Oct 20, 2026News & Insights
Solar Street Light Sizing: Why Watts Lie
Size a solar street light by road width, pole height, nightly hours and autonomy days, not by the watt number. A six-point checklist for project quotes.
Most solar street light enquiries start with a watt number. A buyer asks for 200W because that is what the last quotation said, and the whole negotiation then happens around that figure. Wattage is the least useful number in a solar street light specification. What decides whether a road is actually lit, and whether the lights are still working after the first winter, is pole height, spacing, nightly running hours, days of autonomy and the sun hours available in the weakest month. This guide sets out the sizing sequence we use with distributors and contractors, so a quotation can be built from site data instead of from a competitor's spec sheet.
Why the watt number misleads
The wattage quoted on a solar street light is usually the rated input of the LED chip, sometimes the rated power of the whole luminaire, and occasionally an "equivalent" figure borrowed from conventional lighting. Two fixtures both sold as 200W can put very different light on the road, because LED efficacy, the lens and the mounting height all sit between the watt and the result. When a customer compares watt numbers across suppliers, they are comparing the least comparable figure on the sheet.
It works in reverse too: a better optical design at a lower wattage can meet the same requirement with a smaller panel and a smaller battery, which lowers the total system cost. Distributors who move the conversation from watts to lumens and layout win tenders on price and on field performance at the same time.
Step 1 - Fix the pole height, spacing and road width
Pole height sets how wide an area one luminaire can cover; spacing sets how evenly it is covered. A 6 m pole at 20 m spacing needs a different light distribution from an 8 m pole at 30 m spacing, even at identical wattage. Ask for the road width, whether poles run on one side or both, whether they are opposite or staggered, and the mounting height. A simple sketch is enough to shortlist the optics. Without it, any wattage you quote is a guess.
Step 2 - Convert the requirement into lumens
Lumens describe what the lamp emits. What the driver and the pedestrian actually get is light on the road surface, and that depends on the distribution pattern and the mounting height as much as on the lumen total. Ask your supplier for the rated lumen output and the light distribution type, and compare those two figures rather than the watt rating. Where a project specifies a lighting class or an average maintained lux level, send that requirement together with the layout and let the manufacturer run the photometric calculation - that is the point where a quotation becomes defensible.
Fix the operating schedule at the same stage. Dusk-to-dawn running, a timed cut-off at midnight, and a motion or time-step dimming profile produce three very different energy budgets from the same luminaire.
Step 3 - Size the battery in watt-hours
This is where "wattage vs watt-hour" stops being a technicality. The battery has to carry the lamp through the night and through the cloudy days that follow. Work it out as: lamp wattage x operating hours per night x days of autonomy, divided by the depth of discharge you are willing to accept. The result is the watt-hour capacity the battery must actually deliver - not the nameplate figure printed on a cell.
Depth of discharge is the lever that decides service life. Sizing a battery to run shallow costs more up front and lasts longer; sizing it to use most of its capacity every night looks cheaper on the quotation and comes back as a replacement claim. Agree in writing which one the customer is buying, and make sure the warranty wording matches it.
Step 4 - Size the panel for the worst month
The panel must put back what the lamp takes out, plus system losses, within the sun hours available in the weakest month - not the annual average. An undersized panel leaves the battery in a partial state of charge for months at a time. The lamp still lights up, so nobody notices until the battery has lost capacity and the lights start cutting out before dawn. This is the single most common cause of "they worked fine for six months" complaints.
Confirm the charge controller at the same time: the charge profile has to match the battery chemistry, and temperature compensation matters on sites with real seasonal swings.
Specification checklist
Item | What to confirm | Why it changes the quote |
Road width & pole layout | Width, pole height, spacing, one or both sides | Fixes the light distribution and the unit count |
Lumens, not just watts | Rated lumen output and lens / distribution type | Same watts can mean very different road brightness |
Nightly operating hours | Dusk to dawn, timed cut-off, or dimmed schedule | Drives the battery capacity directly |
Days of autonomy | How many cloudy nights must be covered | The biggest single cost driver in the system |
Local sun hours (worst month) | Peak sun hours in the weakest season | Sets the panel size |
Battery chemistry & DoD | Type and agreed depth of discharge | Decides service life and warranty exposure |
Where projects go wrong
Three mistakes account for most field problems: sizing the panel for average sun rather than the worst month, running a battery deeper every night than it was sized for, and mounting a luminaire at a height its light distribution was never designed for. None of them show up in a showroom or in a sample approval - they appear after the first winter, when the replacement cost is already somebody's problem.
How to quote it as a distributor
Send the six-item checklist above as a one-page form with every enquiry. When the buyer fills it in, you can spec the whole system - luminaire, battery, panel, pole and controller - instead of trading watt numbers against a competitor. It also protects you: if the site data turns out to be wrong later, the record shows where the specification came from.
For private-label or tender supply, agree four things in writing before production: the printed wattage and lumen rating, the battery specification and capacity, the autonomy assumption behind the quotation, and the warranty wording. These four are what get challenged later.
Next step
Send us the road width, pole height and spacing, the nightly operating hours, and the number of autonomy days your market expects. We will match the luminaire, battery, panel and pole to the site and quote against that configuration rather than against a watt number.
Talk to our team on the contact page: /contact-us/ - or message us on WhatsApp for a faster reply.