Installing commercial solar on a flat roof is one of the most rational investments a business can make in its own property: the large flat roof of a factory, warehouse or retail centre usually sits empty, yet it can become an energy-generating asset that pays back in 5–8 years and then keeps cutting the company’s electricity bills for another 15–20 years. In this guide we treat a commercial solar plant not only as an energy project but as a roofing project: how many kWp fit a given roof area, how to calculate payback, how ballasted and penetrating mounts differ on large roofs, why the structural load capacity and waterproofing condition must be checked before installation, how to coordinate warranties and why self-consumption usually beats feeding power back to the grid for a business.
This article focuses on the business decision — scale, ROI and roof readiness for large commercial buildings. If you want the step-by-step technical detail of how modules are fixed to a flat roof, read our separate solar plant foundation article. And if you first want to confirm whether a specific roof is even suitable for a photovoltaic system, see our guide on which roof is suitable for solar panels. Here we concentrate on the commercial logic.
Why commercial solar on a flat roof pays off
Commercial buildings have three features that make solar pay back faster than it does for private homes. First, the large flat roof area allows a powerful system without buying extra land. Second, a company’s electricity demand peaks during the day — exactly when the sun generates most — so almost all the energy is consumed on site. Third, businesses buy electricity at higher prices and in larger volumes, so every self-generated kilowatt-hour delivers a tangible saving.
A flat roof is an almost ideal base for this. Modules sit on tilted support frames and can be aimed precisely south at the optimum angle, regardless of how the building itself is oriented. Ballasted mounting often avoids penetrating the membrane, preserving the integrity of the waterproofing. But precisely because of the large scale, two things that homeowners often overlook become critical in commercial projects: the structural load capacity and the remaining service life of the roof covering.
How many kWp fit: sizing by roof area
A quick rule-of-thumb calculation lets you estimate the system size before any design work. On a flat roof modules are laid out in rows with gaps so the rows don’t shade each other, so roughly 50–60 % of the total roof area is actually usable. On average, 1 kWp occupies about 5–7 m² of roof (including row spacing). In Lithuanian conditions 1 kWp generates around 950–1000 kWh per year.
| Roof area (m²) | Approx. capacity (kWp) | Annual yield (kWh) | Typical building |
|---|---|---|---|
| 500 | ~50 kWp | ~48,000 | Small warehouse, workshop |
| 1,000 | ~100 kWp | ~96,000 | Medium production hall |
| 2,500 | ~250 kWp | ~240,000 | Retail centre, logistics |
| 5,000 | ~500 kWp | ~480,000 | Large warehouse / factory |
| 10,000 | ~1,000 kWp (1 MW) | ~960,000 | Industrial complex |
These are indicative figures only — the exact number depends on roof geometry, parapets, ventilation stacks, rooflights and existing equipment that “eat into” the usable area. A real project always starts from the roof plan and an on-site survey. For more on how large flat roofs are built and maintained, see our flat roof installation article.
Payback and ROI: calculating the commercial benefit
For a business the key metric is how many years the investment takes to pay back and what the return (ROI) will be. The installed cost of a commercial solar plant in Lithuania in 2026 typically ranges around €700–1000/kWp (excl. VAT; the per-kWp price falls in larger systems). Payback is driven by three things: the size of the investment, the share of electricity consumed on site, and the electricity price you thereby avoid paying.
| Capacity | Approx. investment | Annual saving (at ~80% self-consumption) | Payback |
|---|---|---|---|
| 50 kWp | ~€40,000 | ~€6,000–7,000 | ~6–7 years |
| 100 kWp | ~€80,000 | ~€12,000–14,000 | ~6–7 years |
| 250 kWp | ~€190,000 | ~€30,000–34,000 | ~6 years |
| 500 kWp | ~€360,000 | ~€60,000–68,000 | ~5–6 years |
The numbers are indicative and depend on the electricity price, consumption profile and any available funding (such as investment support for commercial solar). The point is this: when a large share of the generated electricity is consumed on site, payback for a business usually falls within 5–8 years, after which the system generates electricity almost for free for another 15–20 years (only maintenance and an inverter replacement remain). But this calculation only holds if the roof lasts as long as the modules — so roof preparation is part of the ROI, not a separate cost.
Planning commercial solar on your roof?
We assess the roof's structural load capacity and waterproofing condition, renew the covering where needed and prepare the roof for solar so the system lasts as long as the modules do. Up to 20-year warranty on roofing work.
Self-consumption or feed-in: what is better for a business
This is one of the most important economic decisions. For a business, consuming self-generated electricity (self-consumption) is almost always more beneficial than selling it to the grid, because you avoid paying the full electricity price with all taxes and grid charges, whereas exported electricity is bought back at a much lower rate. The practical rule: the higher the self-consumption share, the faster the payback.
Commercial buildings have an advantage over homes here — factories, refrigerated warehouses, offices and retail centres use electricity precisely during the day, when the sun is generating. The system is therefore often sized to match daytime demand rather than the maximum roof area. If you want to use surplus in the evening, a storage battery can be considered, but its payback is calculated separately. Either way, the system scale must be matched to the company’s real consumption profile, not just to the roof size.
Ballasted or penetrating mounting on large roofs
The choice of mounting method is the most important roofing decision in the whole project — it determines both the reliability of the covering and the load placed on the structure. On large commercial roofs both systems have their place.
Ballasted mounting
Modules are held down by weight (concrete blocks or ballast trays), without penetrating the membrane. This is a big advantage for the waterproofing — there are no penetrations through which water could enter. The downside is significant added load (20–40 kg/m²), so ballast suits only a sufficiently strong structure. On large roofs ballast is often the first choice when the load capacity allows.
Penetrating (mechanical) mounting
Fixings are mechanically anchored through the membrane into the load-bearing structure, with the penetrations sealed watertight. The system is much lighter, so it suits weaker structures (such as trapezoidal steel-sheet roofs or timber rafters). The trade-off is critical sealing precision: every penetration is a potential leak point, so the work must be done by an experienced roofer. On PVC or TPO membranes, weldable fixing elements are often used to reduce both ballast and the number of penetrations.
| Criterion | Ballasted | Penetrating |
|---|---|---|
| Waterproofing risk | Low (no penetrations) | Medium (depends on sealing) |
| Added load | High (20–40 kg/m²) | Low (12–18 kg/m²) |
| Suits structure | Strong (reinforced concrete) | Light (steel sheet, timber) |
| Wind resistance | Set by ballast weight | Set by fixing points |
| Installation speed | Faster | Slower (precise sealing) |
The final decision is made by the roofer together with the solar installer and structural engineer, having assessed the load capacity, covering type and local wind load. For the underlying flat-roof covering build-up, see our flat roof covering section.
Roof load capacity and structural check
This is a critical and often-forgotten aspect of commercial projects. A solar plant adds a permanent load that acts on the structure for 25+ years. A ballasted system adds 20–40 kg/m², a penetrating one 12–18 kg/m². This weight is summed with the existing loads: the structure’s own weight, snow load (1.2–1.6 kN/m² in Lithuania depending on region) and wind.
Many commercial buildings — especially warehouses and production halls — have light trapezoidal steel-sheet or steel-truss structures designed with a minimal load reserve. For such roofs, added ballast can exceed the design capacity. A structural load capacity assessment by an engineer, based on the building design or an on-site survey, is therefore essential before installation. Three solutions are possible: lighter penetrating mounting instead of ballast, structural strengthening, or reduced module density in certain zones. This step cannot be skipped — an overloaded structure poses both structural and safety risks.
Remember that the solar load is not just dead weight: a ballasted array also changes how snow and wind act on the roof. Modules trap drifting snow between rows and create wind-suction zones at their leading edges, so the engineer must combine the ballast with snow and uplift loads in the worst-case combination – not simply add 20–40 kg/m² to the existing figures.
Waterproofing first: why the roof must be ready before the modules
This is perhaps the most important message of this article for businesses. A solar plant completely covers the roof membrane beneath it for 25–30 years. If the covering only has 5–8 years of life left, it will start to leak within a few years — and then, to re-cover the roof, you have to dismantle the entire plant, store it safely, renew the covering and reinstall and reconnect the system. For a large commercial system this downtime and double work can cost tens of thousands of euros.
So the rule is simple: the covering under a solar plant must have a remaining service life similar to the modules (at least 20–25 years). A practical assessment:
- New or nearly new covering — you can install immediately.
- Middle-aged covering (10–20 years left) — it often makes sense to re-cover so the roof and solar cycles align.
- Old or defective covering — the roof must be renewed first; installing over an old covering programs in an expensive dismantling.
How to recognise an ageing covering (blisters, cracks, lost mineral granules, leaks) is described in our bituminous roof repair article. And if it turns out the whole roof needs renewing, you will find the basics in our roof renovation section. The difference between a simple re-cover and a deep renovation is explained in our re-covering vs renovation comparison.
Covering for a commercial solar roof: what to choose
For large commercial roofs carrying solar, one of these waterproofing systems is usually chosen:
- Torch-on SBS bituminous membrane — reliable, durable and easy to repair locally; with ballasted mounting there is no need to penetrate the covering. More in our bituminous flat roof covering section.
- PVC and TPO membranes — popular for large areas: light, quick to lay, and they allow fixing elements to be welded on, so less ballast is needed (an advantage for light structures).
- EPDM membrane — elastic and durable; mounting is usually ballasted or with bonded elements.
Which covering suits a specific building depends on the structure type, mounting method and budget. We provide commercial roof installation and covering prices in our flat roof cost price lists, and torch-on membrane prices in the torch-applied covering price list.
Coordinating warranties: roof plus solar plant
In a commercial project several warranties meet: the modules (typically a 25–30 year output warranty), the inverter (5–12 years), the installation work and — separately — the warranty on the roof covering and waterproofing work. For a business it matters that these warranties don’t clash. A common problem: if one company installs the solar plant by penetrating the membrane while another provided the roof warranty, each points at the other when a leak appears.
It is therefore recommended that one responsible roofer coordinate the roofing work and the solar mounting details, and that the mounting solution (ballast or penetrations) be aligned with the covering warranty terms. Eurokas provides up to a 20-year warranty on roofing work, so it is worth planning the solar plant so that the roof and plant work are coordinated. This protects against the “warranty gap” where no one is responsible for a leak at the modules.
How a commercial solar installation proceeds
A business solar project follows a clear sequence — from roof audit to commissioning. Below is a typical flow in which roof preparation is integrated into the overall process.
How to install commercial solar on a flat roof
Roof audit and structural check
The roofer and engineer assess the covering’s condition, remaining service life and the structure’s load capacity. They determine whether the roof can be used immediately or whether the covering must be renewed or the structure strengthened first.
Roof preparation and, if needed, covering renewal
If the covering is old or the structure weak, the waterproofing is renewed and the structure strengthened. This step is done first, so the already-installed plant doesn’t have to be dismantled later.
Sizing the system to consumption
The optimum capacity (kWp) is calculated from the roof area and the company’s daytime consumption profile; the orientation, row layout and mounting method (ballast or penetrations) are selected.
Coordinating mounting details and waterproofing
The roofer and solar installer coordinate the mounting details so the waterproofing is not damaged and drainage is not obstructed. Cable runs and the inverter location are planned.
Installation and connection
The support frames, modules and inverter are installed and the system is connected to the grid. Watertightness at every penetration is checked.
Commissioning and warranty registration
The system is commissioned and output is verified. The coordinated roof and plant warranties are registered and a maintenance instruction is handed over.
Drainage and maintenance with solar
On a large commercial roof the modules must not obstruct water flow. Rows are laid out so they don’t block the outlets and downpipes, and walkways are left between rows for inspection and cleaning. If modules block the drainage, ponding forms, the covering ages faster, and extra snow load accumulates in winter. The drainage scheme is therefore assessed during design.
Maintenance of a commercial roof with solar means periodic inspection of the covering (at mounting details and parapets), cleaning the drainage and checking the modules. A flat roof with safe walkways can be walked on, so inspection is simple — the key is that it is regular. We cover typical flat-roof defects and their prevention in our bituminous roof repair article.
New building: plan for solar at the design stage
If you are building a new commercial building — a warehouse, factory or retail centre — the best moment to think about solar is the design stage. By providing for the system in advance, the structural engineer immediately includes the added ballast load in the calculations, so the deck does not have to be strengthened later. At the same time, cable runs, the inverter location and the mounting zones are planned so they don’t damage the waterproofing or obstruct drainage.
Such a “solar-ready from the start” roof is more reliable and cheaper in the overall calculation: no separate strengthening work is needed, and the mounting details are integrated into the roof build-up. If the building is designed with a future plant in mind, it is worth discussing this with the roofer before choosing the covering type. For more on compact flat-roof solutions, see our compact flat roof installation guide, and for a broader view of commercial roofs in Lithuania, our flat roof services section.
Common mistakes in business solar projects
Below are the most common mistakes that slow the payback of commercial solar or cause roof problems:
- Installing over an old covering — within a few years you must dismantle the whole plant and re-cover the roof; the most expensive mistake.
- Not assessing the load capacity — ballast overloads a light steel-sheet or truss structure.
- Sizing by area rather than consumption — excess capacity sold cheaply lengthens the payback.
- Uncoordinated warranties — when a leak appears, the roofing and plant contractors point at each other.
- Blocking drainage with modules — ponding and faster covering ageing.
- Penetrating the covering without proper sealing — leaks already in the first years.
Most of these mistakes are avoided when the roof audit is done before installation and one responsible roofer coordinates the roof and solar work. You will find all services in our contacts section.
Frequently asked questions about commercial solar on a flat roof
When a large share of the generated electricity is consumed on site (usually 70–90% in commercial buildings), payback typically falls within 5–8 years. After that the system generates electricity almost for free for another 15–20 years. The exact period depends on the electricity price, consumption profile and investment size.
On average 1 kWp occupies about 5–7 m² of roof (with row spacing), and roughly 50–60% of the total area is actually usable. So a 1,000 m² roof fits about 100 kWp, and a 5,000 m² roof about 500 kWp. The exact number depends on parapets, ventilation, rooflights and existing equipment.
Ballasted mounting does not penetrate the covering and reduces leak risk, but adds 20–40 kg/m² — it suits a strong structure. Penetrating mounting is much lighter (12–18 kg/m²) and suits light steel-sheet or timber structures, but requires precise sealing. The decision is driven by the structure’s load capacity.
If the covering’s remaining service life is shorter than the modules’ (less than 20–25 years) or there are defects, the roof must be renewed first. Otherwise, within a few years you will have to dismantle the whole plant, re-cover the roof and reinstall — which for a large system costs tens of thousands of euros and causes downtime.
Not always. Trapezoidal steel-sheet and steel-truss roofs are often designed with a minimal load reserve, so a ballasted system can exceed the design capacity. For such roofs a lighter penetrating mount is usually chosen, or the structure is strengthened. An engineering load capacity assessment is essential.
Self-consumption is almost always more beneficial, because it lets you avoid paying the full electricity price with taxes and grid charges, while exported electricity is bought back much more cheaply. Commercial buildings have an advantage because they use electricity during the day — exactly when the sun is generating.
It depends on how the warranties are coordinated. If one responsible roofer does the roofing work and the mounting details, liability is clear. If another company installs the plant by penetrating the membrane, a “warranty gap” can arise. That is why it is recommended to coordinate the roof and solar work through a single contractor.
In Lithuania in 2026 the indicative installed cost is around €700–1000/kWp (excl. VAT), with a lower per-kWp price in larger systems. The cost of preparing or renewing the roof may be added if the covering is old — so we always give an exact quote after a roof survey.
Leave the roof preparation to the professionals
Commercial solar only pays off when the roof beneath it lasts as long as the modules. Eurokas assesses the condition of the commercial roof covering and the structure’s load capacity, renews the waterproofing or strengthens the structure where needed, and prepares the roof for solar so the mounting details don’t damage the covering and the warranties are coordinated. We work with factories, warehouses and retail facilities throughout Lithuania and Europe and provide up to a 20-year warranty on roofing work. Review all our services or get in touch for a commercial roof survey on +370 698 013 23.
