Part IX: Tools, Calculators, and TestingChapter 15. Design Tools and Data
Solar Radiation Data and How to Use It
Where monthly sun data comes from and how to apply it to a tilted surface without over-precision.
- Difficulty
- Intermediate
- Cost
- Free
Sun data is a description of past weather, arranged so you can plan. It is not a promise about next January. The useful trick is to take horizontal measurements and turn them into the sun that falls on the tilted surface you will actually build, and then to round the answer until it is no more precise than your roof.
As of 2026 the public production calculator most households should start with is the national laboratory PVWatts tool at pvwatts.nlr.gov. The underlying radiation set is the National Solar Radiation Database at nsrdb.nlr.gov. Older pages and older books still say NREL. The tool is the same job: a typical year, an hour-by-hour estimate, and a warning that the result has assumptions in it. Read that warning. PVWatts is aimed at photovoltaics. A thermal collector wants the same kind of plane-of-array sun, and it does not want a kilowatt-hour output copied from a module model.
Array size from your bills, which consumes a peak-sun-hour number, is the sizing guide. A three-dimensional shadow is the modeling guide. Data discussions in the links below are other writers' notes.
Difficulty
- Level
- Intermediate
- Time
- An hour in a calculator, and a habit of rounding
- Skills
- Telling a horizontal map from a south-facing roof
Cost estimate
Free
The public data sets and PVWatts do not charge. A false third digit is free too, and it is the expensive kind of free if you buy modules to match it.
Materials
| Item | Qty | Notes |
|---|---|---|
| A location, a tilt, and an azimuth | 1 | True south, not a compass south. The compass error is the true-south guide. |
| A typical-year data set or PVWatts | 1 | Write down which data set and which year of the tool. 'The internet said 4.5' is not a citation. |
Three flavors of sun
Global horizontal irradiance is the sun on a flat surface, direct plus diffuse. Direct normal is the beam on a surface kept facing the sun. Diffuse is the sky's share. A roof is none of these until you tilt and aim it. A south-facing plane in winter sees a different mix than a flat roof. A west plane sees its sun late, which matters for heat you did not want and for time-of-use rates. If the map in your hand does not say "plane of array" or ask you for tilt and azimuth, it does not describe your roof yet.
A typical meteorological year is a constructed year of months chosen to look ordinary. It is a planning climate, not the average of every hour you will live through, and not a forecast. Real years swing around it. PVWatts will show a band for that swing on photovoltaic output. Believe the band more than the single annual number. Shade, snow, and a module that runs hot are still your job to represent. The default loss percentage is a starting point the tool explains, not a measurement of your gutters.
Peak sun hours, the number the sizing guide wants, are the daily plane-of-array energy in kilowatt-hours per square meter. A day with 4.5 peak sun hours has the energy of 4.5 hours at 1000 watts per square meter, spread across whatever longer day you actually had. It is not "the sun was up for 4.5 hours."
How precise to be
Round to something you could feel. A tenth of a peak sun hour is plenty for a house array. Three decimals on a monthly insolation table will not survive the vent you have to walk around or the tree the shade page is about. If your town is not a station, a nearby site in the same climate is a start, and a valley versus a ridge, or a coastal fog belt versus ten miles inland, can be a real difference. When those descriptions do not match, do not average them and feel scientific.
Thermal collectors and photovoltaic modules both begin from this sun. They do not share an efficiency. Do not take a PVWatts kilowatt-hour result and call it heat from an air heater. Take the sun, then use the collector's own test or the efficiency sketch.
Composite: the flat-map roof
Invented town, no station name, not a real address. A household reads a horizontal annual map, finds a cheerful number, and buys an array sized as if the roof were flat and unshaded. The roof faces west and is shaded at 3 p.m. The typical year was never asked about tilt or azimuth. Type the plane you have into the tool, then round down for shade you did not model. The further-reading links are where other people show their data files.
Get plane-of-array sun, not a flat map alone
Tilt and true azimuth. PVWatts or the NSRDB viewer, and write down which.
Treat a typical year as a climate, not a forecast
Read the uncertainty note. A single annual kilowatt-hour is the center of a band.
Round
A tenth of a peak sun hour is enough. Shade and walkways eat more than the third digit.
Do not reuse a photovoltaic output as heat
Both start from the same sun, but each has its own efficiency. Collector heat gets its own test.
Further reading
These links go to outside sources. This book cites them by link and does not copy their words or pictures.
- https://www.builditsolar.com/References/Calculators/
- https://www.builditsolar.com/References/Source/TMYWeather/TMYWeather.htm
- https://www.builditsolar.com/References/Source/RadOnColProgram/RadOnCollector.htm
- https://www.builditsolar.com/References/SketchUp/SketchUpEx.htm
- https://www.builditsolar.com/References/Suppliers/SolarHeating.htm
- https://www.builditsolar.com/References/HowMuch/HowMuch.htm
- https://www.builditsolar.com/References/BestRefsRS.htm
General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.