At a glance
| Tool | Type | Price | Best for |
|---|---|---|---|
| PVGIS | Web app and API | Free | Free PV yield estimates and typical-year solar data for almost any location, including an EPW export. |
| PVWatts Calculator | Web calculator and API | Free; the API needs a developer key | Quick estimates of a grid-connected PV system's production, by month or by hour. |
| Global Solar Atlas | Web map, site reports and GIS downloads | Free | Comparing long-term solar resource and PV potential between places or countries, with free GIS layers. |
| NASA POWER | Data access viewer and API | Free | Free global time series of solar and meteorological data for any point, through a map viewer or an API. |
| C4B | Web app, in the browser | Free plan; paid plans from €59/month (cancellation takes effect after 12 billing cycles) or €599/year, with a 7-day free trial | A first look at solar radiation, façade exposure and PV potential for any address at concept stage. |
| Ladybug Tools | Plugins for Grasshopper (Rhino), open source | Free and open source; Rhino needs its own licence | Radiation, sunlight-hours and shading studies on your own 3D geometry in Rhino and Grasshopper. |
| Forma Site Design | Cloud app for site planning and early environmental analysis | Subscription, standalone or in Autodesk's AEC Collection; free trial | Comparing sun hours and solar energy potential between massing options in their real site context. |
| Meteonorm | Web app and API | CHF 320, 980 or 4,200 per year; education plan CHF 260 per year; free demo | Typical and actual-year solar and climate files for PV and building simulation software, in many formats. |
| Solargis | Web apps (Prospect and Evaluate) and data services | Prospect from €2,400 per year; Evaluate €12,000 subscription | Bankable solar resource data and PV yield simulation for commercial and utility-scale projects. |
How to choose
Be clear about the question. A building designer usually wants to know how much sun reaches the site and its façades, which orientation and tilt suit PV, and roughly what a roof array could produce. A PV developer needs bankable yield estimates with known uncertainty. The first is well served by free tools; the second is what commercial datasets such as Solargis and Meteonorm are built for.
Look at the data source and resolution. Most solar datasets are derived from satellite observations (such as PVGIS-SARAH3, Solargis and NASA POWER's 1° solar grid) or from reanalysis (such as ERA5), and some tools still use station files. Resolution ranges from about 250 m in Solargis and the Global Solar Atlas to 1° in NASA POWER, and the periods covered differ, so two tools can give different numbers for the same place.
Check the output. Long-term averages and monthly totals are enough to compare sites and orientations. Building energy simulation needs hourly typical-year (TMY) data, which PVGIS can export as an EPW file. Series of actual years are needed to study variability between years.
Read the model's assumptions. PV calculators simplify: PVWatts warns that its estimates do not reflect differences between PV technologies or site characteristics beyond its inputs. For radiation on specific roofs and façades, with neighbouring buildings casting shade, you need a tool that works on geometry, such as Ladybug Tools or Forma Site Design.
PVGIS
Free PV yield estimates and typical-year solar data for almost any location, including an EPW export.
- Solar radiation and PV performance for any location in the world except the poles, with no registration.
- Radiation databases include PVGIS-SARAH3 for Europe, Africa and Asia and PVGIS-ERA5; PVGIS-NSRDB covers the Americas in version 5.2.
- Tools for grid-connected, tracking and off-grid PV, hourly series, and a typical meteorological year exportable as CSV, JSON or EnergyPlus EPW.
- Current production version 5.3, with a PVGIS 6 beta whose solar data run from 2014 to 2024.
Good to know
- The API is limited to 30 calls per second per IP address and cannot be called from a web page through AJAX.
PVWatts Calculator
Quick estimates of a grid-connected PV system's production, by month or by hour.
- Inputs include system size, module type, fixed or tracking array, losses, tilt, azimuth, DC-to-AC ratio, albedo and bifacial modules.
- Version 8 uses typical-year data from the NLR National Solar Radiation Database (NSRDB) where it has coverage (a 2020 TMY for the Americas and Asia-Pacific, a 2022 TMY for Europe and Africa), and international station data elsewhere.
- Monthly or hourly results can be downloaded; the calculator, the API and the System Advisor Model (SAM) share the same model.
Good to know
- PVWatts cautions that its predictions do not differentiate between PV technologies or site characteristics beyond its inputs, and points to SAM for more detailed modelling.
- NREL was renamed the National Laboratory of the Rockies in December 2025; PVWatts is now at pvwatts.nlr.gov.
Global Solar Atlas
Comparing long-term solar resource and PV potential between places or countries, with free GIS layers.
- Provided by the World Bank Group as a free service, with a simple PV output calculation for any location it covers.
- Layers include global horizontal, direct normal, diffuse and optimally tilted irradiation, optimum tilt, PV power potential, air temperature and elevation.
- GIS data in GeoTIFF and ESRI ASCII grid formats; solar layers at about 250 m and PV and temperature layers at about 1 km (release 2.13, May 2026, with data to 2025).
Good to know
- Long-term averages (monthly for some parameters), not hourly time series or typical-year files.
NASA POWER
Free global time series of solar and meteorological data for any point, through a map viewer or an API.
- Solar data on a 1° grid from NASA's SRB archive and CERES from 1984; meteorology from MERRA-2 on a 0.5° × 0.625° grid.
- Hourly, daily, monthly and climatological data as CSV, JSON, NetCDF or ASCII; hourly data start in 2001.
- Hourly EPW files from 2001 onwards for its Sustainable Infrastructure (formerly Sustainable Buildings) community.
Good to know
- Coarse grids: values describe large areas rather than a site, and NASA warns against using the daily energy-flux series for trends that span a change of data source.
C4B
We make C4BA first look at solar radiation, façade exposure and PV potential for any address at concept stage.
- Direct normal, diffuse horizontal and global horizontal radiation, with a compass to read the radiation on vertical surfaces facing any direction.
- Yearly PV potential across a sweep of tilts and azimuths, calculated with pvlib using an isotropic sky model and 19% module efficiency, with the optimal tilt highlighted.
- Sun path as a polar ('globe') diagram, a section and a 3D view, and monthly daytime cloud cover, in the same place.
- Paid projects add the optimal PV panel orientation and can use site-specific reanalysis data.
Good to know
- An early-stage estimate with a fixed 19% efficiency and no model of shading from neighbouring buildings; use a dedicated PV tool for system design.
- There is no CSV or JSON data export.
Ladybug Tools
Radiation, sunlight-hours and shading studies on your own 3D geometry in Rhino and Grasshopper.
- Radiation studies quantify the solar energy falling on your geometry; sunlight hours, shadow, solar access and solar envelope studies are also included.
- Honeybee adds Radiance-based radiation studies that account for reflected light.
- Works from EPW weather files, so results follow the file you choose.
Good to know
- Requires Rhino, Grasshopper and visual-scripting skills.
Forma Site Design
Comparing sun hours and solar energy potential between massing options in their real site context.
- Analyses include sun hours, shadow, daylight potential and solar energy potential, alongside wind, noise and microclimate.
- Every proposal is geolocated, with site context such as terrain and surrounding buildings.
- Results of different options can be compared side by side.
Good to know
- Autodesk aims it at the pre-design and schematic design phases.
Meteonorm
Typical and actual-year solar and climate files for PV and building simulation software, in many formats.
- Irradiance parameters including GHI, DNI, DHI and tilted irradiance, plus PV production, among about 50 parameters.
- Built on data from 10,000 meteorological stations, five satellites and climate reanalysis; typical (TMY) and actual (AMY) years.
- Exports CSV, JSON, EPW, PVsyst, TMY3, PV*SOL, Polysun, PHPP and other formats; future scenarios SSP1-2.6, SSP2-4.5 and SSP3-7.0.
Good to know
- Plans differ in actual-year requests: 2, 30 or unlimited per year.
Solargis
Bankable solar resource data and PV yield simulation for commercial and utility-scale projects.
- Prospect: more than 30 map layers, site comparison and PV yield estimates for pre-feasibility, with solar data at 250 m resolution.
- Evaluate: PV design and energy yield simulation with 15-minute time series and TMY data covering more than 30 years.
- Solargis produced the solar database behind the World Bank's Global Solar Atlas.
Good to know
- Prospect provides monthly averages and 24 × 12 profiles; time series and TMY data are part of Evaluate.
See your site's climate before design begins
Pin any location and get climate, comfort and preliminary energy insight your whole team can read. Start with a 7-day free trial, or book a 30-minute demo.
Frequently asked questions
Where can I get solar radiation data for a location for free?
PVGIS, NASA POWER, the Global Solar Atlas and PVWatts are free. PVGIS gives PV yield estimates, hourly series and a typical year that can be exported as EPW; NASA POWER gives global time series through an API; the Global Solar Atlas gives long-term averages and GIS layers; PVWatts estimates PV production.
What is the difference between GHI, DNI and DHI?
Global horizontal irradiance (GHI) is the total solar radiation reaching a horizontal surface. Direct normal irradiance (DNI) comes straight from the sun's disc, measured on a surface facing the sun. Diffuse horizontal irradiance (DHI) is the radiation scattered by the sky and clouds onto a horizontal surface.
PVGIS or PVWatts: which should I use?
Use the one whose data covers your site best. PVWatts version 8 uses NSRDB typical-year data where the NSRDB has coverage (2020 for the Americas and Asia-Pacific, 2022 for Europe and Africa) and international station data elsewhere; PVGIS uses SARAH3 satellite data for Europe, Africa and Asia and ERA5 reanalysis worldwide. Both are free, so running both is a quick sense check.
Do I need hourly TMY data or long-term averages?
Long-term averages are enough to compare sites and orientations. Hourly typical-year (TMY) data are needed for building energy simulation and hourly PV modelling; PVGIS, Meteonorm and Solargis Evaluate provide them.
Is C4B's PV estimate enough to size a system?
No. C4B uses pvlib with an isotropic sky model and a fixed 19% module efficiency to compare tilts and orientations and gauge a site's potential at concept stage. Sizing a system needs a dedicated PV design tool with real module, inverter and shading inputs.
Sources
We make C4B. Information about the other tools comes from their official websites and documentation, checked on 18 September 2026. Products and prices change, so check each vendor's site for the latest details. All product names belong to their respective owners.
- JRC: Photovoltaic Geographical Information System (PVGIS)
- JRC: PVGIS API non-interactive service
- PVWatts Calculator
- NLR Developer Network: PVWatts V8 API
- NLR: Energy Department renames NREL
- Global Solar Atlas
- World Bank: Global Solar Atlas 2.0 technical report
- Global Solar Atlas: data outputs
- Global Solar Atlas: release notes
- NASA POWER
- NASA POWER: Data Access Viewer quick start
- NASA POWER documentation: data sources
- NASA POWER documentation: hourly API
- NASA POWER documentation: user communities
- Ladybug Tools
- Ladybug Tools: Ladybug features
- Ladybug Tools: Honeybee features
- Rhino pricing, North America
- Autodesk: What is Forma Site Design? (August 2026)
- Meteonorm web application and plans
- Solargis: Prospect pricing
- Solargis: Prospect
- Solargis: Evaluate
- Solargis: Evaluate pricing
Related on C4B
Solar radiation and PV potential
Direct, diffuse and global solar radiation, yearly PV potential, optimal panel orientation and geothermal ground temperatures.
Sun path diagram
Sun path diagrams in 2D, 3D and globe views, together with monthly daytime cloud cover.
Preliminary energy analysis
An hourly preliminary energy balance and orientation and aspect-ratio optimization for early design.
C4B vs Meteonorm
Meteonorm supplies climate data files for many simulation tools; C4B turns a site's climate into design analyses and a report.
C4B vs Ladybug Tools
Ladybug is a free, flexible Grasshopper toolkit; C4B is a ready-made browser workflow from an address to a climate report.
C4B vs Forma Site Design
Forma Site Design analyses massing models in the Autodesk cloud; C4B profiles a site's climate from an address, before any model exists.