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Rooftop Solar for Factories by the Numbers

Cells, modules, strings, inverters, roofs, monitoring and the business case for a factory's own solar array

How solar cells turn sunlight into power and how to size, check, mount, monitor and pay for a factory rooftop array with numbers: yield, strings, shading, wind and cost.

10 chapters · About 101 minutes · Free to read · Intermediate engineering and facilities management

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KEVOS® LibraryENGINEERINGRooftop Solarfor Factories bythe NumbersKEVOS® ORIGINAL BOOK10 CHAPTERS

About this book

A solar module on a factory roof is a large, flat semiconductor device. It turns part of the sunlight falling on it into direct current, and an inverter turns that into alternating current the factory can use. How much it delivers over a year depends on physics in the silicon, on the sun and the roof, on how the modules are wired, mounted and kept clean, and on how much of the output the factory uses itself. A proposal for a roof array condenses all of that into a few figures: kilowatts, kilowatt-hours a year, a price and a payback. This book shows how to check each of those figures.

It starts inside the cell, with photons, the bandgap and recombination, because those set the limits on efficiency and explain why output falls as the cell heats. It then works from the module datasheet to the yearly yield of a roof, through strings, voltage limits and the inverter, shading and bypass diodes, roof loads and wind, inverter heat and reliability, and monitoring, soiling and degradation. It finishes with the business case: self-consumption, tariffs, certificates, finance and depreciation.

It is for manufacturing, facilities and maintenance engineers, operations managers and owners. It assumes engineering or trade training and comfort with power, energy and simple algebra; the semiconductor physics is explained as it is used. Each chapter works the running case with real numbers, includes one plausible mistake and how it was found, links the KEVOS® tools that do the chapter’s calculations, and ends with an exercise and its worked answer.

The running case

Every chapter follows one array. This is an illustration. A stainless steel food-equipment fabricator in Shepparton, Victoria, runs a day shift and part of an afternoon shift in a 72 × 36 m portal-frame workshop with a trapezoidal steel deck roof at 5°, ridge running east to west. A single-storey office annex has a flat concrete roof. The business uses about 420 MWh of electricity a year, mostly between 7 am and 5 pm.

The array, called SF-90 in this book, is:

  • Modules: 216 monocrystalline silicon modules of 420 W each, 90.72 kW in total at standard test conditions. Each module is 1,722 × 1,134 mm with 108 half-cut cells and three bypass diodes.
  • Layout: 180 modules flush-mounted on rails on the north face of the workshop roof, and 36 on ballasted frames at 10° on the office annex roof.
  • Strings: 12 strings of 18 modules.
  • Inverters: two 40 kW three-phase inverters, each with three maximum power point trackers taking two strings, in a ventilated room beside the main switchboard.

Irradiation, prices, tariffs, test results and failure records are the business’s working figures or are calculated from stated assumptions. They do not describe KEVOS® work or any real business, installer, module maker, inverter maker, network or retailer. Prices are in Australian dollars excluding GST, because a GST-registered business claims the GST back on its costs; where a price includes GST, the text says so.

How to use the book

Read Chapters 1 to 3 first if the physics is new; they explain the module datasheet. Chapters 4 to 7 deal with designing and checking an array, Chapters 8 and 9 with running it, and Chapter 10 with paying for it. Chapters stand on their own once the case is familiar.

Each chapter links the KEVOS® calculators it uses. The tools do the arithmetic; the chapter explains what the inputs mean and where they come from. Every result in the text was checked by calculation, and the numbers in figures match the text.

Limits and responsibility

This book is educational. A grid-connected solar array is electrical installation work and must be designed and installed by appropriately accredited and licensed people, to the applicable Australian standards and wiring rules and to the local network’s connection requirements. Roof structures must be checked by a qualified structural engineer. Standards are named by number and scope only. Datasheet values, irradiation and costs in the case are illustrations; use measured data and the manufacturer’s documents for real work.

Who this book is for

Manufacturing, facilities and maintenance engineers, operations managers and owners who are buying, checking, running or paying for a solar array on a factory roof and want to judge the numbers in a proposal for themselves.

What you will learn

  • Explain how a silicon cell turns light into current and voltage, and why bandgap, recombination and temperature limit what it can deliver.
  • Read a module datasheet and an IV curve, and work out output at real irradiance and cell temperature.
  • Estimate the yearly yield of a roof array from its irradiation, orientation and loss chain, and check a supplier's yield claim.
  • Check string voltages against inverter limits, and judge shading, bypass diodes and hot-spot risk on a roof.
  • Check wind uplift and ballast, inverter cooling, spares and monitoring data, and build the business case with self-consumption and tariffs.

Inside this book

Chapters

  1. 01Sunlight, photons and the bandgapHow a silicon cell absorbs sunlight photon by photon, why its bandgap sets which light it can use and how much is lost as heat, and what limits module efficiency.
  2. 02The solar cell as a diodeHow a p–n junction separates light-generated carriers, how recombination sets the voltage, and how to read a module's IV curve, fill factor and resistances to find faults.
  3. 03Temperature and module outputWhy a silicon module loses power as it heats, how to estimate cell temperature from irradiance, air temperature and mounting, and how temperature changes yield and string voltage.
  4. 04Sun on the roof and the yearly yieldHow to turn solar irradiation, roof tilt and orientation into a yearly yield, build the loss chain and performance ratio, and check a supplier's yield estimate line by line.
  5. 05Strings, voltage limits and the inverterHow to size strings against an inverter's voltage and current limits at the coldest and hottest cell temperatures, choose the DC to AC ratio, and estimate cable, inverter and clipping losses.
  6. 06Shading, bypass diodes and hot spotsWhy shading one cell can cost far more than its share of the light, how bypass diodes and hot spots work, how to space rows to avoid shading, and how to find a shading fault.
  7. 07Roof loads, fixings and windHow to estimate the dead load of an array, the wind pressure and uplift on flush-mounted and ballasted modules, the loads on clamps and brackets, and the wind risk to modules during installation.
  8. 08Inverter heat, reliability and sparesHow to ventilate an inverter room so the inverters do not derate, read inverter failure records with confidence bounds, and decide whether a spare inverter is worth holding.
  9. 09Monitoring, soiling and degradationHow to monitor an array's performance ratio, check the irradiance sensor's calibration, measure soiling with a cleaning trial and a t-test, and separate soiling from module degradation.
  10. 10Self-consumption, tariffs and the business caseHow to value a factory array's energy by self-consumption and export, build the business case with NPV, IRR and payback, test it at a P90 yield and a shorter stay, and plan the loan.

Apply This in Practice

Use these KEVOS® tools to try the method from this reading with your own numbers.

References and source notes

The manuscript and worked examples are original KEVOS® writing. References inform particular concepts; their authors do not endorse this book. Web references were consulted on 1 November 2026.

  • J. Singh, Cambridge University Press. Electronic and Optoelectronic Properties of Semiconductor Structures (2003). Copyright work, used only as a map of ideas: bandgaps and why direct and indirect gaps absorb light differently, carrier generation and recombination, radiative and nonradiative recombination, the temperature dependence of intrinsic carriers and charge injection across a junction. No text, equations as set out there, figures or problems are reproduced, and all cases and calculations are KEVOS® originals.
  • W. Shockley and H. J. Queisser, Journal of Applied Physics 32, 510–519. Detailed balance limit of efficiency of p–n junction solar cells (1961). Named in Chapter 1 for the detailed-balance limit on the efficiency of a single-junction cell.
  • A. Richter, M. Hermle and S. W. Glunz, IEEE Journal of Photovoltaics 3(4), 1184–1191. Reassessment of the limiting efficiency for crystalline silicon solar cells (2013). Named in Chapter 1 for the limiting efficiency of a silicon cell once Auger recombination is included.
  • M. A. Green, Solar Energy Materials and Solar Cells 92, 1305–1310. Self-consistent optical parameters of intrinsic silicon at 300 K including temperature coefficients (2008). Source of the order of magnitude of silicon's absorption coefficient at several wavelengths, quoted as rounded values in Chapter 1.
  • E. Yablonovitch, Journal of the Optical Society of America 72, 899–907. Statistical ray optics (1982). Named in Chapter 1 for the light-trapping limit of a textured cell with a reflecting rear.
  • Bureau of Meteorology. Climate Data Online: solar exposure (2026). Named in Chapters 4 and 9 as a source of measured daily solar exposure for Australian sites. The irradiation figures in the case are assumed, labelled values, not taken from it.
  • Clean Energy Regulator. Small-scale Renewable Energy Scheme (2026). Named in Chapter 10 for small-scale technology certificates. Eligibility, thresholds and the number of certificates are set by the regulator and must be checked for a real project.

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