How to Size a Solar Street Light System: The Complete Sizing Worksheet

Most solar street light quotes that go wrong go wrong for the same reason: someone picked the wattage based on the road width alone, and ignored three other variables. Then the fixture is either too dim (and the customer complains for 5 years), or massively over-sized (and the customer paid 40% more than they needed to).

This worksheet walks you through every variable that matters, in the order they actually matter, so you can size any project correctly — whether it is a 50-meter residential driveway or a 10-kilometer highway.

The 5 Variables You Need to Know

Before you start calculating, you need five numbers from the project site:

1. Road width (meters)
2. Pole height (meters)
3. Pole spacing (meters)
4. Number of consecutive cloudy days in your region (autonomy requirement)
5. Mounting location (does the pole get shade from trees or buildings?)

That is it. If you do not have these five, the worksheet will not give you a real number.

Step 1: Wattage from Road Width

This is the most important decision. The light on the ground, not the wattage on the label, is what matters.

The IES (Illuminating Engineering Society) recommendation for road lighting is:

• Residential street: 3-5 lux average, 0.5-1.5 uniformity
• Collector road: 5-8 lux
• Main road: 8-15 lux
• Highway: 15-30 lux

A 170 lm/W LED fixture at 6 meters height will produce approximately these lux levels on a flat road:

Wattage	6m pole, 5m road	8m pole, 7m road	10m pole, 10m road
30W	6 lux (residential)	not enough	not enough
60W	12 lux (main road)	8 lux (residential)	not enough
100W	20 lux (highway)	13 lux (main road)	8 lux (residential)
150W	30 lux (highway+)	20 lux (highway)	13 lux (main road)
200W	too much	27 lux (highway)	17 lux (main road)

The right pick depends on your road type, not the wattage that “looks right” in a catalog.

Step 2: Pole Spacing from Road Uniformity

The uniformity ratio (U0) is the ratio between the dimmest point and the brightest point on the road. Anything below 0.4 is unacceptable for driving. The pole spacing that achieves U0 ≥ 0.4 is roughly 3× the pole height for a one-sided layout, or 5× the pole height for a staggered two-sided layout.

Pole height	1-sided spacing	2-sided staggered spacing
5m	15m	22m
6m	18m	25m
7m	21m	28m
8m	24m	32m
10m	30m	40m
12m	36m	48m

If your project has tighter spacing than these numbers, you can drop one wattage tier. If it is wider, you need to step up.

Step 3: Battery Capacity from Autonomy

This is the step most calculators get wrong, because they ignore temperature. A LiFePO4 battery rated at 50Ah at 25°C only delivers about 42Ah at 0°C and 38Ah at -10°C. If you are deploying in a cold region, multiply the calculated capacity by 1.15-1.25.

The formula:

Required capacity (Wh) = LED wattage × night hours × autonomy days × 1.2 (depth of discharge buffer)

For a 60W LED, 11-hour nights, 2 days autonomy, moderate climate:

60 × 11 × 2 × 1.2 = 1,584 Wh

At 12.8V system voltage: 1,584 / 12.8 = 124 Ah battery

This is a large battery for an all-in-one fixture. Most 60W all-in-ones ship with 30-40Ah batteries, which gives 1 night of real autonomy. To get 2 nights, you typically need to step up to a 80W or 100W fixture with a bigger battery bay, or move to a split system.

Step 4: Solar Panel Wattage from Location

The solar panel must recharge the battery during the day. The formula:

Required solar panel wattage = (Required Wh per day) / (Peak sun hours × 0.85 system derate)

Peak sun hours by region (annual average):

• North Africa, Middle East: 5.5-7.0 hours
• Sub-Saharan Africa: 4.5-6.0 hours
• Mediterranean: 4.0-5.5 hours
• South Asia: 4.5-5.5 hours
• Southeast Asia: 3.5-4.5 hours
• Northern Europe: 2.5-3.5 hours
• Pacific Islands: 4.0-5.0 hours

For our 60W LED example, 11-hour night, 1 day autonomy, Sub-Saharan Africa (5 peak sun hours):

Daily consumption: 60 × 11 = 660 Wh
Required panel: 660 / (5 × 0.85) = 155 W

This is why our T6 60W all-in-one ships with an 80W panel — it does not actually meet the 155W spec. It assumes motion-sensor dimming, which reduces average consumption to about 30W, requiring only 80W of panel. If you do not use the motion sensor, you are under-sized for the Sub-Saharan case.

This is also why the standard TR408 60W all-in-one is a safer pick for projects with average solar irradiance.

Step 5: Adjust for Shading and Soiling

Real-world conditions knock 10-30% off the panel output:

• Tree shade for 2 hours/day: -15%
• Heavy dust (desert, construction site): -10% (cleaning every 3 months)
• Bird droppings on panel: -5% (cleaning every 6 months)
• Snow cover (winter): -30% (if you do not tilt the panel)
• Coastal salt spray on panel: -3% (rain usually washes it off)

If your site has any of these, multiply the panel wattage by the reciprocal. For a shaded site: 155 × 1.15 = 178W panel minimum.

The Quick-Reference Sizing Table

If all that math is too much, here is the cheat sheet we use internally:

Road type	Road width	Pole height	Spacing	Wattage	Panel	Battery (1 day)
Residential driveway	3-4m	4-5m	15m	30W	40W	20Ah
Residential street	5-6m	6m	18m	40-60W	60-80W	30-40Ah
Side road	6-7m	7m	21m	60W	80W	40Ah
Main road	7-9m	8-9m	25m	80-100W	100-120W	50-60Ah
Highway	10m+	10-12m	30-35m	120-150W	150-180W	80-100Ah
Parking lot (single)	15m × 30m	8m	area	100W flood	120W	60Ah
Parking lot (array)	per 100 m²	8m	area	150W flood	180W	80Ah

This table assumes Sub-Saharan / South Asian / Middle Eastern irradiance (4.5-5.5 peak sun hours), motion sensor dimming, and 1-day autonomy. For different conditions, scale the panel and battery columns.

What If the Math Says You Need 200W?

When the math pushes you above 150W, an all-in-one fixture is usually the wrong answer. The battery and panel sizes get too big to mount on a single pole-arm without structural concerns. Move to a split system: solar panel on a separate ground-mount or roof, battery in a ground box, LED on the pole. The installation is more work, but the engineering is more honest.

For a downloadable Excel version of this worksheet, send us a request and we will email it to you the same day.