Procurement teams routinely compare cell efficiency numbers when evaluating suppliers — 22.5% versus 23.1%, and so on. But a cell’s rated efficiency is measured before it’s anywhere near a finished module. Once that cell is encapsulated in glass, EVA, backsheet and frame, its contribution to the module’s actual output power shifts — sometimes down, occasionally up. Understanding why is essential to reading a module datasheet correctly.

Quick Answer

Cell-to-module (CTM) loss is the difference between the sum of a module’s individual cell power ratings and the module’s actual measured output power after assembly. In real manufacturing, CTM values typically range between 97% and 99% — meaning a well-designed module retains 97–99% of its cells’ combined rated power, while poor design choices can push that ratio lower. The gap is driven by optical, electrical, and geometric effects introduced during lamination and interconnection — and, notably, good module design can sometimes turn a “loss” into a net gain.

What CTM Actually Measures

The encapsulation of solar cells into a photovoltaic module introduces optical loss mechanisms, and typically the output power of the module is less than the total sum of individual cells — this difference is referred to as cell-to-module (CTM) losses.The additional electrical and optical effects introduced during the manufacturing of a module result in power loss — or, sometimes, gain — compared to the solar cells used to make it; the difference between input power and output power is the CTM conversion loss, which manufacturers work to minimise.

Put simply: if you buy cells individually rated at a combined 550W and the finished module tests at 535W, roughly 15W has gone somewhere during assembly — and CTM analysis is the discipline of identifying exactly where.

The Main Loss (and Gain) Mechanisms

Optical, geometrical and electrical effects interact and result in a cell-to-module ratio in terms of power and efficiency — analysis breaks these effects down to the material level to assess the potential for optimisation. A rigorous CTM analysis typically calculates around 15 distinct loss and gain factors based on material properties and module setup. Broadly, they fall into three categories:

Optical effects — reflection and absorption losses introduced by the front glass, the encapsulant (typically EVA or POE), and the anti-reflective coating. Some of these can actually be gains: anti-reflective coating on the cover glass can produce a power gain, alongside a power loss from junction box resistance — meaning module-level effects aren’t uniformly negative.

Electrical effects — resistive losses in the interconnection ribbons, solder joints, and junction box, plus current mismatch losses when cells with slightly different electrical characteristics (see our companion piece on solar cell binning and quality control) are wired together in a series string.

Geometric effects — the inactive space between cells, and the inactive margin around the module’s edge. Losses from the inactive module margin and the cell and string spacing areas are described by specific factors in CTM modelling that account for geometrical losses of inactive areas that don’t contribute to power generation. Every millimetre of gap between cells is silicon area not being used, and it directly drags down the module’s overall power-per-area.

CTM Loss Factors at a Glance

Why Half-Cell Design Can Push CTM Above 100%

This is the counterintuitive part that’s genuinely useful for procurement teams to understand: CTM isn’t a fixed ceiling that only ever subtracts from a module’s power. Comparisons of cell-to-module results show that while full-cell modules had CTM power ratios ranging between 93% and 97%, CTM power ratios over 100% were achieved for half-cell modules made from the same materials.

The reason is that cutting cells in half reduces the current flowing through each half-cell for the same voltage output, which cuts resistive losses in the interconnection ribbons roughly in proportion. Since resistive loss scales with the square of current, halving current per cell meaningfully reduces one of the largest electrical loss terms in the CTM equation — enough, in well-optimised designs, to offset the optical and geometric losses entirely and produce a module that outputs more than the simple sum of its individual cell ratings.

This is also part of why the industry’s shift toward half-cut and shingled cell architectures over the last several years wasn’t purely a wafer-size story — it was, in significant part, a CTM optimisation story.

Why This Matters for Procurement Decisions

Comparing cell efficiency alone misrepresents module performance. Two suppliers offering cells with identical rated efficiency can deliver meaningfully different finished module wattage depending on their module design and assembly quality — the CTM ratio, not just cell efficiency, determines what you actually receive.

CTM ratio is a proxy for manufacturing process discipline. A manufacturer consistently achieving high CTM ratios has tight control over lamination temperature/pressure, interconnection quality, and cell binning — all indicators of a mature, well-controlled production line rather than a loosely managed one.

Module design choices (half-cell, cell spacing, encapsulant type) are legitimate technical differentiators. When comparing module datasheets that look similar on cell technology and wafer format, ask specifically about module architecture — half-cell vs. full-cell, cell gap dimensions, encapsulant type — since these design choices materially affect the final CTM outcome.

Websol Energy System’s M10 Bifacial Mono-PERC solar cells are engineered for use in half-cell, multi-busbar module architectures — the design category most associated with favourable CTM outcomes in current industry data. Cell-level specifications are available on the Websol solar cell page, and our detailed look at solar cell conversion efficiency and project ROI covers how cell-level efficiency numbers should — and shouldn’t — be read by developers evaluating project returns.