Answer
A production forecast can be distorted when roof geometry, shading objects, orientation, equipment assumptions, weather data, losses, or operational constraints are incomplete or mismatched. Protect the forecast by documenting each input and updating the model after material site changes.
A production estimate can look precise while representing the wrong roof plane, missing an obstruction or using assumptions from an earlier layout. Start by tracing the result back to the site and configuration it describes rather than adjusting the annual figure until it looks plausible.
This desk-research checklist covers the inputs behind a proposed system’s energy estimate. It does not establish a surveyed roof, engineering approval or guaranteed output. An annual estimate based on a weather dataset is also different from a forecast for tomorrow’s weather. Identify which question the model answers before comparing results.
Roof form changes the available design
Multiple planes, pitches, parapets, dormers, skylights and mechanical zones can change the feasible array. Record which planes are included, how their dimensions were established, and the tilt and orientation used for each modeled group. Treat an imagery-derived dimension as an estimate until the applicable verification is complete.
If modules move to another plane, compare the accepted configuration with the model inputs. A roof image and an energy report should describe the same option. Keep unresolved geometry visible instead of converting a preliminary drawing into a final site fact.
Classify obstructions before choosing evidence
A tree can require attention to seasonal foliage and growth assumptions. A chimney or vent needs a position and geometry. A neighboring structure may create a nearby shadow or obstruct the distant horizon. Record what is observed, what is assumed and what evidence is still needed to represent the object.
A single shadow image does not establish an annual energy loss. The selected model must connect location, solar positions, geometry and its electrical treatment of partial shading. SAM’s shading documentation distinguishes external shading from row-to-row self-shading and describes method-specific assumptions and limits. Its modeled shading inputs are not a site verification or a SurgePV feature claim.
The same documentation warns that enabling multiple shading tables can produce an unrealistic representation. As a proposed review check, identify each applied shading input and its source so an imported loss is not silently combined with another representation of the same effect.
Record the weather basis and equipment configuration
Keep the location, weather dataset, period, time convention and model version with the result. Changing those inputs while changing the layout makes the comparison harder to interpret. When investigating one design change, hold the other relevant inputs constant where the method allows it and document exceptions.
Record module and inverter identity, DC capacity, configuration and the assumptions the selected model can actually represent. A calculation with a broad module category does not prove the behavior of an exact product or detailed string arrangement.
PVWatts exposes system size, array type, tilt, azimuth, losses and inverter-related inputs. Its published caution says site-specific characteristics are represented only to the extent its inputs capture them. Use an appropriate modeling method for the decision rather than treating a simplified estimate as detailed design validation.
Give each loss assumption a name
Maintain the basis for shading, soiling, mismatch, wiring, temperature, inverter conversion, clipping, availability and curtailment where those effects apply to the chosen method. Some are calculated from equipment or weather inputs; others may be entered as assumptions. Do not put every effect into a generic percentage and then apply it again elsewhere.
For each assumption, record its source, applicable period, configuration and owner. A utility export limit is not automatically the same as an annual curtailment percentage. Determine how the actual operating constraint is represented before using it to adjust generation.
Where evidence is incomplete, identify the unresolved assumption and the decision it affects. A named sensitivity scenario can show the consequence of a changed input; it does not establish a probability, bankable uncertainty range or guaranteed minimum output.
Keep meter and load data separate from generation
A well-modeled array can still support a misleading savings estimate if consumption timing, tariffs, demand charges or export treatment are wrong. Keep physical generation inputs distinct from the financial scenario’s meter, load and commercial inputs.
A revised electricity tariff may require recalculating the financial scenario without rerunning PV generation. A changed export constraint may affect operation and therefore require both reviews. Ask the responsible model owner which dependency changed instead of triggering every calculation for every commercial edit.
Maintain a forecast-input register
The following is a proposed team worksheet, not an industry-mandated document set.
| Input group | Evidence and revision to record | Question before issue |
|---|---|---|
| Roof geometry | Measurement or imagery source, planes, dimensions, tilt and orientation | Does the model use the current accepted geometry? |
| Obstructions | Position, height basis, object type and observation date | What is confirmed, assumed or missing? |
| Array and equipment | Layout revision, equipment identity and electrical basis | Does this match the option being presented? |
| Weather and method | Location, dataset, period and model version | Are compared results on a consistent basis? |
| Losses and constraints | Source, representation and applicable period | Is any effect omitted or counted twice? |
| Result and review | Output revision, reviewer, limitations and disposition | Is this result suitable for its stated use? |
Keep the actual files or records identifiable. A checked box without the evidence revision cannot explain which configuration was reviewed.
Use changes as review triggers
A new photo, equipment substitution, revised layout, discovered object or operating constraint should prompt an input-impact check. Record the changed fact, affected inputs, reviewer and disposition: rerun, unchanged with reason, or pending evidence. Preserve the prior issued result and its relationship to the replacement.
For the wider downstream effects, use the panel-placement change checklist. It extends the review to drawings, materials, pricing and customer documents.
Tell the customer what the number means
Present annual modeled AC energy with its period, configuration and assumptions. Distinguish it from rated DC capacity, self-consumption and bill savings. Describe unresolved site facts and meaningful limitations beside the result rather than hiding them in an unrelated conclusion.
To evaluate the energy and financial modeling workflow, ask a presenter to change a representative site input and show which calculations and customer-facing values need review. Confirm the actual workflow in a guided demo; the software does not replace qualified site or project review.
Frequently asked questions
Which site features can affect a solar production estimate?
Roof geometry, tilt and orientation, nearby shading objects, row arrangement and site conditions can affect the modeled result. Equipment, weather data, loss assumptions and operating constraints also matter. Their relative importance depends on the site and model; this guide does not rank their frequency.
Is a generation estimate the same as a savings estimate?
No. A generation estimate models energy output. A savings estimate also depends on consumption, tariff, export treatment, financing and other financial assumptions. State the period and assumptions of each result.
When should an energy model be updated?
Review the affected inputs after a material change to layout, site geometry, obstruction information, equipment, weather basis, losses or operating constraints. A tariff or financing change can require updating the financial scenario without changing the physical generation model. Record the disposition and responsible reviewer.
Sources
Primary research and reference material used for this desk-research article.
Where this fits
This article is part of SurgePV's Solar Design hub, which works through the topic from first principles to the decisions a project team actually has to make.
