What is the difference between true south and magnetic south?
True south is geographic south based on Earth's rotation axis, while magnetic south is the direction indicated by a magnetic compass. The difference between them is magnetic declination.
True south belongs to geography. It follows a meridian toward the geographic South Pole. Solar position calculations use geographic location, date, and time because the sun path is defined by Earth's rotation and orbit.
Magnetic south belongs to Earth's magnetic field. A compass needle aligns with local magnetic field direction, not automatically with geographic south. The angle between true direction and magnetic direction is magnetic declination.
According to NOAA/NCEI World Magnetic Model documentation, WMM computes geomagnetic components including magnetic declination, also called magnetic variation. That official model exists because magnetic direction changes with place and date.
Why do solar panels use true south?
Solar panels use true south in the Northern Hemisphere because true south aligns panel direction with the geographic sun path. Magnetic south can point away from true south by a location-specific declination angle.
Solar orientation uses true azimuth. NREL PVWatts defines azimuth as a PV input with valid values from 0 deg to less than 360 deg. In the common PVWatts compass convention, 180 deg represents south-facing direction.
True south is the correct reference because solar position is geographic. NOAA Solar Calculator provides solar position values by place, date, and time. A magnetic compass reading that has not been corrected can place the panel direction several degrees away from the true bearing used by solar calculators.
The hemisphere rule matters. Northern Hemisphere fixed panels generally use true south as the main reference. Southern Hemisphere fixed panels generally use true north. Direction still needs roof and shade review because the available roof plane can constrain the usable azimuth.
Solar panels use true direction because PV tools use geographic coordinates. According to NREL PVWatts documentation, latitude and longitude are location inputs and azimuth is a separate array input. The location tells the tool where the sun path belongs. The azimuth tells the tool which direction the panel surface faces.
Magnetic south is not wrong for navigation; it is wrong as an uncorrected solar calculator entry. The compass is measuring a magnetic field direction. The solar calculation is using a geographic direction. Magnetic declination is the bridge between those two systems.
How does magnetic declination change a compass reading?
Magnetic declination changes a compass reading by adding or subtracting the local angle difference between magnetic north and true north. The correction turns magnetic bearing into true bearing.
Magnetic declination is location-specific. NOAA/NCEI states that WMM uses latitude, longitude, altitude, and date as input parameters. Those inputs explain why one correction number cannot cover every ZIP code or year.
The direction of the correction depends on whether magnetic north sits east or west of true north at the site. A compass reading without the local declination value is incomplete for solar orientation. A corrected true bearing is the value that belongs in a solar orientation calculator.
What inputs define magnetic declination?
Magnetic declination is defined by position and date. Latitude and longitude locate the site. Altitude refines the field calculation. Date matters because Earth's magnetic field changes over time.
NOAA/NCEI documentation states that WMM2025 was released on December 17, 2024 and remains valid until late 2029. The current model window matters because old declination values drift out of date as the magnetic field changes.
Why does declination change over time?
Magnetic declination changes over time because Earth's magnetic field changes. NOAA/NCEI describes WMM as a model updated every 5 years to address changes in Earth's magnetic field.
Solar panel planning does not require daily magnetic-field tracking for ordinary home direction checks. Solar panel planning does require current enough declination data when a compass bearing is used. A stale map legend or old compass app setting can create avoidable direction error.
How do you convert a magnetic compass direction to true direction?
A magnetic compass direction converts to true direction by applying local magnetic declination to the compass bearing, then using the corrected true bearing as the solar azimuth value.
The practical workflow has 5 steps:
- Measure the roof or panel face with a compass.
- Find local magnetic declination from a current source.
- Apply the declination correction to the compass bearing.
- Confirm the corrected bearing on a map or roof image.
- Enter the true bearing into the orientation calculator.
The correction sign depends on local convention and source display. Some tools show east declination as positive and west declination as negative. The user-facing rule is simple: do not enter an uncorrected magnetic bearing into a tool that asks for true azimuth.
Map-based measurement often avoids part of the compass problem. A satellite map or roof drawing can show geographic direction directly when the map is aligned to true north. The map still needs careful roof-plane selection because the opposite roof face points in the opposite direction.
The conversion also needs the correct bearing type. A compass direction written as "southwest" is too broad for a calculator field. A calculator field usually needs degrees. Southwest is near 225 deg on a true azimuth scale, but a real roof face can sit at 210 deg, 225 deg, or 240 deg. The degree value carries the measurement precision that directional words lack.
Declination correction belongs before calculator entry. A user measuring 183 deg with a magnetic compass does not automatically have a true 183 deg roof. The local declination value decides the corrected true bearing. After correction, the value can be compared with 180 deg true south or entered into an orientation tool.
What mistakes create wrong south-facing panel direction?
Wrong south-facing panel direction comes from using magnetic south as true south, measuring the wrong roof plane, ignoring roof pitch, mixing azimuth with tilt, and ignoring shade.
The magnetic-south mistake is the core issue. A compass can point to magnetic south while the solar calculator expects true south. The difference is small in some places and large in others. The correction comes from magnetic declination.
The wrong-roof-plane mistake flips direction. A roof with 2 slopes has 2 different azimuth values. Measuring the north-facing plane and installing on the south-facing plane gives a 180 deg error. Roof orientation checks must identify the exact panel surface.
The tilt-and-azimuth mistake mixes 2 separate measurements. Roof pitch or panel tilt describes slope from horizontal. Azimuth describes compass direction. NREL PVWatts treats tilt and azimuth as separate inputs because both values describe different geometry.
The shade mistake treats direction as the complete answer. A true-south roof plane with heavy tree shade can be weaker than a less centered but clearer roof plane. Direction belongs inside site review, not outside it.
Phone settings create another direction mistake. Some phone compass apps display true north after location correction. Other apps display magnetic direction or change behavior by setting. A solar direction workflow records the app setting, verifies the bearing against a map, and uses one coordinate system consistently.
Map rotation creates another mistake. A map that is rotated on screen can make a roof appear to face a different direction. The map must be north-up or explicitly measured with a bearing tool. Visual guessing from a rotated satellite image is not a true azimuth measurement.
Street-grid assumptions also create error. A house on a north-south street does not automatically have a south-facing roof. Roof ridges and roof planes can be rotated relative to the street. The measured roof face is the entity that matters, not the road direction.
How does true south connect to solar azimuth planning?
True south connects to solar azimuth planning because it gives the geographic reference for a south-facing panel value. The true azimuth value then works with tilt, roof pitch, shade, and mount type.
Solar azimuth is the compass direction of the sun or panel surface. A true-south panel in the Northern Hemisphere is commonly represented near 180 deg in azimuth systems that start at true north. That value becomes meaningful only when the measurement reference is true direction.
True south is not a guarantee of final site suitability. Roof pitch controls panel tilt for flush mounts. Shade controls sunlight access. Mount type controls whether direction and tilt can be changed. Local structure, roof condition, access, and code requirements still require site review.
The cleanest planning sequence is true direction first, roof plane second, tilt third, shade fourth, calculator entry fifth. This order keeps magnetic correction in the right place and prevents compass readings from becoming incorrect solar azimuth values.
True south also sets the language for nearby orientation pages. South-facing solar panels, east-versus-west comparisons, north-facing exceptions, and roof-orientation articles all depend on true azimuth. Without that reference, the site can mix magnetic and geographic bearings across pages. Consistent true-direction language keeps the entity graph clean.
True south does not remove local judgment. A roof plane at 175 deg true azimuth and another at 190 deg true azimuth are both close to true south. A roof plane at 145 deg or 225 deg is more southeast or southwest. The useful question is not only whether the roof is exactly 180 deg. The useful question is which available roof plane has the best combination of true direction, tilt, shade, space, and access.
Use one tool after this page: Check Panel Orientation.
Source Notes
- C001-C004: NREL PVWatts V8 documents tilt, azimuth, location, and array type inputs.
- C005: NOAA Solar Calculator provides solar position values by place, date, and time.
- C007-C009: NOAA/NCEI World Magnetic Model documents magnetic declination, model validity, and location/date inputs.
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