8 Perspectives for Comparing Power Generation Forecasts in Solar PV Simulations

When reviewing solar PV generation simulations, simply lining up and comparing annual generation figures does not lead to correct decisions. Even for the same building or site, generation forecasts can vary widely depending on installed capacity, installable area, irradiance, orientation, tilt, shading, loss rates, and assumptions about self-consumption. A proposal with higher generation is not necessarily better; proposals that reflect on-site conditions may appear more conservative. This article explains eight perspectives for comparing generation forecasts aimed at practitioners who search for "solar PV generation simulation."

Table of Contents

• Basics when comparing generation forecasts

• Perspective 1: Align installed capacity and installable area

• Perspective 2: Check assumptions about irradiance and regional conditions

• Perspective 3: Examine differences by orientation, tilt, and mounting surfaces

• Perspective 4: Check how shading, obstacles, and surrounding environment are reflected

• Perspective 5: Check loss rates such as temperature, soiling, and snow

• Perspective 6: Look at monthly generation and time-of-day generation curves

• Perspective 7: Compare by self-consumption and surplus electricity

• Perspective 8: Confirm long-term degradation, maintenance, and performance management

• Decisions to avoid when comparing generation forecasts

• Summary

Basics when comparing generation forecasts

The purpose of comparing generation forecasts from solar PV simulations is not simply to choose the proposal with the largest generation. It is to determine which forecast is closest to reality for the installation site, under what conditions generation will be stable, and how much of that generation can be utilized on-site. Forecasted generation depends not only on installed capacity and irradiance but also on site geometry, surrounding environment, shading, temperature, soiling, snow, maintainability, and the facility’s electricity usage patterns.

When comparing multiple proposals, annual generation can differ. One proposal may show higher generation while another may look more conservative. However, without separating whether the difference comes from better design, different installed capacity, or different assumptions about shading and loss rates, you cannot make a correct judgment.

For example, increasing installed capacity generally increases annual generation. But if capacity is increased by using shaded or hard-to-maintain areas, generation per unit capacity may decline. Conversely, a proposal with slightly lower generation that avoids shading, secures maintenance access, and realistically reflects site conditions may be closer to post-installation performance.

When comparing forecasts, it is important to look at the same basis. Whether you compare at the same installed capacity, the same roof or land area, whether you prioritize self-consumption or total generation—the evaluation axes change. Comparing proposals with different objectives as-is will make the higher-generation proposal look better.

Forecasts are not only for pre-installation decisions but also connect to post-installation performance management. If the forecast’s assumptions are clear, when actual generation is lower than expected after installation, it becomes easier to check causes such as weather, shading, soiling, snow, equipment downtime, or demand changes. Organizing assumptions at the comparison stage also improves post-installation management accuracy.

The basic rule when comparing generation forecasts is to examine the assumptions behind the numbers, not the magnitude of the numbers themselves. Below are eight perspectives practitioners should check, organized in order.

Perspective 1: Align installed capacity and installable area

The first perspective is to align installed capacity and installable area. When comparing generation forecasts, looking only at annual generation favors proposals with larger installed capacity. But that does not mean higher generation efficiency; it may simply mean more panels are installed.

For rooftop projects, separate the roof’s total area from the actually usable area. Roofs have rooftop equipment, piping, drains, inspection hatches, penthouses, handrails, waterproofing clearance, and maintenance pathways. A roof that looks spacious on drawings may have limited area usable for PV equipment. Proposals that overestimate installable area can appear to generate more in initial simulations but may require capacity reductions after site surveys or before construction.

The same applies to land projects. Even with a large total site area, fences, slopes, elevation differences, trees, drainage channels, existing structures, maintenance paths, potential interconnection points, and snow storage areas limit usable area. Predictions that place panels across the entire site may show high generation but can be impractical for management and construction.

When comparing, look not only at total generation but also at generation per unit capacity. High generation per capacity can indicate efficient use of areas with good irradiance. Low generation per capacity may indicate using shaded or unfavorably oriented areas. It is important to confirm not only total generation but how efficiently the installed capacity generates power.

Also, comparing the same area versus placing the same capacity yields different meanings. Comparing the same capacity makes differences in irradiance, orientation, and shading clearer. Comparing the same area yields a realistic comparison including layout efficiency, row spacing, and maintenance path allocation. Be clear which standard you are using for comparison; otherwise, you cannot correctly understand differences in generation forecasts.

Aligning installed capacity and installable area is the starting point for generation comparisons. Comparing annual generation without this alignment risks incorrect installation decisions.

Perspective 2: Check assumptions about irradiance and regional conditions

The second perspective is to check assumptions about irradiance and regional conditions. Solar generation depends largely on how much irradiance the installation site receives. Even with identical installed capacity, forecasts vary by regional irradiance conditions, temperature, cloudiness, snow, terrain, and surrounding environment.

When comparing forecasts, confirm which regional conditions are used. Using conditions close to the candidate site versus broad regional averages affects forecast reliability. Irradiance conditions can vary within the same administrative area in mountainous regions, coastal areas, basins, snowy regions, or areas with frequent fog or cloudiness.

Check not only annual irradiance but monthly irradiance patterns. Even if annual generation is the same, differences in monthly generation change the perceived benefits. Determine whether generation is higher in summer or drops significantly in winter, and how the rainy season or cloudiness affects generation. If monthly generation is not provided, seasonal variation is hard to assess.

Temperature effects are also important. Even with high irradiance in summer, panel temperature rise can reduce output. For proposals that show very high summer generation, confirm the extent of temperature losses assumed. Predictions based only on irradiance risk overestimating summer generation.

In snowy regions, consider not only winter irradiance but generation losses due to snow accumulation or residual snow on panels. For proposals with high winter generation, confirm whether snow impacts are sufficiently reflected. Slope for snow shedding, snow storage areas, and ease of snow removal and inspection also affect winter generation.

Irradiance and regional conditions form the foundation of generation forecasts. If assumptions differ by proposal, do not compare annual generation directly. Separating whether differences arise from design or weather assumptions improves comparison accuracy.

Perspective 3: Examine differences by orientation, tilt, and mounting surfaces

The third perspective is to examine differences by orientation, tilt, and mounting surfaces. Which direction panels face and at what angle affects annual generation, monthly generation, and time-of-day generation curves. Forecasts can vary substantially with orientation and tilt even for the same installed capacity.

Generally, south-facing surfaces tend to yield higher annual generation. However, in practice south-facing is not always optimal. East-facing arrays generate more in the morning, west-facing in the afternoon. Depending on the facility’s usage hours, east- or west-facing arrays may be advantageous for self-consumption. When comparing forecasts, check how much capacity is assigned to each orientation.

Rooftop projects often have multiple roof surfaces. Combining generation from south, east, west, north-leaning, and flat roof sections into one figure makes it hard to see which surfaces contribute. Reviewing installed capacity, generation, and generation per capacity by mounting surface reveals which surfaces are favorable and which require caution.

For land projects, racking orientation and terrain conditions matter. Even land that can be oriented southward may be constrained by site shape, maintenance paths, drainage, or interconnection points. Sloped sites or sites with embankments and elevation differences change actual irradiance and shading patterns. Simulations treating terrain as flat may diverge from reality.

Check tilt angles as well. For rooftops, tilt often follows existing roof pitch, so the ideal angle may not be selectable. For flat roofs and land, racking angle can be set, but increasing angle affects row-to-row shading, wind load, spacing, and maintainability. Small gains in generation from a steeper angle are not realistic if they compromise construction or maintenance.

When comparing forecasts, evaluate orientation and tilt under conditions that are actually constructible, not idealized. Clarifying differences by mounting surface makes it easier to explain why generation is higher or lower.

Perspective 4: Check how shading, obstacles, and surrounding environment are reflected

The fourth perspective is to check how shading, obstacles, and the surrounding environment are reflected. Forecasts that do not sufficiently reflect shading tend to diverge from actual performance after installation. When comparing multiple forecasts, always confirm the extent to which shading is considered.

Sources of shading include neighboring buildings, rooftop equipment, penthouses, handrails, piping, HVAC equipment, exhaust devices, trees, utility poles, signs, embankments, and terrain elevation differences. Shading changes by time of day and season. Shadows may be short in summer but extend long in winter when solar altitude is low. Morning and evening shadows are also easy to overlook.

A useful way to compare shading treatment is to look at the difference between shaded and unshaded generation estimates. Proposals that show conservative generation after accounting for shading may be closer to reality. Conversely, proposals that show high generation despite many shading factors may be underestimating shading.

For rooftops, rooftop equipment shading is important. HVAC units, penthouses, piping, handrails, and exhaust devices can cast shadows at short distances. For land projects, trees, neighboring buildings, utility poles, embankments, and nearby structures create shading. Consider future tree growth as well as current height.

The surrounding environment affects not only shading but also soiling and maintainability. Nearby trees can increase leaf drop and bird activity. Unpaved ground or roads nearby can increase dust and sand. Roofs with a lot of equipment affect not only shading but maintenance access and space.

When comparing forecasts, confirm how thoroughly shading and obstacles are reflected as site conditions. Separate preliminary estimates made without site surveys from estimates revised after site surveys. For final decisions, use effective generation that reflects shading and obstacles.

Perspective 5: Check loss rates such as temperature, soiling, and snow

The fifth perspective is to check loss rates such as temperature, soiling, and snow. In solar PV simulations, various losses are subtracted from ideal generation to estimate effective generation. Differences in handling these loss rates change forecasts even with the same installed capacity.

Temperature loss is the reduction in output due to panel temperature rise, and is especially important in summer and for rooftop installations. Even with high irradiance seasons, elevated panel temperature can prevent expected generation gains. When comparing forecasts, confirm that summer generation is not overly optimistic.

Soiling loss is the generation decline caused by dust, pollen, leaves, bird droppings, exhaust-related grime, and other deposits on panel surfaces. In locations with many trees nearby, unpaved ground, or high dust generation, standard loss assumptions may not fit reality. Ease of cleaning and inspection also affect long-term generation.

In snowy regions, estimate generation declines from snow. Accumulated snow on panels creates periods without generation. Snow-shedding slope, snow storage areas, and ease of snow removal and inspection, as well as snow-load capacity, all relate. For forecasts that show high winter generation, confirm whether snow and residual snow are sufficiently reflected.

Also check wiring and power conversion losses. Electricity generated by panels passes through wiring and equipment to be used in the facility, incurring losses. Confirm whether the forecasted value is panel-side generation or usable electricity after conversion.

Loss rates are sometimes presented as an aggregate figure. However, a total loss rate alone can obscure what is included. Check whether temperature, shading, wiring, conversion, soiling, snow, and aging are included, and align assumptions across proposals.

A proposal with a low loss rate may show high generation but may be optimistic relative to site conditions. In comparing forecasts, prioritize whether loss rates are realistic rather than simply favoring lower loss rates.

Perspective 6: Look at monthly generation and time-of-day generation curves

The sixth perspective is to look at monthly generation and time-of-day generation curves. Annual generation alone does not show how generation is distributed. Determining which months have higher generation and which times of day generate power helps assess forecast practicality.

Monthly generation reveals seasonal variability. Summer may have high irradiance but suffer temperature losses. Winter has shorter daylight and lower solar altitude, causing generation to fall. In snowy regions, winter generation can drop further. During rainy seasons or extended cloudy periods, generation may stagnate.

When comparing monthly generation, overlay the facility’s monthly electricity usage. If high-generation months align with high demand, self-consumption becomes easier. If generation peaks when demand is low, surplus energy may increase. Even with equal annual generation, differences in monthly generation versus demand change the installation’s benefit.

Time-of-day generation curves are also important. Nearly south-facing arrays tend to peak around midday; east-facing arrays produce more in the morning and west-facing more in the afternoon. Depending on whether a facility’s demand is higher in the morning, around midday, or in the afternoon, the optimal layout and orientation assessment changes.

Examining generation curves reveals times prone to surplus. If generation around midday greatly exceeds facility demand, surplus increases. Combining east and west faces can smooth generation peaks and reduce surplus. However, balance with total generation and installed capacity is necessary.

Time-of-day generation also helps identify shading effects. A failure to ramp up in the morning suggests east-side shading; an early drop in the evening indicates west-side shading; unnatural midday dips can indicate shading from rooftop equipment or nearby structures. You can detect generation irregularities not visible from annual totals.

When comparing forecasts, examine not only annual totals but monthly and time-of-day generation patterns. This lets you evaluate not only how much can be generated but when it can be used.

Perspective 7: Compare by self-consumption and surplus electricity

The seventh perspective is to compare by self-consumption and surplus electricity. Total generation alone is insufficient to judge the effectiveness of a solar PV installation. How much of the generated electricity can be used on-site is crucial. When comparing forecasts, always separate self-consumption and surplus electricity.

Self-consumption refers to the portion of generated electricity consumed within the facility. It directly reduces purchased electricity and is central to explaining installation benefits. Surplus electricity is the portion not used within the facility at the same time it is generated. Whether surplus is exported, stored in batteries, or curtailed changes the evaluation.

A forecast showing large generation but small self-consumption may yield limited benefit. This is particularly true if facility demand is concentrated at night or if weekend operating hours are low—daytime generation may go unused. Confirm whether monthly and time-of-day usage are reflected, not just annual consumption.

Relying solely on self-consumption ratio is also risky. Smaller installed capacity tends to yield a higher self-consumption ratio but may still produce little absolute self-consumption. Larger capacity may lower the ratio but increase absolute self-consumption. Compare self-consumption ratio, self-consumed energy, and surplus electricity together.

Comparisons for different installed capacities are also useful. When capacity increases, check whether the additional generation serves self-consumption or becomes surplus. If surplus increases disproportionately beyond a certain capacity, reconsider installed capacity. Maximizing land or roof use is not always optimal.

For proposals including batteries, compare results with and without batteries separately. Batteries can increase self-consumption but involve charge–discharge losses and capacity limits. Results that include batteries alone can obscure the standalone solar surplus risk. Compare battery and non-battery scenarios separately.

When comparing forecasts, focus on usable energy within the facility, not just potential generation. Separating self-consumption and surplus electricity allows realistic comparisons of installation benefits.

Perspective 8: Confirm long-term degradation, maintenance, and performance management

The eighth perspective is to confirm long-term degradation, maintenance, and performance management. Solar PV systems are long-lived assets, and comparing only first-year generation risks misjudging long-term benefits. When comparing forecasts, adopt a long-term operational perspective.

First, check aging assumptions. Panels, power conversion equipment, wiring, connections, and racking may require inspection, repair, or replacement over long-term operation. High first-year generation does not guarantee that state will persist. When comparing long-term revenue, confirm assumptions about annual degradation and equipment replacement.

Maintainability is also important. Verify whether inspection pathways, cleaning routes, equipment access, drainage, weed control, and snow storage areas are secured. Layouts that maximize generation but are hard to maintain may become difficult to sustain long-term. Compare generation under maintainable layouts.

Also consider long-term changes in soiling and shading. Trees can grow and increase shading, neighboring buildings may be constructed, rooftop equipment may be added, or surrounding conditions may change to increase dust and leaf fall—each affects generation. While predicting all changes precisely is difficult, known site risks should be organized during comparison.

It is important that forecasts support post-installation performance management. Forecasts that preserve monthly generation, hourly generation, generation by mounting surface, self-consumption, and surplus electricity as baselines make it easier to compare actual performance after installation. If generation falls, it becomes easier to identify whether weather, shading, soiling, snow, equipment outage, or demand changes are the cause.

When comparing forecasts, assess not only first-year figures but whether the plan can maintain generation over the long term. Forecasts that ignore long-term degradation and maintenance may look good short-term but pose greater post-installation risks.

Decisions to avoid when comparing generation forecasts

When comparing generation forecasts, avoid simply judging the proposal with the highest annual generation as the best. Annual generation is important, but it is easily increased by enlarging installed capacity. Confirm whether high generation is due to larger capacity, better irradiance assumptions, lower loss rates, or insufficient shading consideration.

Also avoid using forecasts that do not reflect on-site conditions as comparison targets. Preliminary estimates based only on drawings or aerial photos can change after site surveys. Compare forecasts that reflect rooftop equipment, drains, maintenance paths, site boundaries, trees, slopes, elevation differences, and interconnection points.

Avoid judging based solely on total generation without checking self-consumption. High generation may still result in excess electricity if it cannot be used on-site. Unclear handling of surplus leads to unstable benefit assessments. Separate self-consumed energy and surplus electricity.

It is also dangerous to assume that a proposal with low loss rates is automatically favorable. Low loss rates make generation look higher but may omit realistic considerations of temperature, soiling, snow, shading, and aging. In comparisons, prioritize whether loss rates match site conditions over simply aiming for a low loss rate.

Additionally, be cautious about comparing only results that include batteries. Batteries can increase self-consumption but without viewing standalone solar generation and surplus reality, it is hard to judge installed capacity. Compare battery and no-battery scenarios separately.

When comparing generation forecasts, the important attitude is not to seek the best numbers but to confirm the conditions under which those numbers are valid. Comparing site conditions, generation losses, self-consumption, and long-term operations reduces post-installation gaps.

Summary

When comparing generation forecasts from solar PV simulations, do not focus solely on annual generation. Confirm installed capacity, installable area, irradiance, regional conditions, orientation, tilt, shading, obstacles, loss rates, monthly generation, time-of-day generation curves, self-consumption, surplus electricity, long-term degradation, maintainability, and performance management.

Perspective 1 aligns installed capacity and installable area: check generation per capacity and realistic installable range, not only total generation. Perspective 2 checks irradiance and regional assumptions: confirm whether site-specific meteorological conditions, monthly generation, temperature, snow, and cloudiness effects are reflected.

Perspective 3 examines orientation, tilt, and mounting surface differences: identify which surfaces contribute to generation and whether orientations and angles are constructible. Perspective 4 checks shading, obstacles, and surrounding environment: compare shaded vs. unshaded differences and evaluate winter and morning/evening shading and the influence of trees and rooftop equipment.

Perspective 5 reviews loss rates such as temperature, soiling, and snow: low loss rates make generation look larger but may diverge from site realities. Perspective 6 looks at monthly and time-of-day generation curves: check when generation occurs to evaluate compatibility with self-consumption.

Perspective 7 compares by self-consumption and surplus electricity: judge based on usable energy within the facility, not just the total possible generation. Perspective 8 confirms long-term degradation, maintenance, and performance management: assess whether the plan can sustain generation over time, not just first-year figures.

The basis for improving forecast comparison accuracy is accurate on-site information. If installable areas, rooftop equipment, obstacles, trees, site boundaries, orientations, tilts, maintenance access, and interconnection points are accurately captured, the assumptions in solar PV generation simulations become clearer and proposals can be compared fairly.

If you want to enhance the accuracy of comparing generation forecasts by precisely recording installable areas, rooftop equipment, obstacles, trees, site boundaries, orientations, tilts, maintenance access, and interconnection points on-site, using LRTK, an iPhone-mounted GNSS high-precision positioning device, is effective. High-precision site positioning makes it easier to organize shading and obstacles, installable areas, cable routes, and maintenance access, and supports consistent comparison of vendor proposals, pre-construction verification, and post-installation performance management. To correctly compare generation forecasts in solar PV simulations, it is important to not rely only on desk-based generation figures but to accurately capture the site and produce comparison materials based on the same assumptions.