Five Free Tools Compared for Solar Power Generation Simulation

For practitioners who want to run solar power generation simulations using free tools, the initial point of confusion is often not so much “which tool to use” as “at what stage and to what level of accuracy should I aim.” Free tools for estimating generation include those for checking irradiance data, those that estimate annual generation from system capacity and tilt angle, map-based tools for checking installation candidates, spreadsheet tools for organizing conditions, and tools that allow trial use of some design support features. Explanatory materials published by public and professional organizations also present the idea that estimating solar power generation involves inputting irradiance, installation conditions, system capacity, and loss conditions to check generation. ([pvwatts.nlr.gov][1])

Table of Contents

• Basics to grasp before comparing free tools

• Comparison 1: Irradiance database type tools

• Comparison 2: Simple generation calculation tools

• Comparison 3: Map-linked potential assessment tools

• Comparison 4: Spreadsheet template tools

• Comparison 5: Design-support tools with free scopes

• Practical checklist when comparing free tools

• Causes that reduce the accuracy of generation simulations

• How to think when using free-tool results in internal proposals

• Summary: Use free tools according to purpose

Basics to grasp before comparing free tools

When comparing free tools for solar power generation simulation, it’s important to note that it’s not simply a question of “whether a number for generation comes out.” In practice, the required accuracy and the input items change depending on purpose: initial screening of candidate sites, checking rough profitability, comparing design conditions, internal explanations, customer explanations, on-site checks before construction, and reconciliation with post‑completion generation performance. While free tools are easy to use, they often simplify the inputs and calculation models, so it is important not to take the output generation figure as the final decision but to position it according to the stage of consideration.

First, understand that a generation simulation does not provide a guaranteed value of “if installed, it will definitely generate that amount.” Solar generation varies with many factors: irradiance, temperature, panel orientation, tilt angle, shading, system capacity, conversion losses, wiring losses, soiling, aging, curtailment, snow, and surrounding environment. Annual and monthly generation from free tools are estimates modeled under certain assumptions about these conditions. Therefore, what should be compared is not only absolute values but also what assumptions can be entered, which losses are considered, and what uses the tool is suitable for.

When practitioners choose a free tool, it helps to first separate uses. When candidate sites are not yet decided, irradiance database or map-linked tools are useful. Once a candidate site and rough system capacity and tilt are known, simple generation calculation tools become handy. For comparing multiple proposals internally, spreadsheet templates are convenient. If you want to check conditions closer to design stage, design-support tools with free scopes make it easier to delve into layout and equipment configuration considerations.

Also, don’t overlook “ease of use” and “explainability” when comparing free tools. In practice, you must be able to explain results not only to yourself but also to supervisors, customers, construction and maintenance personnel, financial institutions, and local government officials. Whether the output screen is clear, whether you can check monthly generation, whether it’s easy to compare results from changed conditions, and whether input values can be tracked later greatly affects how well simulation results are utilized. Don’t just use a free tool once and stop; keep a record of the study history, standardize the conditions, and enable comparisons under the same criteria for practical use.

Comparison 1: Irradiance database type tools

Irradiance database type tools are easy-to-use entry points among free solar power generation simulation tools. Their primary role is to grasp the irradiance conditions around candidate installation sites and to check regional differences and seasonal variations. Even with the same system capacity, annual generation can vary greatly depending on regional irradiance. Regions with many clear days, regions where winter irradiance drops sharply, and regions affected by snow or the rainy season will show different generation for the same system. Therefore, it is very important to check irradiance trends before running generation calculations.

The strength of this type of free tool is that by simply specifying the installation location, you can easily check irradiance and monthly solar energy trends. In initial studies, panel counts, equipment configuration, and detailed layout are often undecided, so simply understanding whether the region is suitable for solar generation and which months tend to have lower generation through the year provides useful judgment material. Especially when comparing multiple candidate sites, being able to view irradiance differences under the same standard helps in the early stage of site selection.

On the other hand, judging final generation based solely on irradiance database tools is risky. Even in high-irradiance regions, actual generation will drop if there is a lot of shading on the roof or land. If orientation or tilt is unfavorable, generation can vary even within the same region. Influences from surrounding buildings, trees, utility poles, mountains, and adjacent structures cannot be fully reflected by wide-area irradiance data alone. Therefore, it’s appropriate to position this type as a “tool for checking regional potential.”

In practice, use irradiance database tools to check annual irradiance trends, monthly variations, and differences due to the tilt of the installation surface for each candidate site, and then connect those results to the next calculation steps. For example, when comparing multiple regions with the same system capacity, you need to distinguish whether differences in annual generation are due to irradiance conditions or installation conditions. Irradiance database tools are the first step in that separation.

From a free-tool comparison perspective, irradiance database tools can be summarized as “strong for initial surveys but limited in accuracy for individual design.” They are suitable when you want quick prospects for candidate sites, broad-area comparisons, or to prioritize candidate sites internally. Conversely, if you want to see roof-by-roof shading, panel layout, equipment selection, or detailed loss analysis, you will need to combine them with other types of tools.

Comparison 2: Simple generation calculation tools

Simple generation calculation tools are among the easiest free solar power generation simulation tools for practitioners to use. By inputting installation location, system capacity, panel tilt, orientation, and system loss rate, you can check estimated annual and monthly generation. Because the number of input items is relatively small, they are user-friendly even for those without deep expertise and are suitable for initial proposals and rough estimates.

The advantage of this type is that you can quickly see how generation changes when you change system capacity. For example, you can estimate how much annual generation will increase if you increase capacity at the same site. Also, changing tilt or orientation and comparing results makes it easier to see the impact of installation conditions on generation. In practice, when installation area is limited or a roof has multiple orientations, comparing multiple scenarios with simple calculation tools helps decide the direction of the study.

However, the weakness of simple calculation tools is that their simplicity makes it difficult to reflect detailed on-site conditions. In particular, the treatment of shading, surrounding obstacles, snow, soiling, temperature conditions, detailed equipment characteristics, mismatch due to panel layout, and curtailment varies greatly between tools. When loss rates are entered as an aggregate value, if the practitioner cannot set an appropriate loss rate, the tool may produce numbers that look calculated but are far from reality.

When using these tools in practice, it is important to compare multiple scenarios under the same rules. If one scenario uses a lower loss rate and another uses a higher one, differences in results will reflect the user’s assumptions rather than differences in system conditions. The more you rely on free tools, the more necessary it is to record input values and standardize comparison conditions. Especially for internal or customer presentations, prepare to explain not only generation numbers but also input system capacity, orientation, tilt, loss rate, and how meteorological data were handled.

Simple generation calculation tools can be used from initial study to rough proposals, but additional verification is needed for final design or detailed profitability decisions. Among free tools, they are “the fastest way to get an estimate of generation, but on-site specific risks should be checked separately.” They are useful for practitioners who want to compare multiple scenarios quickly, but do not overtrust the results and use them in combination with site surveys and detailed design.

Comparison 3: Map-linked potential assessment tools

Map-linked potential assessment tools use map or aerial photo displays to check the location, roof shape, and surrounding environment of candidate installation sites while assessing the potential for solar generation. Among free tools, they are visually easy to understand and suitable for initial candidate checks and customer explanations. Showing a site on a map makes it easier for stakeholders to visualize than showing generation numbers alone.

The strength of this type is that it links location information with generation assessment easily. Where a system is installed is extremely important in solar generation simulations. Even on the same building, generation varies with the orientation of roof faces, and on the same land, results change depending on whether there are obstructions on the southern side. Using map-linked tools, you can roughly assess a candidate’s suitability while checking surrounding buildings, roads, site boundaries, roof orientations, and terrain context.

However, what you can see on maps is limited. If aerial photos are outdated, the current condition of buildings or trees may differ. Roof pitch, roofing material, structural constraints, equipment load conditions, wiring routes, and maintenance access cannot be judged from maps alone. Also, depending on the map’s positional accuracy and display resolution, it may be difficult to accurately judge area and orientation. Therefore, map-linked tools should be used for hypothesis building before site surveys, not as a substitute for onsite inspection.

In practice, an effective workflow is to narrow candidate areas with map-linked tools, then perform on-site checks to measure shading and installation surfaces. Particularly when comparing multiple buildings or sites, confirm candidate installation surfaces on the map and prioritize them based on rough generation estimates. In initial customer meetings, it’s easy to have conversations like “this face has a good orientation,” “we need to check for shading in this direction,” and “we should measure the installable area in this part” while looking at the site together.

From a free-tool comparison viewpoint, map-linked tools are “easy to explain and strong for candidate checks, but detailed accuracy depends on site inspection.” Because they are visually intuitive, they are good for communicating with stakeholders unfamiliar with solar generation simulations, but relying only on visible information can lead to oversights. In practice, approximate areas on maps and actual constructible areas may not match, so combine map checks with early site measurement and photo documentation.

Comparison 4: Spreadsheet template tools

Spreadsheet template tools are among the free solar power generation simulation options and are particularly useful for practitioners to organize conditions and compare internally. Unlike dedicated web calculators, you can manage your own input items and formulas, making it easy to compare multiple projects, patterns, and conditions side by side. Rather than producing highly accurate generation calculations, they are suited to visualizing study conditions and organizing the basis for comparisons.

A benefit of spreadsheet templates is their high transparency of input conditions. You can see which cell contains system capacity, which cell holds annual irradiance, and which cell contains loss rates, making later checks easier. Free web tools sometimes hide calculation model details, but with spreadsheets you can at least trace the coefficients and assumptions your company set. They are useful when you want to standardize in-house study rules or have multiple staff use the same format.

Spreadsheet templates are also effective as a receptacle for results obtained with free tools. By compiling regional conditions from irradiance database tools, annual generation from simple calculation tools, and installable area from map-linked tools into the same sheet, it becomes easier to compare projects. Recording not only generation but also system capacity, installable area, assumed losses, monthly variation, confirmation date, input conditions, and whether site checks were done makes it easier to track differences when conditions change later.

On the other hand, spreadsheet templates carry risks of input errors and broken formulas. If cell references are off, coefficients are outdated, or units are inconsistent, results can change dramatically. Mixing units like kW, kWh, irradiance, area, conversion efficiency, and loss rates can produce plausible-looking but incorrect outcomes. When using spreadsheet templates, separate input and calculation cells, keep change logs, and provide explanatory fields for key coefficients to improve safety.

In free-tool comparisons, spreadsheet templates are “excellent in flexibility and recordkeeping, but calculation quality depends on the creator’s management.” They are better used to organize and compare results from other free tools and as material for decision-making rather than as standalone high-precision simulation tools. For practitioners, they add value as auxiliary tools to enhance reproducibility and accountability in studies.

Comparison 5: Design-support tools with free scopes

Design-support tools with free scopes handle content closer to practical design in addition to generation simulation: panel layout, equipment configuration, shading checks, wiring and design condition organization, etc. Although full detailed design often requires paid features or specialized environments, the free features can still help learn initial design thinking and concretize system conditions.

The strength of this type is that it makes it easy to check the relationship between generation numbers and design conditions. Solar generation is not determined by system capacity alone. Panel arrangement, orientation, tilt, shading patterns, equipment combinations, interaction with the grid, and division of installation surfaces all affect results. Design-support tools allow you to check these conditions on screen while approaching a more realistic estimate of generation, making it easier to study aspects close to actual construction and design.

However, free features may be limited. There may be limits on the number of projects you can save, the reports you can output, the system sizes you can handle, the device conditions you can input, the detail of shading analysis, and data export. Therefore, in practice, you need to know in advance how far the free scope lets you go. Even if the interface looks professional, using it without checking output conditions and calculation assumptions may not reach the expected level of study accuracy.

Design-support tools’ free scopes are particularly good for practitioners’ learning. They let you see which input conditions the simulation is sensitive to. By trying changes in tilt and seeing effects, changing orientation to see monthly generation differences, or adding shading to see how much generation drops, you improve not just numeric comparisons but design judgment.

From a free-tool comparison perspective, design-support tools with free scopes are “approaching detailed study, but it is essential to check free-scope limitations.” They are suitable when you want to advance beyond initial proposals, explain differences in design conditions, or reflect on-site findings. For final design documents, contract decisions, performance guarantees, and construction plans, combine them with professional verification, paid detailed design environments, on-site measurements, and technical staff checks.

Practical checklist when comparing free tools

When comparing free solar power generation simulation tools, don’t just look at the annual generation output; check what items can be input and how results will be used. First, confirm the accuracy of location specification. Tools differ in how you specify location: address, latitude/longitude, map selection, or region name. Because irradiance data greatly influence generation estimates, it’s important whether calculations can be done using conditions close to the candidate site. In mountainous, coastal, snowy, or urban areas, even slight changes in location can alter conditions.

Next, check whether orientation and tilt angle can be input. For roof installations, buildings often prevent ideal orientation and tilt. Some free tools assume standard tilt and orientation, so verify whether they match actual installation conditions. Orientation and tilt directly affect generation for east‑west faces, low‑slope roofs, flat roofs, and multi-face installations.

Also check how loss conditions are handled. Solar irradiance received by panels does not directly become electrical energy. There are many losses: temperature-related output reduction, conversion losses, wiring losses, soiling, shading, mismatches from panel layout, equipment efficiency, and aging. Determine whether a free tool allows individual settings for these, treats them as a single aggregate loss rate, or automatically uses standard values, as this changes how to read the results.

It is also important in practice whether monthly generation can be checked. Annual generation alone does not show seasonal variation. For self-consumption systems, alignment of generation with demand over time is critical. Even if annual generation seems sufficient, if high-demand periods do not coincide with high generation periods, expected benefits may not materialize. Monthly generation reveals summer/winter differences, rainy season dips, and snow-season effects clearly.

Check the ease of saving and sharing outputs. Viewing results on screen only may prevent later reproduction. Can you record input conditions, export results, easily copy into internal materials, and compare before/after condition changes? Usability in practice depends on this. When handling multiple projects, keeping the calculation date, input conditions, responsible person, and whether site checks were done will make later explanations easier.

Causes that reduce the accuracy of generation simulations

Many causes of reduced accuracy in free-tool solar generation simulations stem from coarse input conditions rather than the tools themselves. A common problem is entering orientation and tilt without measuring the installation surface. A roof may appear south-facing but actually be slightly off to the east or west. Tilt cannot be accurately known without drawings or site measurement. Small differences can affect annual and monthly generation.

Overlooking shading is another major factor. Even short-duration shading can affect generation. Shading sources include surrounding buildings, trees, signs, poles, antennas, rooftop equipment, railings, and mountain shadows. If a free tool cannot input shading details, you need to estimate shading losses separately. In winter, when the sun’s altitude is lower, shading that is negligible in summer can have a large impact.

Be careful about system capacity inputs. Solar capacity involves panel-side capacity and inverter-side capacity, and interpretation differs depending on which is used as a reference. Some free tools simplify the meaning of capacity inputs. In practice, organize panel capacity, inverter capacity, the concept of overloading, and possible output limits before input. Increasing capacity alone will not produce expected generation if installable area, equipment conditions, and grid constraints are inadequate.

How meteorological data are handled also affects results. Many free tools estimate generation based on representative historical weather or average irradiance data. But actual years can vary significantly in weather: some years are sunnier, others have long rain or heavy snow. Treat simulation results as long-term guidelines, not as figures that will exactly match single-year actuals.

Free tools may also fail to reflect site‑specific construction conditions sufficiently. Panel spacing, wiring length, equipment locations, maintenance access, rooftop obstacles, ground conditions, racking conditions, and responses to wind and snow loads affect not only generation but also constructability. Even if generation numbers look good, strict construction conditions may require plan revisions. Simulations are only part of the study; decide in conjunction with design, construction, and maintenance perspectives.

How to think when using free-tool results in internal proposals

When using free-tool simulation results in internal proposals, be careful how you present numbers. If you highlight an estimated annual generation value, stakeholders may take it as a fixed value. In internal documents, clarify that generation figures are estimates based on input conditions and assumptions. Especially at the initial stage, treat them as ranges rather than fixed numbers.

Explaining why you used a particular free tool increases persuasive power. If you used an irradiance database for site comparison, explain that; if you used a simple calculator for rough generation, explain that; if you used a map-linked tool for on-site explanation, say so. When comparing multiple tools’ outputs, numbers may not match exactly; these differences stem from input assumptions and calculation models. Instead of hiding differences, explain that multiple perspectives exist during the study stage to increase transparency in decision-making.

In proposal documents, record generation along with input conditions, system capacity, orientation, tilt, loss rate, calculation date, and whether a site check was performed. This makes it easy to trace which assumption changed when conditions later change. For example, if you initially assumed full-roof installation but had to avoid part of the roof after a site check, you can explain why generation decreased. Free-tool results become practical documentation only when paired with records.

When explaining to customers, don’t just show free-tool screens or numbers; translate technical terms into plain language. Equipment capacity, annual and monthly generation, loss rate, orientation, and tilt may be hard for non‑experts to understand. Carefully explain that some months produce more generation than others, that shading and weather affect actual output, and that final judgments require a site inspection to avoid excessive expectations or misunderstandings.

To use free-tool results effectively in practice, separate roles by stage: initial study, rough comparison, pre‑detailed‑design checking, and post‑site‑survey correction. Free tools are convenient but not a single tool to complete everything. Create a workflow: check irradiance, estimate rough generation, confirm candidate sites on maps, organize conditions in spreadsheets, and, when needed, use design-support tools to concretize—this makes free tools viable for practical comparative studies.

Summary: Use free tools according to purpose

This article organized five types of free solar power generation simulation tools: irradiance database type, simple generation calculation type, map-linked potential assessment type, spreadsheet template type, and design-support tools with free scopes. No single tool is always optimal; it is important to use them according to the stage and purpose. If you want to see regional differences, use irradiance database tools; if you need a quick annual estimate, use simple calculation tools; if you want to share location or roof images, use map-linked tools; if you prioritize internal comparison and condition organization, use spreadsheet templates; and if you want study closer to design, use design-support tools with free scopes.

The biggest advantage of free tools is quickly forming hypotheses in early stages. You can rapidly check what happens when you change system capacity, how orientation and tilt affect generation, and how irradiance conditions differ by site. This lets you quickly separate projects that should proceed to detailed surveys from those that need condition re-evaluation. For practitioners, improving the speed of initial study is a major benefit.

At the same time, do not overtrust free-tool results. Generation simulations are heavily influenced by input conditions. With coarse inputs for installation location, orientation, tilt, shading, loss rate, system capacity, and meteorological data handling, accuracy will not improve regardless of tool clarity. In particular, on-site shading, installable area, roof and ground constraints, maintenance access, and surrounding environment cannot be fully confirmed by free tools alone. It is more important to understand the assumptions behind numbers than to simply produce numbers.

To achieve practical results, tie free-tool outputs to on-site information. If you can manage estimated generation, candidate surfaces on maps, installable area, shading status, positioning information, and photo records together, planning accuracy will greatly improve. In particular, obtaining accurate on-site positioning information and keeping a record of the basis for candidate sites and equipment layouts strengthens the assumptions behind generation simulations.

As a way to make on-site confirmation more efficient, using LRTK (iPhone-mounted high-precision GNSS positioning devices) helps record installation candidate locations, measurement points, boundary proximity, shading occurrence points, and equipment layout checkpoints with high accuracy. Linking rough solar generation estimates from free tools with accurate on-site positioning and photo records makes it easier to elevate desk simulations into practical study material. Start quickly with free tools, verify with on-site data, and feed findings into design decisions—this workflow is a shortcut to success in solar power generation simulation.