How much do east- or west-facing panels lose?

Far less than the 50% many people assume — typically about 17% versus true south at a normal roof tilt (as little as 8% on a low-slope roof, more on a steep one). The exact figure depends on your latitude and tilt; enter your ZIP above for the computed number. West-facing can even be preferable under time-of-use rates that pay more in the late afternoon.

Do I need to correct for magnetic declination?

Yes, if you aim with a phone compass. A compass points to magnetic north, which is offset from true north by the magnetic declination — 10-20° in parts of the US. This tool computes your declination from the NOAA World Magnetic Model and tells you the exact compass heading to aim at for true south.

Can north-facing panels work?

They produce the least (worst case in the Northern Hemisphere — about 47% of south on a steep roof, but up to ~83% on a low-slope one), yet they can still pay off where electricity is expensive or where it's the only usable roof. The calculator shows the exact north-facing percentage for your location so you can judge it against your rates.

Solar Panel Direction Calculator: Which Way To Face Panels + True-South Correction (2026)

Enter your ZIP and the direction your roof faces. This tool computes your annual output as a percentage of the true-south maximum — from NREL PVWatts v8 for your exact location — and gives you the magnetic-declination correction so your phone compass actually points at true south.

Solar Panel Direction Calculator

Your ZIP code

Roof tilt: 20°

Panel direction

Assumptions: output figures come from NREL PVWatts v8 (NSRDB) for a fixed roof mount, 14 % system losses (~0.83 derate), and a 1 kW reference array. Declination is computed locally from the NOAA/NCEI World Magnetic Model (WMM2025, epoch 2025.0). ZIP-to-coordinates via Zippopotam.us (public-domain U.S. Census data).

What Direction Should Solar Panels Face?

In the Northern Hemisphere the answer is true south (180° azimuth) — that maximizes annual production. But "south" on your phone compass is magnetic south, which can be 10-20° off true south depending on where you live. The calculator above resolves both: your yield at the direction you actually face, and the compass heading that points at true south.

East-Facing And West-Facing Roofs

This is where expectations are usually wrong. An east- or west-facing array at a normal tilt keeps about 83% of true-south output — a loss of roughly 17%, not 50% (as little as 8% on a low-slope roof). The calculator gives your exact figure. West can even beat east if your utility uses time-of-use rates that pay more for late-afternoon energy. See whether the economics still work on the cost and worth-it pages.

The full picture across roof directions and tilts — the same numbers behind the calculator — is:

Output by roof direction and tilt — % of a true-south array

Roof direction	Low tilt (10°)	Mid tilt (25°)	High tilt (40°)
South toward the equator	100%	100%	100%
SE / SW ±45° off south	98%	95%	94%
East / West ±90° off south	92%	83%	78%
NE / NW ±135° off south	86%	69%	57%
North 180° off south	83%	62%	47%

NREL PVWatts v8 (NSRDB), annual output as a share of a true-south array at the same tilt, averaged across the contiguous US. Direction matters more the steeper the array: a near-flat roof is far more forgiving than a steep one — north-facing keeps ~83% at 10° but only ~47% at 40° (and spans ~55–71% at 25° depending on latitude). Your exact figure is computed for your ZIP by the calculator above.

These are contiguous-US averages; the calculator above returns the exact figure for your location, which varies with latitude and climate — so a slightly different number there is expected, not a bug.

North-Facing Panels

North is the worst case in the Northern Hemisphere, but how bad depends almost entirely on tilt: a north-facing array keeps only about 47% of true south on a steep (40°) roof, but roughly 83% on a low-slope (10°) one. So it is not automatically a dealbreaker — in high-rate areas, or when it's the only viable roof plane, a shallow north-facing array can still pay back. Read your exact north-facing percentage from the calculator and weigh it against your local rates and peak sun hours.

Split East/West Arrays

Splitting panels across east- and west-facing roof planes spreads production across the morning and afternoon. Total annual energy lands between a pure-east and pure-west array (the calculator reports the split-E/W figure), and the flatter daily curve can pair well with batteries or time-of-use billing.

Magnetic South vs True South — The Phone-Compass Trap

Most people aim panels with a phone, which reads magnetic north/south. The gap between magnetic and true is the magnetic declination. Aim naively at magnetic south and your array is misaligned by the full declination — but the honest news is that the annual cost of that error is usually small (the calculator computes the exact percentage for your ZIP). Use the computed "aim your compass at X°" heading and you remove the error entirely for free.

How Tilt Changes The Azimuth Penalty

Azimuth and tilt interact: the flatter your array, the less direction matters. A near-flat array is close to omnidirectional, while a steep array is much more sensitive to facing the wrong way. Re-run the calculator at a lower tilt to see the curve flatten, and use the tilt angle calculator to find your best angle. For whole-system sizing, the solar panel calculator ties it together.