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For a business, choosing between south-facing and east-west solar starts with a practical question: when does the site need electricity? Higher annual generation, more installed capacity on the roof and greater direct consumption can favour different designs. A useful comparison needs a common basis: the same roof, weather data, business load profile and clearly stated technical constraints. Here is how to define that comparison and assess the recommendation in a project proposal.
Define what the orientation should improve
Start with the decision criterion. For a factory operating daytime shifts, it may be the amount of solar electricity used directly by equipment as it is generated. For a building with limited usable roof space, the number of modules that actually fit also matters. Where surplus electricity can be exported, assess the applicable arrangements separately. An exported kilowatt-hour and one that avoids a grid purchase do not necessarily have the same value.
Agree on the main measure before receiving calculations. Otherwise, one supplier may compare annual generation, another installed capacity and another the self-consumption percentage. Each figure could be correct while answering a different question. A useful decision summary shows available generation, direct consumption, remaining grid purchases and potential surplus together.
Orientation does not remove the need to check system size. If demand is low relative to both options, adding modules may mainly increase surplus. Establish a suitable capacity range before comparing the layouts within it.
How direction changes the generation profile
Under suitable conditions, south-facing rows produce a more pronounced contribution around solar noon. East-facing modules shift their contribution towards the morning, while west-facing modules contribute more later in the day. Combining the two directions can create a broader generation profile. That is a starting principle, rather than a ready-made curve for a particular building: season, cloud cover, tilt and shading affect the result.
The design brief should identify the direction, tilt and module count of each group. An east-west array does not necessarily have equal capacity on both sides. A roof may be rotated relative to the cardinal directions, and one section may have more obstructions. Agree on the azimuth convention too, because modelling tools can use different reference directions.
Mounting a rectangular module in portrait or landscape does not itself determine which way its surface faces. Nor does a low tilt make every layout an east-west design. Ask for a plan and a cross-section showing the surface directions, row heights and spacing.
Use two clear comparison bases
The first comparison keeps total module capacity equal. This helps assess orientation and its associated losses through generation per unit of installed capacity, time profiles and direct use. The location, weather period, module characteristics and treatment of losses should be consistent. If inverter capacity or tilt also changes, show those differences explicitly.
The second comparison uses the same available roof area. For each layout, determine module count after allowing for access routes, obstructions, clearances and structural constraints. An east-west arrangement can sometimes accommodate more modules, but a drawing must establish that for the particular roof. There is no universal multiplier for capacity or usable space.
If one layout contains more modules, higher total generation does not demonstrate higher output from each module. Report installed capacity, annual generation and generation per unit of capacity together. Keep the gross roof area distinct from the area actually available to both layouts: using different denominators can distort the comparison.
Request the model file with coordinates, weather dataset, calculation period and assumptions so the result can be reproduced after a layout change. Screenshots from different tools do not establish comparability simply because both show the same nominal capacity.
Establish the usable roof area
A building's roof area is not automatically space available for modules. Mark ventilation equipment, rooflights, parapets, drainage, maintenance zones and access to equipment on the plan. Then assess shading between rows and shadows from obstructions. The same roof outline does not ensure identical conditions on its eastern and western sections.
For shadows from specific rooftop structures, review the obstruction map and industrial roof shading model.
A compact arrangement may fit more modules while making inspection or repair harder. Assess the layout alongside access for cleaning, mounting-system maintenance and cable routing. Blocking a component that staff must regularly service is a poor trade for an attractive capacity figure on an initial drawing.
Weight, wind and snow effects, roof condition and attachment design need checking for both options. Module direction alone does not establish structural suitability. The article on checks before installing ballasted solar mounting covers flat-roof preparation in more detail. Here, those checks identify feasible layouts before their generation is compared.
Match generation to the business schedule
Each option needs a generation time series that can be matched to metered consumption. Align interval duration, dates and time zone. If the meter uses daylight saving time and the model uses a different time basis, reconcile the timestamps first. A shift of even one interval can create an apparent match with the beginning of a production shift.
A working day does not represent the whole year. Include weekends, seasonal shutdowns, changing operating hours and anticipated new loads. A factory with a lunch shutdown and a warehouse with continuously running refrigeration will use the same solar profile differently. An average monthly electricity bill does not reveal that difference.
Without storage, direct use within an interval cannot exceed either available generation or demand. Calculate both at the same electrical boundary, including conversion to alternating current. The guide to sizing commercial solar from a load profile explains data preparation and capacity selection. Use one agreed dataset for the orientation comparison.
A hypothetical six-interval example
The following values are invented to explain the method. They are not a forecast for a region, a particular roof or an installed capacity. Two hypothetical generation shapes are labelled south-facing and east-west. Each interval lasts two hours, with generation and load power assumed constant throughout it. We compare AC energy at a common metering boundary, without a battery. Generation is available energy before any curtailment. No financial value is assigned to surplus.
| Time | Demand | South-facing | East-west |
|---|---|---|---|
| 06:00-08:00 | 30 | 10 | 25 |
| 08:00-10:00 | 45 | 40 | 50 |
| 10:00-12:00 | 45 | 80 | 60 |
| 12:00-14:00 | 45 | 90 | 60 |
| 14:00-16:00 | 45 | 50 | 50 |
| 16:00-18:00 | 30 | 10 | 25 |
| Total | 240 | 280 | 270 |
For each row, take the smaller of demand and generation. The south-facing profile supplies 10 + 40 + 45 + 45 + 45 + 10 = 195 kWh of direct use. The east-west profile supplies 25 + 45 + 45 + 45 + 45 + 25 = 230 kWh. Grid purchases are therefore 240 - 195 = 45 and 240 - 230 = 10 kWh respectively.
Potential surplus is 280 - 195 = 85 and 270 - 230 = 40 kWh. Without permitted export or storage, this would need to be curtailed. In this example, the profile with lower total generation matches demand better. The conclusion applies only to the stated intervals and assumptions.
A different load can reverse the result
Keep the generation values but change demand to 0, 0, 90, 100, 0 and 0 kWh across the same six intervals. Total demand is now 190 kWh. The south-facing profile directly supplies 80 + 90 = 170 kWh, while east-west supplies 60 + 60 = 120 kWh. Grid purchases become 20 and 70 kWh respectively.
The hypothetical generation profiles have not changed, yet the preferred option for direct consumption has reversed. A conclusion without the business schedule is therefore incomplete. Multiplying one illustrative day's result by the number of days in a year would also be inappropriate: seasonal solar profiles and operating patterns differ.
In a real assessment, shorter intervals better represent rapid changes in load and cloud cover. Taking the minimum of two totals over a long period can overstate simultaneous consumption. Constant power within each interval makes this example reproducible, but that assumption must be checked before using it with metered data.
Check the electrical design
Groups facing different directions need their connections to inverter inputs and maximum power point tracking channels, or MPPTs, checked. The arrangement depends on the specific equipment, modules per string, voltage at design temperatures and permitted currents. Having east-facing and west-facing surfaces does not by itself determine the number of inverters required.
Do not automatically reduce inverter capacity because a broader generation profile is expected. Model the overlap between the two groups, conversion limits and energy lost through clipping. Compare any equipment saving with energy the system cannot use. The documentation should identify electrical limits as well as restrictions on export to the grid.
If the site includes battery storage, its operating strategy changes the comparison and requires a combined model. Orientation also does not provide backup power: operation during a grid outage depends on the system architecture and protection arrangements.
What to request in the final proposal
Ask for two layouts on the same roof plan and one consistent set of assumptions. Include module count and capacity, directions, tilt, inverters, shading, losses and the demand time series. Identify which parameters remain the same and which change between options.
- Monthly results and interval data suitable for comparison with the meter.
- Direct consumption, grid purchases, surplus and possible generation restrictions.
- A check using an alternative operating schedule or expected load change.
- An explanation of how the recommended layout meets the business criterion and roof constraints.
To prepare a commercial solar design, provide the roof plan, available structural information and interval consumption data. If you are considering a grid-tied solar power plant, specify the arrangements for surplus electricity from the outset. This makes the orientation decision traceable to an assessable result for your site.