Calculator
Example data table
| Location | Date | Window | Shade | Cloud | Effective sun hours | Classification |
|---|---|---|---|---|---|---|
| 31.5204, 74.3587 | 2026-02-25 | 06:00–18:00 | 15% | 20% | ~5.10 | Partial Sun |
| 24.8607, 67.0011 | 2026-02-25 | 07:00–17:00 | 35% | 10% | ~4.60 | Partial Sun |
| 35.6762, 139.6503 | 2026-02-25 | 08:00–16:00 | 60% | 30% | ~1.90 | Dappled Light |
Example outputs are illustrative and will change with season and inputs.
Formula used
The calculator samples the sun’s position across your time window using a compact solar-position method based on Julian day, solar declination, equation of time, and hour angle.
- Solar elevation comes from the solar zenith angle: elevation = 90° − zenith.
- Incidence on surface uses a dot product between the sun vector and the surface normal: cos(θ) = s · n.
- Intensity index scales to 0–100: Index = 100 × max(0, cos(θ)).
- Realism factors reduce direct sunlight: Index × (1−shade) × (1−0.75×cloud) × canopy.
- Effective sun hours integrate intensity over time: Σ(Δt_hours × Index/100).
This is a planning tool; nearby terrain and microclimates can shift results.
How to use this calculator
- Enter your garden’s latitude and longitude.
- Select the date you want to plan for.
- Set the time window you care about, like morning.
- Adjust shade, clouds, and canopy to match conditions.
- Press Calculate and review classification and sun hours.
- Download CSV or PDF to keep a project record.
For plant matching, aim for Full Sun ≥ 6 effective hours.
Daily sun hours and growth response
Most edible and flowering crops perform best when the site delivers enough effective direct sun. As a practical guide, many “full sun” plants need about 6+ effective hours, “part sun” 4–6 hours, “part shade” 3–4 hours, and “full shade” under 3 hours. Fruiting crops often lose yield below ~4 hours, while leafy greens commonly tolerate 2–4 hours.
Why effective hours differ from clock hours
Sunlight intensity changes through the day because solar elevation and surface orientation change. A south-facing wall or slope can boost midday exposure, while a north-facing aspect can reduce it. The intensity index used here weights stronger sun more than weak-angle sun, producing “effective” hours that better match plant photosynthesis than a simple sunrise-to-sunset count. Latitude and season also matter: winter sun stays lower and casts longer shadows.
Shade, clouds, and canopy as reduction factors
Real gardens have obstructions. Permanent shade from buildings and trees can cut direct sun by 10–80%, and cloudier seasons can further soften light. The shade, cloud, and canopy controls let you model those reductions so the output reflects what reaches leaves, not what reaches the top of the atmosphere. Use canopy reduction for pergolas, netting, or dense branches overhead.
Turning results into plant placement decisions
Use the classification and effective hours to plan beds and containers. Reserve the brightest zones for tomatoes, peppers, melons, sunflowers, and most herbs. Medium zones suit lettuces, spinach, many brassicas, and shade-tolerant ornamentals. Low zones are better for ferns, hostas, and mossy groundcovers. If you are borderline, prefer morning sun for tender foliage and afternoon sun for heat-loving crops. Save a few runs with different time windows to build a simple sun map.
Improving exposure without moving the garden
Small changes add up. Prune selectively to lift tree canopies, add light-colored surfaces to reflect light into beds, and rotate containers seasonally. For sloped sites, align rows to reduce self-shading and keep tall crops on the north edge. Recheck results in different months because the sun path shifts between seasons.
FAQs
1) What is “effective sun hours”?
It is sun time weighted by intensity. Strong, high-angle sun counts more than weak-angle sun, so the total better represents usable light for plant growth than a simple clock-hour estimate.
2) How do I estimate shade percentage?
Observe the site during the chosen window. If direct sun is blocked about half the time by buildings, fences, or trees, use ~50%. If only brief shadows pass, use 10–25%.
3) Does cloud cover mean my garden is “shade”?
Not exactly. Clouds reduce direct-beam intensity, but bright overcast can still support many shade-tolerant plants. Use the cloud slider to reflect typical conditions, then rely on the classification as a planning guide.
4) Why do results change by month?
The sun’s path shifts seasonally. In winter the sun stays lower, shadows lengthen, and effective hours drop. In summer the sun climbs higher, shortening shadows and increasing usable light in open areas.
5) What inputs matter most for balconies or walls?
Orientation and tilt are key. A south-facing wall can receive strong midday sun, while an east-facing balcony favors morning light. Use the surface angle and direction to match your actual exposure.
6) Can I use this for solar panels instead of plants?
It gives a simplified sunlight intensity index, which can help compare locations. For design-grade solar work, use a dedicated PV tool with irradiance data, shading geometry, and system losses.