is almost exclusively due to the optical losses introduced by the module materials. This gives you a hard number for your optical loss factor.

Conducting these kinds of Material Testing & Lamination Trials is essential for validating supplier datasheets against real-world performance after the lamination process.

Pinpointing Electrical Losses: The Hidden Resistance

Once light is converted into electricity, that energy has to travel out of the module. Every step of this journey introduces electrical losses—primarily as resistance, which generates heat and reduces the final power output.

Major sources of electrical loss include:

• Resistive Losses: Current flowing through the thin metal fingers on the cell surface, the solder-coated ribbons, and the larger busbars all generate resistive heat. Research shows these interconnection losses alone can account for 1-3% of total power loss.
• Mismatch Losses: No two solar cells are perfectly identical. When cells with slightly different electrical characteristics are wired together in series, the string’s overall performance is often limited by the weakest cell. This is like a traffic jam where the slowest car sets the pace for the entire lane.
• Junction Box and Diodes: The final connections in the junction box and the bypass diodes also add their own small but measurable resistance to the circuit.

Now that you’ve done the hard work of isolating optical losses, calculating the electrical losses is remarkably simple. Using a straightforward formula, you can determine what was previously hidden:

Electrical Loss = Total CTM Loss – Optical Loss

This simple subtraction gives you a clear, actionable number, separating the two biggest culprits behind your power loss.

„Many teams chase marginal gains in cell efficiency, but often, the most significant and cost-effective improvements are found by systematically tackling the electrical and optical losses during module assembly,“ notes Patrick Thoma, PV Process Specialist at PVTestLab. „A 0.5% reduction in resistive loss can be more valuable than a 0.1% increase in cell efficiency.“

Bringing It All Together: A Holistic Optimization Strategy

With CTM losses deconstructed, you can move from guessing to data-driven decision-making. The systematic approach looks like this:

1. Establish Baseline: Measure your individual cells to get your theoretical maximum power.
2. Measure Final Module: Find your Total CTM Loss.
3. Isolate Optical Loss: Build and test a single-cell coupon.
4. Calculate Electrical Loss: Subtract the optical loss from the total.

Now you know exactly where to focus your efforts.

• High Optical Loss? It’s time to evaluate your glass supplier, check the integrity of your AR coating, or experiment with a different encapsulant.
• High Electrical Loss? Your focus should shift to your interconnection technology, soldering parameters, cell sorting (binning) strategy, or ribbon design.

This approach transforms the complex „black box“ of CTM loss into two distinct problems that can be solved independently, making Prototyping & Module Development faster and more effective.

Frequently Asked Questions About CTM Loss

What is a „good“ CTM loss percentage?
While this depends on the technology, the industry trend is moving downward. A few years ago, 3-4% was common. Today, high-efficiency manufacturers aim for 2% or less, with best-in-class targeting below 1.5%. If your losses are above 3%, you have significant room for improvement.

Can I just use supplier datasheets to calculate optical loss?
Datasheets are a good starting point, but they represent ideal laboratory conditions. The high heat and pressure of the lamination process can slightly alter the properties of encapsulants and coatings. The only way to know the true loss is to measure a physical sample built with your exact process parameters.

Is CTM loss the same as degradation?
No, and this is a critical distinction. CTM loss is a power loss that is „built-in“ during the manufacturing process. It’s the immediate difference between cell potential and module reality. Degradation refers to additional power loss that occurs over time once the module is installed in the field, caused by factors like UV exposure (LID), high voltage stress (PID), or material aging.

How does cell technology (e.g., PERC, TOPCon, HJT) affect CTM loss?
Different cell technologies have unique sensitivities. For example, heterojunction (HJT) cells are sensitive to high lamination temperatures, which can increase electrical resistance if not controlled properly. TOPCon cells may require specific encapsulants to prevent moisture ingress. Every new technology requires a re-evaluation of the CTM loss profile to ensure materials and processes are fully optimized.

From Data to Decision

Deconstructing Cell-to-Module power loss is not just an academic exercise—it’s a fundamental discipline for any team serious about creating competitive, high-performance solar modules. By moving beyond a single, monolithic loss figure and adopting a systematic approach to separate optical and electrical factors, you unlock a powerful diagnostic tool.

This clarity allows you to invest resources intelligently, solve problems faster, and incrementally close the gap between your design’s potential and its real-world performance.

Ready to turn these insights into action? Exploring a structured approach to PV process optimization is the next logical step in transforming your module concepts from theory into high-performance reality.