While machinery, cooling or ventilation systems operate inside a factory, the roof above them could supply part of their electricity. For the owner, the question is: how much of that potential can become measurable business value?
The answer starts by separating three things: roof area, solar capacity and the value of the electricity generated.
From 2,000 m² of roof to module area
Imagine an industrial building with 2,000 m² of gross roof area. Skylights, ventilation and other equipment occupy some of the space. Maintenance access, safety requirements, tilt and shading also shape the layout. A 2,000 m² roof therefore does not mean 2,000 m² of modules.
Consider two hypothetical layouts: 1,000 m² of modules in the first and 1,300 m² in the second. These are assumptions for this example, not recommended roof coverage ratios.
| Indicator | Scenario A | Scenario B |
|---|---|---|
| Gross roof area | 2,000 m² | 2,000 m² |
| Module surface area | 1,000 m² | 1,300 m² |
| Share of gross area | 50% | 65% |
| Rated DC capacity | 210 kWp | 273 kWp |
| Annual generation | 231,000 kWh | 300,300 kWh |
Module surface area differs from the footprint of the complete array. Tilted modules and gaps between rows require a geometric layout to establish the space needed. Neither scenario has been confirmed as technically feasible.
How does area become capacity?
At an assumed module efficiency of 21% and test irradiance of 1,000 W/m², each square metre of module provides 210 W of rated capacity. Thus 1,000 m² corresponds to 210 kWp, and 1,300 m² to 273 kWp. Standard test conditions include a module temperature of 25°C. [1, section 4.1]
1,000 m² × 1 kW/m² × 21% = 210 kWp
210 kWp × 1,100 kWh/kWp per year = 231,000 kWh per year
kWp describes module rated capacity; kWh measures energy. The 21% efficiency is an assumption. An actual project uses the selected manufacturer's specifications. Rated capacity does not mean constant operating power.
Why 231,000 kWh is not yet your site's forecast
Applying the same annual yield of 1,100 kWh per kWp gives 231,000 and 300,300 kWh for the two scenarios. The difference is 69,300 kWh per year, under that shared assumption.
This yield was not obtained from a simulation of a known location. A real estimate must reflect location, orientation, tilt, temperature and losses. The European Commission's PVGIS supports such estimates and reports year-to-year variability. Local obstructions and shading between modules also require assessment. [1]
When comparing proposals, request the inputs and monthly generation profile. An annual total alone does not show winter output, weekends or factory shutdowns.
Three checks before a roof becomes a power plant
1. Structural capacity and roof condition
Review the covering's age and condition, the structure and mounting compatibility. U.S. Department of Energy guidance highlights structural capacity, roof and mounting compatibility, and wind exposure. Qualified specialists must assess a Croatian site against the requirements that apply to it. [2, Rooftop PV Systems]
A practical question: if refurbishment is due soon, how should its timing and cost be coordinated with the solar installation? Resolve this before the final equipment choice.
2. Connection and export capability
Module capacity does not establish how much power the site may export. HEP ODS publishes connection forms and procedures; the applicable procedure and permitted export must be established for the proposed project. [3] Keep module capacity separate from grid connection conditions.
3. Timing of consumption
In a model without storage, electricity used directly within each interval cannot exceed either solar output or site consumption in that interval. Compare both profiles over matching intervals. Annual bills alone cannot demonstrate a high direct self-consumption share.
The same system, a €4,620 annual difference
Return to 210 kWp and an assumed 231,000 kWh per year. For comparison, assume €0.14/kWh of genuinely avoided variable purchasing costs and €0.06/kWh for exported electricity. These are not current tariffs, a quotation or a price forecast.
| Energy flow | 75% self-consumption | 50% self-consumption |
|---|---|---|
| Direct use | 173,250 kWh | 115,500 kWh |
| Grid export | 57,750 kWh | 115,500 kWh |
| Gross annual value | €27,720 | €23,100 |
At 75% direct self-consumption, 173,250 kWh is used on site, worth €24,255, and 57,750 kWh is exported, worth €3,465. The total is €27,720. At 50%, the total falls to €23,100.
The €4,620 difference arises because another 57,750 kWh avoids purchases at €0.14 instead of being exported at €0.06. Generation stays the same; how the energy is used changes. HEP Opskrba explains the contractual framework for surplus purchases. The specific terms must be checked in the contract. [4]
These figures show gross annual energy value, not net profit or investment payback. They exclude capital costs, financing, maintenance, insurance, degradation, equipment replacement and taxes. Fixed and other bill components do not automatically disappear. Prices are applied uniformly to all kWh; actual settlement may depend on timing and contract terms. Export assumes suitable technical and contractual conditions.
Should you cover the whole roof?
For the larger layout, ask what the additional 69,300 kWh contributes relative to the additional investment. If all of that extra electricity were exported at the assumed €0.06/kWh, its gross annual value would be €4,158. This is a separate calculated scenario, not a forecast of the larger system's operation.
Compare feasible layouts against time-based consumption profiles. Phased construction, rescheduling certain loads or storage may merit investigation. Each option requires its own assessment of additional investment and losses.
For more on system sizing, see How much solar capacity does your business actually need?
What should you prepare for a useful first discussion?
- Site and roof: drawings or dimensions, photographs, and information about the covering's age and structure.
- Consumption: the last 12 months of bills, available interval data, shifts, non-working days and seasonal shutdowns.
- Connection: existing documentation and approved capacities.
- Plans: new machinery, electrification, EV charging or working-hour changes that may affect consumption.
A roof's value emerges when its area is connected with how the business operates. A sound initial assessment should explain feasibility, expected generation and the assumptions behind its business value.
Explore your roof’s energy potential
Do you own an industrial building, logistics centre or larger commercial property? Tell us about the location, consumption and investment objective. EMISSIO TRADE develops initial project concepts, compares technical and economic options and coordinates the involvement of appropriate specialist partners.
Discuss your project with us →Sources and methodology
All numerical scenarios are editorial calculations based on explicitly stated assumptions. No site-specific PVGIS simulation was carried out. Roof coverage, module efficiency, annual yield and prices are not presented as market standards or guaranteed results. The U.S. source supports technical principles, not interpretations of Croatian regulations.