Understanding "surface area" and "default output" in photovoltaics: what are we really talking about?
Usable surface area vs total surface area: what changes in the roof layout
In photovoltaics, the total surface area describes the roof section "on paper." The usable surface area, on the other hand, corresponds to what you can actually install after safety margins, edges, ridges, access zones, and obstacles (mechanical ventilation, roof windows, chimney). It's this usable surface area that drives the layout and the number of modules, sometimes with spacing required by the mounting system and ventilation.
Nominal output (Wp), efficiency, orientation: the basics that affect sizing
A module's output is given in Wp, measured under standard test conditions. On the roof, production also depends on the panel's efficiency, temperature, shading, and then orientation and tilt. The same Wp figure can produce very differently depending on the site. This is where sizing and inverter choice come into play.
Why a "default" value exists: starting assumptions and the risk of error if you keep them as-is
Software tools set a "default" output to get started quickly. They often assume a favourable orientation and "average" losses. Handy, but risky if you don't correct it with a site visit, photos, and the real constraints. Otherwise, you overestimate production, or propose an output that's impossible to actually install.
Photovoltaic sizing: field methods to avoid over- or undersizing
Starting from usage (kWh/year) and the consumption profile: self-consumption, selling, a mix
Start with the bills. Note the kWh/year and, if possible, the Linky hourly curve. In self-consumption, proper sizing follows daytime usage (domestic hot water, heat pump, EV). For selling everything back, the focus is mainly on annual production and orientation. For a mix, set a target self-consumed share, then adjust the output to avoid an unrewarding surplus.
Translating into a number of panels and surface area: simple rules + points to watch (shading, constraints)
Translate the target output into panels. Example: 3 kWp is often 7 to 8 modules of 400 to 450 W. For surface area, count on around 1.7 to 2.2 m² per panel. Check for shading (chimneys, trees), ventilation zones, load-bearing capacity, and planning limits before locking in the layout.
Securing electrical consistency: inverter/micro-inverters, strings, protections, safety margin
Ensure DC/AC consistency. A central inverter needs homogeneous strings and a voltage within the MPPT range. Micro-inverters help when orientations differ. Plan for an isolator, surge arrester, earthing and AC protections according to current regulations. Keep a margin on output and currents to limit trips and overheating.
"Default" parameters: settings to check before sending a preliminary quote
Unit module output: 2026 standards and the immediate impact on the stated surface area
Before quoting, lock down the unit output you're using. In 2026, many residential projects use modules around 400 to 500 Wp. If your tool is still set to 330 or 375 Wp, the stated surface area balloons and the client thinks "it won't fit." Conversely, overestimating output shrinks the surface area and throws off the budget, logistics and layout. Keep an up-to-date reference library, with actual Wp and dimensions.
Tilt, azimuth, obstructions: how a default assumption skews estimated production
A preliminary photovoltaic quote can look "good" while the assumptions are wrong. A roof at 15 degrees doesn't have the same output as a 35-degree pitch. An East or West azimuth lowers annual production and changes self-consumption. And obstructions (trees, chimneys, neighbouring buildings) can hurt the curve without it showing on an annual average. It's better to enter a cautious assumption and state it clearly. To go further on these settings, see tilt, azimuth, obstructions.
Losses and assumptions: cables, temperature, soiling, ageing, grid availability
Check the default losses. Between cables, inverter, temperature, soiling and ageing, a simulation that's too "optimistic" costs you when it comes time to meet client expectations. Add a margin for grid unavailability and injection limits when relevant. In practice, document two sets of parameters: "standard" and "degraded." You stay in control, without overselling the sunshine.
How Argile saves you time on photovoltaic sizing (without losing reliability)
Quick energy assessment: PV scenarios consistent with the whole-house retrofit project
With Argile, you set the framework in a few minutes. You test photovoltaic scenarios consistent with the whole-house retrofit. After insulation and a heating system change, consumption shifts. Argile helps you adjust the target output and self-consumption without starting from a blank sheet.
Feasibility analysis: identifying address-specific constraints (roof, environment) and prioritising
At the address, Argile quickly identifies the points that waste time. Usable surface area, orientation, shading, roof constraints, urban context. You know earlier whether PV is straightforward, borderline, or should be ruled out. The result: a quick decision and clear priorities for the technical site visit.
Quotes and grants: preliminary pricing, MaPrimeRénov'/CEE integration when relevant, and securing the client conversation
Argile generates a preliminary price estimate and a readable quote. When MaPrimeRénov' and CEE apply to the rest of the works bundle, the impact is factored in. You secure the client conversation, particularly on the grants actually available in 2026, and keep a controlled margin.
RGE tradesperson checklist: validating surface area, output and assumptions before signing in 2026
Questions to ask during the technical site visit: roof, access, structure, panel, uses and future changes (heat pump, EV)
Check the actual heated surface area, room by room. Confirm the condition of the roof, access, load-bearing capacity, and whether future photovoltaics or a buffer tank are planned. On the electrical side, look at the panel, earthing, available space, and plan ahead for a heat pump and EV charging.
Documents and evidence to keep: photos, measurements, obstruction notes, diagrams, compliance criteria
Keep dated photos before, during, after. Note the measurements (surface areas, thicknesses, references), obstructions and obstacles, and a simple diagram of the networks. Also file away compliance evidence and manuals, useful for audits.
Common errors linked to default values: how to spot and fix them quickly
Classic defaults come from an overestimated surface area, a forgotten setpoint, or a future use that wasn't factored in. Cross-check your assumptions against the bills, the measured volumes and the real needs. Correct them before quoting, otherwise output and grants can go off the rails. To avoid confusion, make sure to clarify the difference between living area and heated floor area right from the technical site visit.
Key figures
0.45 kW
Default output
18 to 22%
Cell efficiency
1.0 m²
Default surface area
Frequently asked questions
Use a realistic baseline: 400 to 450 Wp per module for residential projects, with a unit surface area of around 1.7 to 2.2 m² (depending on the 108/120/144 half-cell format). Adjust as soon as you have the exact model, since a 0.1-0.2 m² gap per panel can cost you a whole module on a roof section. Also lock down the number of modules per row based on the mounting system and the ventilation spacing.
Louis Airy
COO of Argile
