Solar roof estimator
See roughly how many panels a commercial roof could take, what the array might generate in a year, and what to check on the roof before anything goes on it.
Generation figures follow the MCS method for estimating solar output. Whether the roof and structure can take an array is a question for a pre-solar roof survey and a structural engineer.
The roof
The area free for panels, and which way it faces.
Leave out edges, rooflights, plant and access routes. Don’t know the area? Measure the roof.
On a flat roof, east and west facing panels pack closer together. South facing rows produce more per panel but need gaps between rows so they don’t shade each other.
Estimate
An indication of what the roof could take and produce.
- Array size
- Panels
- Estimated generation
- Indicative added weight without ballast
- Ballast, where used
- about 5 to 10 kg/m² more
Check the roof first. An array is expected to stay in place for decades. If the roof covering has less life left than that, it will have to be lifted off for repairs or a recover, which costs far more than surveying the roof before it goes on.
Before panels go on
- Remaining life of the roof covering, against the life of the array. Roof condition survey
- Wet insulation under the membrane, which an array would hide. Moisture mapping
- The structure, for the array’s weight, ballast, wind and snow, confirmed by a structural engineer.
- The roof warranty, and whether the installation affects it.
- Drainage, so the array and its ballast don’t block outlets. Gutter and outlet checker
- Fire and access, with routes left clear for maintenance and the fire service.
These results are a guide only and don’t replace a survey or professional advice.
Sources
- MCS, MIS 3002, The Solar PV Standard (Installation), Issue 6.0, 18 March 2026, Appendix B, the standard estimation method and its table of postcode zones
- MCS, Irradiance Datasets, Version 2.0, the kWh per kWp for each postcode zone, pitch and orientation
- Renusol, FS Pro 10-EW System Datasheet, January 2024, for the east and west layout, its tilt and its weight
- Renusol, FS10-S, FS10-S XL and FS18-S XL System Datasheet, November 2022, for the south facing rows, their tilt and their weight
- Renusol, FS Pro 10-S System Datasheet, January 2024, its weight
- IBC Solar, Installation instructions IBC AeroFix / AeroFlat, Version 19.01, June 2019, its weights and the ballast to expect
- Ernst Schweizer, Product brochure MSP-FR-EW, December 2023, its typical roof surface load
- Esdec, Manual ClickFit EVO hanger bolt, for the gaps between panels on a pitched roof
- LONGi, LR5-54HTH 420-440M datasheet, for the panel the estimator starts with
For panel sizes other than the default, the spacing between rows is scaled in proportion to the panel’s size.
How to use it
Enter the usable area
The roof area free for panels, once edges, rooflights, plant and access routes are left out. The roof measurement tool gives the overall area.
Describe the roof
Its postcode area, whether it’s flat or pitched, and how the panels would face. Change the panel figures to match a quote if you have one.
Read the estimate
The array size, the number of panels and a yearly generation figure, with the checks to make on the roof before an installer starts.
What it tells you, and what it cannot
The estimate is for planning a conversation with an installer, not for signing one off.
It tells you
- Roughly how many panels the usable area could take
- The array size in kilowatts peak
- An estimate of yearly generation for the location, layout and pitch
- What to check on the roof before the array goes on
It cannot tell you
- Whether the structure can carry the array, which is a structural engineer’s question
- Shading from nearby buildings, trees or plant on the roof
- How much of the power the building would use itself, or what it would save
- The condition of the roof the panels would sit on
Questions about solar on commercial roofs
Why survey the roof before installing solar?
An array is fixed to the roof for many years. If the covering fails during that time, the panels have to come off before it can be repaired or replaced, and that work costs far more than checking the roof first. A survey shows whether the roof should be repaired or recovered before the installer arrives.
How is the yearly generation worked out?
The tool follows the MCS method: the array size multiplied by a figure for the location, the direction the panels face and their pitch. The MCS figure assumes no shading, so a real roof with plant or nearby buildings will produce less.
East and west, or south facing?
On a flat roof, panels laid east and west in low pairs fit more onto the same area, because they need little gap between rows. South facing panels produce more each, but have to be spaced so each row doesn’t shade the next. Which gives more power overall depends on the roof.
How much weight does an array add?
It depends on the system. Ballasted systems on flat roofs use weights to hold the array down instead of fixings through the membrane, and add more load than fixed systems. A structural engineer confirms whether the roof can carry it, including wind and snow.
Does solar affect the roof warranty?
It can. Fixings through the membrane, ballast on it and foot traffic during installation can all affect what a roof manufacturer’s warranty covers. Check with the warranty holder before the installation is designed.
Get the right survey for your roof
Whether you’re investigating a problem or planning ahead, we’ll help you choose the right survey for your roof. Get in touch for a free quote.
Pre-solar roof surveys
The roof’s condition, moisture and remaining life, checked before an array goes on.
About roof condition surveysAll measurements, calculations and estimates provided by Roofcheck’s online tools are for guidance purposes only and should not be relied upon as definitive or technically accurate. Results should be independently verified through a professional roof survey or assessment before undertaking any works or making decisions. Roofcheck accepts no liability for any loss, damage or costs arising from reliance on these results, to the fullest extent permitted by UK law.