Gather site data that affects PV output
Solar irradiation in the area: why the same kWp does not produce the same output in every region
| Data to collect | Unit | Reference source |
|---|---|---|
| Annual irradiation | kWh/m²/year | PVGIS, solar atlas |
| Array azimuth | ° | Roof survey |
| Array tilt | ° | Roof survey |
| Horizon masking angle | ° | Site survey |
For a reliable output calculation, start from irradiation from a local reference source (such as PVGIS). Avoid using a single kWh/kWp ratio. Between two regions, the difference can range from simple to nearly double.
Orientation and tilt: measure the real impact on annual production
Measure azimuth with a corrected compass, or from plans and landmarks, then tilt with a clinometer. Feed both values into your solar panel production calculation. An east-west roof should be treated as two slopes, not averaged.
Obstructions and shading: identify losses before you quote a number
Map near obstructions (parapet, chimney, trees, neighboring building) and far obstructions (horizon line). Qualify the period (winter, morning, late day) and the string type (strings, optimizers). Without shading quantification, the estimate remains fragile.
Build the PV output calculation step by step up to kWh/year
Step 1: start from annual irradiation (kWh/m²/year) and usable area
| Zone (order of magnitude) | Irradiation on collector plane (kWh/m²/year) |
|---|---|
| North | 1 |
| Centre | 1 |
| South | 1 |
Measure irradiation on the module plane with a PVGIS-type tool, using the real orientation and tilt. Usable area is the sum of the installed module areas, not the roof area. Keep the kWh/m²/year value as the input to the calculation.
Step 2: convert to power (kWp) then estimate AC energy (kWh/year)
Convert to kWp from the module datasheets. P(kWp) = usable area (m²) x STC efficiency. Then run your solar panel production calculation. Theoretical energy (kWh/year) = P(kWp) x irradiation (kWh/m²/year) x 1 m²/kW.
Step 3: apply a realistic loss factor (cables, inverter, temperature)
| Loss item | Typical value (%) |
|---|---|
| Temperature | 5 to 10 |
| Inverter | 2 to 4 |
| DC+AC cables | 1 to 3 |
| Soiling and shading | 2 to 8 |
Translate these losses into a global PR coefficient, often between 0.75 and 0.85 depending on the site and equipment. The estimated AC energy becomes Energy = P(kWp) x irradiation x PR. This PR is easy to justify in technical review and in your calculation note.
Avoid the trap of a single ratio: from one region to the next, or from one installation to another, output can nearly double
Compare output order-of-magnitude by region without locking yourself into one fixed figure
When estimating output, avoid the reflex of saying “1 kWp = X kWh/year”. In practice, output varies strongly with latitude and loss assumptions. Provide an order of magnitude from weather data (PVGIS, Meteonorm), then adjust for the installation.
| Zone (examples) | South-facing 30° output (kWh/kWp/year) | East/West 10° output (kWh/kWp/year) |
|---|---|---|
| North | 900 to 1 40 | 760 to 920 |
| West | 1 000 to 1 200 | 850 to 1 020 |
| Centre-East | 1 150 to 1 350 | 980 to 1 160 |
| Mediterranean | 1 350 to 1 650 | 1 150 to 1 380 |
When orientation or tilt “eats” your margin on the kWh/year promise
The risk is not the installed power; it is the installation delta. In a solar panel production calculation, an unfavorable azimuth, low tilt, poorly ventilated array, or distant obstruction can quickly undermine your promise.
| Parameter | Typical impact on kWh/year (%) |
|---|---|
| East/West vs South | -10 to -20 |
| 10° tilt vs 30° | -3 to -10 |
| Untreated partial shading | -5 to -30 |
How to present a quantified range rather than an overly optimistic figure
State a range, for example P50 and P90, with written assumptions on the quote: location, azimuth, tilt, horizon, and global losses. You keep a contractual margin, and you can justify the gap against a low bidder selling a single ratio.
Turn production into photovoltaic sizing to propose the right system size
Move from the customer’s need (kWh/year) to the installed power (kWp) without oversizing
Start from the annual need and convert it into power by using a local output figure. The solar panel production calculation uses the site irradiation, orientation, tilt, shading, and system losses. Simple calculation basis. Installed power (kWp) = need (kWh/year) / output (kWh/kWp/year).
| Zone (France) | Indicative output (kWh/kWp/year) | Typical system losses (%) |
|---|---|---|
| North, East | 900 to 1 1100 | 10 to 20 |
| Centre, West | 1 050 to 1 250 | 10 to 20 |
| South | 1 200 to 1 600 | 10 to 20 |
Self-consumption, export, storage: what changes in sizing
For self-consumption, you size against the measured daytime load, not the annual total. For export, you usually make the decision based on the grid-connection limit and the PV field/inverter pairing. Storage does not create kWh. It shifts kWh, so the battery is sized on the load profile, not on peak power.
Check consistency with roof and grid-connection constraints
Validate usable area, technical zones, layout, shading, then structural consistency. On the network side, lock in connection capacity, single-phase or three-phase, protections, single-line diagram, and commissioning requirements. Document your assumptions; that is what stands up in counter-expertise.
Full practical case: estimate an installation from A to Z (2026 method)
Starting assumptions: local irradiation, orientation, tilt, shading, equipment
The solar panel production calculation starts from plane-of-array irradiation from a weather database (such as PVGIS), then you fix geometry and equipment. Example below, to be adapted after the shading survey and inverter selection.
| Parameter | Value | Unit |
|---|---|---|
| DC power | 6 | kWp |
| Plane irradiation | 1650 | kWh/m²/year |
| Orientation | South-east | |
| Tilt | 20 | ° |
| Shading (target loss) | 5 | % |
Detailed calculation: losses, final PV output and result in kWh/year
Stack the losses while keeping a coherent global PR. You then get a specific output, then annual energy.
| Item | Value | Unit |
|---|---|---|
| Temperature | 7 | % |
| Inverter | 2 | % |
| Cables and connections | 2 | % |
| Soiling | 3 | % |
| Resulting PR | 0.83 | |
| Specific output | 1330 | kWh/kWp/year |
| Annual energy | 7980 | kWh/year |
Customer handover: calculation table, assumptions, and limits to state in the quote
Attach the table to the quote and list what affects the result. Weather source, shading not finely modelled, grid unavailability, and calculation uncertainty of ±10 kWh/year. Add the module degradation assumed (for example 0.5%/year) and the exact scope of losses included.
