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Solar facade output is estimated by combining the wall surface that can actually carry modules, the modules’ DC rating, the facade’s true tilt and azimuth, local solar data, and time-varying shade. There is no reliable universal “kWh per square metre of wall” figure: two identical walls can produce very different energy in different locations or with different obstructions.
The defensible result is a monthly and annual AC-energy estimate from an hourly PV model. The workflow below shows how to measure usable area, convert it to array capacity, model irradiance on a vertical plane, apply shade and equipment losses, and report assumptions so another designer can reproduce the calculation.
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Use a location-specific PV model. First calculate the area where modules can physically be installed; then determine the array’s DC nameplate capacity from the selected module dimensions and rating. Enter the facade’s actual tilt and azimuth, local weather or solar-resource data, shade geometry, temperature assumptions, and electrical losses into a model such as NREL SAM/PVWatts or the European Commission’s PVGIS. Report the resulting monthly and annual AC energy, not just incident sunlight or DC array energy.
A simple screening expression is:
Annual AC energy ≈ active module area × annual facade-plane irradiation × effective conversion and system-performance factors.
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This is a check on the order of magnitude, not a substitute for an hourly model. Irradiation must be for the facade plane, and shade, temperature, clipping and losses vary by hour and season.
Which wall area should be counted?
Count only the surface on which modules can be mounted with the required clearances. Gross wall dimensions are a starting point, not the PV area.
- Measure the facade width and height for each uninterrupted plane.
- Subtract windows, doors, vents, louvers, parapets, roof overhangs, expansion joints and architectural features that cannot accept modules.
- Exclude maintenance and fire-access zones, structural exclusion areas, drainage paths and any section that cannot support the mounting system.
- Allow for module edge setbacks, gaps between modules, rails, cable routes and access around penetrations.
- Record the result as usable module-covered area, and keep it separate from the gross facade area.
For a wall with several orientations, calculate each plane separately. A north-facing section, a recessed bay and a projecting wing may have different sky view and shade even when they are part of one elevation.
PVWatts’ general sizing estimate does not account for a particular roof or facade’s tilt, azimuth or shading, so a generic wall-area multiplier should not be treated as a facade yield calculation. See the PVWatts Version 5 manual for the distinction between system-size inputs and production modeling.
How do I convert usable wall area into solar capacity?
Use the selected module’s physical dimensions and nameplate power whenever possible. The basic calculation is:
Module count = whole modules that fit within the usable layout
DC capacity (kW) = module count × module rating (W) ÷ 1,000
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If no module has been selected, an area-based estimate can be written as:
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Estimated DC capacity (kW) = usable module area (m²) × assumed module efficiency × 1 kW/m².
The 1 kW/m² term is the standard-test-condition reference irradiance used for rated power; it is not the sunlight a vertical facade receives over a year. PV module ratings are defined under STC, including 1,000 W/m² irradiance and a 25°C module-temperature reference, as described in the PVGIS 5 user manual.
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- Usable area: physical surface available for modules.
- DC nameplate capacity: the sum of module ratings under reference conditions.
- DC energy: modeled output before inverter conversion and downstream losses.
- AC energy: electricity delivered after inverter, wiring and other modeled losses.
How should a vertical facade’s tilt and azimuth be entered?
A true vertical facade is approximately 90° tilt from horizontal. Enter that actual geometry rather than borrowing a nearby roof’s optimized tilt. For a sloped or stepped wall, model each plane’s tilt separately.
State the azimuth convention in every calculation. PVGIS reports azimuth relative to due south, with east and west represented as negative and positive offsets in its documented convention. Other software may use clockwise degrees from north. Also state the hemisphere and whether the value describes the direction the facade faces or the direction of the surface normal. A sign or reference-direction error can reverse an east- or west-facing wall.
Record at least:
- Site latitude, longitude and elevation (if the model requests it).
- Facade tilt from horizontal.
- Facade azimuth and the software’s reference convention.
- Weather-data source and period.
- Whether the wall is flush, recessed, projected or partially obstructed.
Orientation changes when sunlight reaches the modules, not merely the total annual resource. A vertical plane can receive useful low-angle winter sun while losing midday summer irradiance compared with a roof. The model must transpose direct beam, sky-diffuse and ground-reflected radiation onto the facade plane.
What is plane-of-array irradiance, and why does it matter?
Plane-of-array (POA) irradiance is the solar irradiance incident on the module plane, reported in W/m² in SAM results. It is the relevant solar input for a facade; horizontal global irradiance or a roof-plane value is not interchangeable.
An hourly model calculates the sun’s position and separates radiation into beam, diffuse-sky and reflected components before transposing them to the wall’s tilt and azimuth. The resulting POA profile shows when the facade receives sunlight and how strongly. Monthly POA totals help explain seasonal output, but the hourly profile is needed when shade, temperature, inverter limits or time-of-use decisions matter.
NREL’s SAM location and resource guidance describes the weather and solar-resource inputs, while the SAM results documentation identifies POA irradiance and AC-energy outputs. Use those outputs to verify that the model is reporting the intended surface, not a default horizontal plane.
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How does shading change facade production?
Shade reduces direct-beam irradiance and can also reduce the diffuse sky contribution. On a facade, nearby buildings, trees, balconies, overhangs, fins, parapets, reveals and neighboring wings can shade the modules for only part of a day or season. A single annual shade percentage hides these timing effects.
Build a time-dependent shade profile whenever possible:
- Survey the surrounding objects, recording their height, distance, bearing and shape relative to each array surface.
- Separate objects that shade the beam from features that reduce the visible sky or create a local recess.
- Model the sun path by hour and month to determine when each surface is obstructed.
- Apply the shade profile to the correct subarray or electrical section, not to the whole building if only one bay is affected.
- Check whether bypass diodes, module-level electronics, string layout or mismatch assumptions alter the electrical consequence of partial shade.
SAM’s shading and layout documentation describes external-shade objects, beam losses and a diffuse-loss treatment. Its 3D Shade Calculator uses the project location, active array surfaces and surrounding objects to estimate time-dependent losses. A geometric shade mask is more informative than applying a constant loss to every month.
Document whether the model includes seasonal foliage, snow, dirt, facade reflections or future construction. If the shade survey is uncertain, run low, central and high-obstruction cases and show the resulting range instead of presenting one false-precision number.
How do I model temperature, electrical losses and inverter conversion?
After POA irradiance and shade are established, the PV model converts sunlight to DC output using module temperature and electrical characteristics, then applies wiring, mismatch, soiling and other system losses and converts DC to AC through the inverter. The exact loss categories and defaults differ by model, so list them explicitly.
Important inputs include:
- Module behavior: rated power, temperature coefficients and mounting configuration, if available.
- Temperature: ambient conditions, wind and the facade mounting arrangement. A ventilated facade and a close-mounted system can run at different temperatures.
- DC wiring and mismatch: cable resistance, string differences and any modeled availability or degradation assumptions.
- Soiling and snow: use a site-appropriate schedule; do not assume a roof default automatically represents a vertical wall.
- Inverter: conversion efficiency, DC-to-AC ratio, clipping behavior and availability.
- System losses: state each category and percentage or model setting rather than hiding them in one unexplained factor.
Use the model’s AC output as the energy delivered to the AC side under the stated assumptions. If you quote DC energy or POA energy, label it clearly so it cannot be mistaken for usable electricity.
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What inputs should a reproducible facade calculation show?
Put the assumptions in a compact record before presenting results. A useful template is:
| Input | What to record |
|---|---|
| Location and weather | Latitude/longitude, weather-data source and period, and any elevation setting |
| Facade geometry | Gross dimensions, usable module area, tilt and azimuth convention |
| Module layout | Module model, dimensions, count, orientation and DC rating |
| Capacity basis | Measured module count or explicitly assumed efficiency for an area estimate |
| Shade method | Survey or 3D objects, time resolution, diffuse treatment and seasonal assumptions |
| Thermal and loss settings | Mounting/temperature model, wiring, mismatch, soiling, snow and availability |
| Inverter | Model or efficiency curve, rating, DC/AC ratio and clipping settings |
| Software | PVGIS, PVWatts or SAM name and version, plus key model options |
Then report monthly AC kWh, annual AC kWh and, where useful, monthly POA irradiation and shade loss. State that these are modeled estimates; actual production varies with weather, installation quality, operating conditions and future obstructions.
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How can I calculate the result with PVWatts or SAM?
NREL SAM provides detailed PV modeling, while PVWatts offers a streamlined system-design path. The exact interface can change, so preserve a copy of the input file or a screenshot of every key setting.
- Define the site. Enter the project location and select an appropriate weather file. Confirm that the time zone and weather period are correct.
- Define the array. Enter the DC capacity derived from the module layout, not the gross wall area. For multiple facade planes, create separate subarrays when their geometry or shade differs materially.
- Set geometry. Enter approximately 90° tilt for a vertical plane, or the measured tilt for a sloped section. Enter azimuth using the selected software’s documented convention.
- Add shade. Use hourly shade factors or a 3D shade layout for buildings, trees, overhangs and recesses. Confirm that the shade applies to the intended subarray.
- Set module, temperature and losses. Replace generic assumptions with the selected module and inverter data where available. Record any defaults retained.
- Run and inspect outputs. Review POA irradiance, DC energy, inverter output, AC energy and losses by month. A plausible annual total with an implausible monthly pattern often indicates a geometry, azimuth, weather or shade-input error.
- Export the result. Save the model version, inputs, monthly table and sensitivity cases so a designer or owner can reproduce the estimate.
The SAM/PVWatts system-design guidance explains the capacity and system inputs. PVWatts is useful for a first pass, but its generic sizing view should not be used as a substitute for facade-specific orientation and shade modeling.
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How can PVGIS be used for an orientation-specific estimate?
PVGIS accepts installed peak power, system loss and module azimuth among its inputs and provides orientation-specific yield estimates. Enter the calculated DC peak power, the wall’s tilt and azimuth using PVGIS’s south-relative convention, and a documented loss assumption. For a complex facade, model each materially different plane separately, then sum the AC energy.
PVGIS is especially useful as an independent reasonableness check against a SAM/PVWatts run. Differences can arise from weather databases, transposition methods, temperature models and loss defaults. Do not average the two outputs without investigating why they differ; keep the model settings visible.
How should two facade options be compared?
Compare both options with the same location, weather basis, module and inverter assumptions, loss settings and reporting period. Otherwise a better-looking result may simply reflect different inputs.
- Compare usable module-covered area and the resulting DC capacity.
- Compare actual tilt and azimuth, with the same convention and hemisphere noted.
- Compare monthly and annual POA irradiation on each plane.
- Compare the hours and seasons affected by shade, including which subarrays are shaded.
- Compare temperature, wiring, mismatch, soiling, snow and inverter assumptions.
- Compare monthly and annual AC kWh, plus the sensitivity range for uncertain shade or weather inputs.
A facade with less area can still deliver more energy if its orientation and shade profile provide better POA irradiance. Conversely, a large wall can underperform when it is recessed, obstructed or electrically constrained. SAM exposes irradiance, power and shading results; PVGIS provides an orientation-specific yield path for consistent cross-checking.
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What common calculation mistakes should be avoided?
- Using gross wall area as capacity. Subtract unusable sections and account for module packing and clearances.
- Multiplying area by an unexplained kWh/m² figure. Require a location, facade plane, weather basis and model assumptions.
- Reusing a roof estimate. A roof’s tilt, azimuth and shade do not describe a vertical wall.
- Confusing STC irradiance with annual sunlight. 1,000 W/m² is a rating reference, not a facade’s yearly resource.
- Applying one shade percentage to the year. Shade is usually time-dependent and can affect beam and diffuse irradiance differently.
- Reporting DC as delivered AC. Include inverter and system losses before calling a value usable electricity.
- Mixing azimuth conventions. State the reference direction, sign convention and hemisphere.
- Hiding defaults. List weather file, temperature, loss, inverter and shade assumptions.
- Overstating precision. Use scenarios or ranges when obstructions, module selection or weather data are uncertain.
Can on-site irradiance measurements improve the estimate?
A pyranometer or solar-irradiance meter can provide plane-of-array measurements for an existing facade, but it does not calculate annual yield by itself. Measurements are useful only when their orientation, shading, soiling and snow conditions match the modeled array and when the data record is long and representative enough for the intended purpose.
SAM documentation discusses measured POA data and pyranometer inputs. Treat measurements as a calibration or validation aid, not a replacement for a shade survey, long-term weather resource and PV performance model. A short measurement campaign can capture unusual weather and should not automatically be extrapolated to a typical year.
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What should the final result look like?
A professional result starts with a one-line conclusion such as: “Under the stated location, vertical geometry, module layout, shade profile and loss assumptions, the model estimates [monthly table] and [annual AC kWh].” Follow it with the input table, monthly AC results, annual total, and sensitivity cases for the assumptions most likely to change.
Include the model name and version, links to the relevant documentation, and a note that the values are estimates rather than measurements. If the project is still at concept stage, show how the result changes when usable area, shade geometry or module selection changes. That makes the calculation useful for facade design decisions without pretending that a wall label alone determines production.
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How much electricity can solar panels on my wall generate?
The answer depends on location, usable module area, facade tilt and azimuth, shade, module rating, temperature and system losses. A PV model can provide monthly and annual AC kWh once those inputs are defined; wall area alone cannot support a reliable figure.
Is a vertical solar facade always worse than a roof array?
No. A vertical plane usually receives a different seasonal and daily solar profile, and its result depends on direction, latitude and obstructions. Compare plane-of-array irradiance and modeled AC energy with the same module, inverter, weather and loss assumptions rather than assuming one geometry wins everywhere.
What azimuth should I enter for an east- or west-facing wall?
Use the convention required by your software and state it explicitly. PVGIS uses offsets relative to due south, with east and west represented by negative and positive values in its documented convention; other programs may use clockwise degrees from north.
Can a solar meter tell me the annual output of a facade?
No. It can measure irradiance on the instrument’s plane, but annual electricity also requires module behavior, temperature, shade over time, inverter conversion and other losses. Measurements can support a model when their orientation and site conditions are documented.
Why do two PV calculators give different facade results?
They may use different weather databases, irradiance-transposition methods, temperature models, shade treatments or default losses. Keep the location, geometry, capacity, weather basis and assumptions consistent, then investigate those model differences instead of averaging unexplained numbers.
