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Solar calculator

Solar panel tilt angle calculator: the best angle for your exact location

Finds the optimum tilt and orientation by simulating every hour of a real NASA weather year at 30+ panel angles. Includes seasonal angles, roof-pitch losses and an east/west comparison.

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Results

How this tilt calculator works

The textbook answer to “what angle should my solar panels be?” is “your latitude”. That’s a decent starting point and nearly always slightly off, because the real optimum depends on how much of your sunlight is direct versus diffuse, when in the year your clear days occur, how hot the panels run in summer and which way they face. This calculator doesn’t rely on a formula. It tests the angles.

It downloads one full year of hourly NASA POWER satellite weather for your location and runs our complete PV simulation (the same engine as the solar panel calculator) for every tilt from 0° to 90° in 5° steps. The best value is then refined in 1° steps, and the search is repeated for the winter half-year, the summer half-year, each month and thirteen orientations from east through south to west. A typical run performs around 80 complete 8,760-hour simulations. Inverter clipping is switched off, so differences come purely from geometry and climate.

The physics in one formula

G_poa = DNI · cos θ + DHI · [ A·R_b + (1 − A)·(1 + cos β)/2 ] + GHI · ρ · (1 − cos β)/2

cos θ = cos Z · cos β + sin Z · sin β · cos(γ_sun − γ_panel) Annual yield(β, γ) = Σ_hours P_ac( G_poa, T_cell )

β is the tilt, γ the azimuth, Z the solar zenith angle, θ the angle of incidence on the panel, ρ the ground albedo, and A the anisotropy index of the Hay–Davies model (the share of diffuse light that behaves like the sun’s disc). Steeper tilts catch more low winter sun (cos θ) but see less of the sky dome ((1 + cos β)/2); the optimum is where the two effects balance over your year’s weather.

Worked example: Bucharest, south-facing

With the defaults (44.4°N, panels facing due south, TOPCon modules, 14% losses, 2024 weather) the simulation finds:

  • Best annual tilt: 35°, yielding 1,222 kWh/kWp, or 6,110 kWh for a 5 kWp system. The latitude rule of thumb suggests 37°, close but slightly too steep.
  • A typical 30° roof loses only 0.3%; 25° loses 1.1% and 45° loses 1.2%. A flat array loses 13.6%, a vertical façade 32%.
  • Best winter tilt (Oct–Mar): 56°; best summer tilt (Apr–Sep): 21°. Switching between them twice a year gains 3.5%; four positions gain 3.8% and monthly adjustment 4.6%.
  • Facing east at 35° gives about 80% of the optimum, south-east about 95%.

How to read the results

  • Best fixed tilt is the headline for a ground mount or flat-roof frame. On a pitched roof you rarely have a choice, so check your tilt to see how much you lose (usually very little).
  • Monthly optimum tilt swings from about 10–15° in June to 60–65° in December in mid-latitudes. That’s why off-grid systems, which have to get through December, are mounted steeply.
  • Orientation sensitivity shows how fast yield drops as you turn away from the equator. Within ±30° of south you typically lose less than 5%.
  • The tilt × orientation matrix is a quick reference for every roof face of a house.

Typical optimum tilts (fixed, equator-facing)

Location Latitude Best annual tilt Best winter tilt Flat vs. optimum
Singapore 1°N 0–10° 10–15° 0 to −2%
Phoenix, USA 33°N 29–32° 48–52° −11 to −13%
Madrid, Spain 40°N 32–35° 50–55° −12 to −15%
Bucharest, Romania 44°N 33–36° 55–58° −13 to −14%
Berlin, Germany 52°N 35–40° 55–62° −12 to −16%
Helsinki, Finland 60°N 38–44° 62–70° −15 to −20%
Sydney, Australia 34°S 28–32° 48–52° −10 to −12%

Tips

  • Don’t build steep frames on a roof to chase a few percent. Raising panels on a 25° roof to 35° gains about 1%, but adds wind load, cost and visual impact.
  • Use at least 10–15° tilt where possible: rain washes panels clean and snow slides off. Very flat arrays lose an extra 1–3% to soiling in dusty climates.
  • On flat roofs, flatter can produce more per roof. Rows at 10–15° can be packed much closer than rows at 35° without shading each other, so total kWh per square metre of roof is often higher (see the solar roof area calculator).
  • For off-grid, optimise for winter. Select Winter half-year above; the off-grid solar calculator then sizes the array for the worst month.
  • For solar water heating, steeper collectors (latitude + 10°) balance summer overheating against winter gains; see the solar water heater calculator.

Limitations

Results are for one weather year (selectable back to 2001); the optimum tilt typically varies by only 1–3° between years. Shading from the horizon, trees or buildings is not modelled and can shift the optimum; mountains to the south, for example, favour steeper tilts. Snow cover on flat panels, bifacial gains and row-to-row shading on flat roofs are outside the model. NASA POWER data have roughly 50 km resolution, so coastal fog or valley effects are smoothed.

Method & assumptions

One year of NASA hourly irradiance, temperature and wind is downloaded once. The full PV simulation (NOAA sun position → Hay–Davies transposition → ASHRAE incidence-angle modifier → cell temperature → temperature-corrected DC → losses) is then repeated for tilts 0–90° in 5° steps and refined in 1° steps around the best value, for the yearly, winter and summer half-years, and for azimuths ±90° around the equator-facing direction. Inverter clipping is disabled so the comparison reflects only geometry and climate.

Seasonal gains compare re-tilting the panels twice a year, four times a year or monthly with a fixed array at the annual optimum.

Frequently asked questions

What is the best angle for solar panels?
For maximum annual energy with fixed panels, the optimum tilt is usually a little below your latitude: roughly 0.76 × latitude + 3° up to about 50° latitude. That gives about 30–37° in most of Europe and the US, 20–30° in the subtropics and 40–45° in Scandinavia. Cloudy climates favour slightly flatter angles because diffuse light comes from the whole sky. Enter your location above for the simulated optimum.
How much energy do I lose if my roof is not at the optimal angle?
Very little near the optimum. The yield curve is flat: ±10° from the best tilt typically costs only 1–3% of annual energy. A flat installation loses about 10–15% in mid-latitudes, and a vertical façade 30–40%. Orientation matters more than tilt: an east- or west-facing roof loses roughly 15–25% at typical pitches.
What angle should solar panels be in winter?
Steeper, roughly latitude + 10–15°. In Bucharest (44°N) the best fixed tilt for October–March is about 56°, against 35° for the whole year. A steep winter angle also helps snow slide off. Off-grid systems that must work in December are therefore often mounted at 50–65° in Europe.
Is it worth adjusting the tilt of solar panels seasonally?
Adjusting twice a year (a summer and a winter angle) typically gains 3–5% over the best fixed angle; adjusting monthly gains 4–6%. That is worthwhile for small ground mounts or off-grid systems you visit anyway, but rarely for roof systems, where adding one more panel gains more.
What is a 6/12 roof pitch in degrees?
Roof pitch x:12 means x units of rise per 12 units of run, so the angle is arctan(x ÷ 12). 4/12 ≈ 18.4°, 6/12 ≈ 26.6°, 8/12 ≈ 33.7°, 10/12 ≈ 39.8° and 12/12 = 45°. A 6/12 to 9/12 roof is very close to optimal across most of the United States.
Are east-west solar panels a good idea?
An east–west array at a low tilt (10–15°) produces about 10–20% less than a south-facing one, but spreads output over morning and evening and allows panels to be packed tightly on flat roofs without inter-row shading. Where exports earn little, the better match with household demand can make it the more profitable layout.

Sources & references

  1. NASA POWER — hourly solar and meteorological data
  2. Jacobson, M.Z. & Jadhav, V. (2018). World estimates of PV optimal tilt angles and ratios of sunlight incident upon tilted and tracked PV panels relative to horizontal panels. Solar Energy 169
  3. European Commission JRC — PVGIS (optimal inclination and orientation)
  4. Hay, J.E. & Davies, J.A. (1980). Calculation of the solar radiation incident on an inclined surface

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