The Four Factors That Determine Spacing
Every solar street light spacing calculation comes down to four variables. Change any one and the number of lights per kilometer shifts.
Factor 1: Pole Height
Pole height controls the spread diameter of the light cone. A taller pole casts light over a wider area, allowing greater spacing between fixtures. The relationship is roughly linear within the 4-12 m range used for solar street lighting:
- 4 m pole: Light cone diameter approximately 10-12 m. Used for pathways, parks, residential lanes.
- 6-7 m pole: Light cone diameter approximately 16-20 m. The most common height for village roads and secondary roads.
- 8-9 m pole: Light cone diameter approximately 24-30 m. Used for primary roads and collector streets.
The engineering constraint: pole height must match road width. A rule of thumb is that pole height should be 1.0 to 1.2 times the road width for single-side mounting, or 0.7 to 1.0 times for staggered/opposite mounting.
Factor 2: Fixture Wattage and Lumen Output
Higher wattage means more lumens, which means a wider effective illumination area at the required lux level. But wattage alone is misleading — lumen efficacy matters. A 30 W unit produces 2,850-5,700 lumens depending on series. A budget fixture with low-efficiency chips needs double the wattage to match the same light output. The spacing in our table assumes high-efficiency LED chips. For a deeper explanation of why lumens per watt matters more than raw wattage, see our guide on how to choose a solar street light.Factor 3: Road Width and Carriage Type
A 6 m wide single carriageway can be lit from one side. A 12 m dual carriageway needs either taller poles with wider beam angles, or lights on both sides. This directly doubles the quantity:
- Single carriageway (3-7 m): Single-side mounting — one row of lights.
- Dual carriageway (8-14 m): Staggered or opposite mounting — effectively two rows of lights.
Factor 4: Lighting Standard and Uniformity
Different road classes require different minimum lux levels and uniformity ratios. A residential path might need 5 lux average, while a collector road requires 15 lux. Higher uniformity requirements (ratio of minimum lux to average lux > 0.4) demand tighter spacing. National standards vary, but the CJJ 45 and CIE 115 frameworks are the most commonly referenced in our project markets.
Master Spacing and Quantity Table
This table uses data from photometric testing of our three solar street light product lines. Spacing values assume single-side mounting on a road width approximately matching pole height, targeting 15-20 lux average illuminance with uniformity ratio above 0.35.
| Pole Height | Wattage | Spacing | Lights/km (Single Side) | Lights/km (Both Sides) | Model |
|---|---|---|---|---|---|
| 4 m | 12 W | 15 m | 67 | 134 | BF-SSL-20-45W |
| 6 m | 20 W | 20 m | 50 | 100 | BF-SSL-20-65W / BF-SSL-21-65W / BF-SSL-22-60W |
| 7 m | 30 W | 22 m | 46 | 91 | BF-SSL-21-90W / BF-SSL-22-80W |
| 8 m | 30-35 W | 25 m | 40 | 80 | BF-SSL-20-90W / BF-SSL-21-120W |
| 9 m | 40 W | 32 m | 32 | 63 | BF-SSL-22-120W |
Three Mounting Arrangements Explained
The arrangement pattern determines both the quantity of lights and the uniformity of illumination. There are three standard patterns used in road lighting design.
Single-Side Arrangement
All poles are mounted on one side of the road. This is the simplest and most economical layout. It works well when the road width is less than or equal to the pole height.
Road direction →
─────────────────────────────────────
● ● ● ● ← Lights (one side)
═══════════════════════════════════════ ← Road surface
─────────────────────────────────────
Staggered (Zigzag) Arrangement
Poles alternate between left and right sides of the road. This provides better uniformity than single-side at a moderate cost increase. Each side has half the total lights, but with double the spacing between same-side poles.
Road direction →
─────────────────────────────────────
● ● ← Lights (left side)
═══════════════════════════════════════ ← Road surface
● ● ← Lights (right side)
─────────────────────────────────────
Opposite (Bilateral) Arrangement
Poles are placed directly across from each other on both sides. This is used for wide roads or roads requiring high uniformity under strict standards.
Road direction →
─────────────────────────────────────
● ● ● ● ← Lights (left side)
═══════════════════════════════════════ ← Road surface
● ● ● ● ← Lights (right side)
─────────────────────────────────────
Model Selection by Road Type
Choosing the right fixture depends on the road classification and expected traffic. Here is how our three product lines map to common project types.
BF-SSL-20 Series (12-30 W): The most versatile range. The 12 W variant handles pathways and park walkways at 4 m pole height. The 20-30 W variants cover village roads and secondary roads at 6-8 m. This series suits most rural area solar lighting projects where cost efficiency is the priority. BF-SSL-21 Series (20-35 W): Designed for medium to heavy-duty applications. The 35 W at 8 m pole height covers collector roads and school zones. The higher lumen package provides wider beam coverage at each spacing interval. Ideal for village solar lighting projects that serve as main traffic arteries. BF-SSL-22 Series (20-40 W): Our highest output line. The 40 W at 9 m pole height achieves 32 m spacing — the widest in our range. This translates to the fewest lights per kilometer (32 single-side), making it the most cost-effective choice for long-distance highway solar lighting deployments where minimizing pole count is critical.Budget Estimation by Kilometer
Once you know how many lights per kilometer, estimating the total lighting cost becomes straightforward. The budget has three components.
Component 1: Fixture Cost
| Configuration | Lights/km | Unit Price Range (FOB) | Fixture Cost per km |
|---|---|---|---|
| 4 m / 12 W (single side) | 67 | $45-65 | $3,015-4,355 |
| 6 m / 20 W (single side) | 50 | $65-90 | $3,250-4,500 |
| 7 m / 30 W (single side) | 46 | $80-110 | $3,680-5,060 |
| 8 m / 35 W (single side) | 40 | $95-130 | $3,800-5,200 |
| 9 m / 40 W (single side) | 32 | $110-150 | $3,520-4,800 |
Component 2: Pole and Foundation Cost
Poles are typically sourced locally to minimize shipping cost. Budget estimates:
- 4-6 m galvanized steel pole with foundation: $80-150 per pole
- 7-8 m galvanized steel pole with foundation: $120-200 per pole
- 9 m galvanized steel pole with foundation: $150-250 per pole
Component 3: Installation Labor
Installation cost varies by region, terrain, and local labor rates. For budget planning, allocate $20-50 per pole for installation in developing markets (Africa, Southeast Asia) and $50-120 per pole in developed markets.
Total Budget per Kilometer (Single Side)
| Configuration | Fixtures | Poles | Installation | Total per km |
|---|---|---|---|---|
| 6 m / 20 W (50 lights) | $3,250-4,500 | $4,000-7,500 | $1,000-2,500 | $8,250-14,500 |
| 7 m / 30 W (46 lights) | $3,680-5,060 | $5,520-9,200 | $920-2,300 | $10,120-16,560 |
| 9 m / 40 W (32 lights) | $3,520-4,800 | $4,800-8,000 | $640-1,600 | $8,960-14,400 |
The 9 m configuration is often the most economical per kilometer despite higher per-unit cost — fewer poles, fewer foundations, less installation labor. This is the counter-intuitive insight many project planners miss: fewer, more powerful lights often cost less per kilometer than many smaller lights.
Common Calculation Mistakes
After reviewing hundreds of project RFQs, these are the three errors I see most frequently.
Mistake 1: Using generic spacing without checking beam angle. A 30 W fixture from two different manufacturers can have completely different beam angles (60 degrees vs 140 degrees), which changes the effective spacing by 40% or more. Always request the IES photometric file and simulate the actual light distribution. Mistake 2: Forgetting the arrangement multiplier. A project planner calculates 46 lights per km for a 7 m pole and budgets accordingly — then discovers the road is 12 m wide and requires both-side mounting. The quantity doubles to 91. Always confirm road width and arrangement type before finalizing quantities. Mistake 3: Ignoring curves, intersections, and obstacles. Straight-road calculations are the baseline, but real roads have curves that need tighter spacing on the outside edge, intersections that need additional illumination, and trees or structures that block light. Add 10-15% to your straight-road calculation for a realistic total.How to Get an Exact Layout for Your Project
The master table gives you a reliable planning estimate. For a precise, engineering-grade layout, you need a photometric simulation using the actual road geometry and the specific fixture's IES data. This simulation produces a point-by-point lux grid showing exact illuminance values and uniformity ratios at every location on the road surface.
We provide this calculation free of charge for any project inquiry. Send us the following information:- Road length and width
- Road type (single/dual carriageway, vehicle/pedestrian)
- Desired pole height (or ask us to recommend)
- Any applicable national lighting standard
- Site photos or satellite images showing curves, obstacles, intersections
FAQ
How many solar street lights do I need for 1 km of village road?
For a typical village road with 6-7 m poles and 20-30 W fixtures on single-side mounting: 46 to 50 lights per kilometer. This covers road widths of 5-7 m at 15-20 lux average illuminance. If the village road is narrower (3-4 m), a 4 m pole with 12 W fixtures at 15 m spacing (67 per km) is more proportionate and cost-effective.
What is the standard spacing for solar street light poles?
There is no single standard — spacing ranges from 15 m to 32 m depending on pole height and fixture power. The most common configuration in our projects is 20-25 m spacing with 6-8 m poles, which balances illumination quality and cost. National standards like CJJ 45 (China), BS 5489 (UK), and IESNA RP-8 (US) provide spacing guidelines by road class.
Can I reduce the number of lights by using higher wattage fixtures?
Yes, within limits. Higher wattage on taller poles increases spacing and reduces quantity. Our BF-SSL-22-120W (40 W on 9 m pole) achieves 32 m spacing — 32 lights per km versus 50 for a 20 W on 6 m pole. However, the pole height must be appropriate for the road width. Mounting a 40 W fixture on a 6 m pole will not achieve 32 m spacing because the pole height limits the light spread regardless of wattage.
Do solar street lights on both sides of the road need to face each other or be staggered?
Both arrangements work, but they serve different purposes. Staggered (zigzag) provides acceptable uniformity at lower cost — each side has half the lights but the alternating pattern fills gaps. Opposite (face-to-face) arrangement provides the highest uniformity and is required for roads with strict lighting standards. Staggered is the default recommendation for most projects unless the specification demands uniformity ratios above 0.4.
How does spacing change for curved roads?
On curves, the outside edge of the road is farther from a pole mounted on the inside. We recommend reducing spacing by 20-30% on the outside of curves with radius under 100 m, and placing additional lights at the start and end of each curve for transition visibility. Intersections and T-junctions also require supplementary lighting beyond the straight-road baseline.
What spacing should I use if I do not know the local lighting standard?
Use our master table as a starting point — it targets 15-20 lux average illuminance, which satisfies most developing-market standards for secondary roads. For safety-critical roads (highways, school zones, intersections), tighten spacing by 15-20% from the table values. For low-traffic residential paths, you can relax spacing by 10-15%. When in doubt, send us your road dimensions and we will recommend the appropriate standard and spacing.
How do I calculate the total project cost from lights per kilometer?
Multiply the lights-per-km count by three cost components: fixture unit price (FOB), pole plus foundation cost (usually sourced locally), and installation labor. For a 7 m / 30 W single-side deployment at 46 lights per km, budget approximately $10,000-16,500 per kilometer depending on region and pole specification. Our cost breakdown guide details every line item from BOM to landed cost.Does Beamfact provide free lighting layout design?
Yes. We provide free photometric layout design for any project inquiry. Send us your road length, width, type, and any applicable lighting standard. Our engineering team returns a complete layout drawing with pole positions, fixture model selection, quantity bill, and a point-by-point lux distribution map — typically within 48 hours. This service helps you validate quantities before committing to an order.