On a solar street light, the headline watt figure can describe several different quantities: the LED’s power at full output, the rated capacity of the controller, the panel’s rated power, or a comparison with a conventional lamp. What decides how the road looks at 3 a.m. is the light delivered through the whole night, and that depends on three numbers agreeing with each other: panel watts, battery watt-hours and the dimming profile.
On a mains fixture, the watt figure is the power drawn for as long as the light is on. A solar light has a fixed energy budget each night, set by what the panel harvested and the battery stored, and the controller decides how it is spent. Two solar lights with the same watt figure can therefore be built for quite different nights, and both can be described correctly.
This article explains what the watt figure can refer to, shows the energy balance with simple numbers, and gives you a template for comparing two offers on equal terms. The full sizing calculation lives in our guide to sizing solar street lights; here the aim is to read a listing and a quotation correctly.
Key Takeaways
- A solar street light’s watt figure usually describes one operating point, most often the LED’s power at its brightest dimming step, not the average power drawn across the night.
- Panel watts, battery watt-hours and the dimming profile have to balance at the site’s worst-month sun hours; the watt figure alone cannot tell you whether they do.
- Lumens decide how much light reaches the road, so ask for measured luminaire lumens at each dimming step alongside the LED power.
- Battery capacity in amp-hours only becomes comparable once it is converted to watt-hours at the pack’s nominal voltage.
- Two offers with the same watt figure can suit different briefs; compare them on nightly watt-hours, lumen-hours and autonomy days at your own site.
What the Watt Figure on a Solar Street Light Can Refer To
Solar lighting has no single convention for which power figure goes in a product name or listing title. Each of the figures below is a legitimate measurement. They simply answer different questions.
| Figure | What it measures | When it applies | What it tells you | Pair it with |
|---|---|---|---|---|
| LED power at full output | Electrical power into the LED module at the 100% step | Only while the light runs at its brightest setting | The top of the operating range | Power at every other dimming step |
| Nominal LED package rating | Sum of the individual LED packages’ rated power | A design capacity; packages are often driven below it | How much LED the head carries | The actual drive power at each step |
| Controller or driver rating | The maximum output the electronics are designed to deliver | A ceiling, not an operating point | Headroom in the electronics | LED power actually delivered |
| Panel rated power (Wp) | Panel output under standard test conditions | Laboratory conditions, not a winter morning | How much energy the panel can harvest per sun hour | The site’s worst-month peak sun hours |
| Lamp comparison | Output compared with a conventional lamp of a stated wattage | A stated comparison, based on an assumed light output and optic | The replacement the supplier has in mind | Measured lumens and a photometric layout |
Why these figures differ
The LED figures and the panel figure measure opposite ends of the system. The panel figure is power in, measured at standard test conditions: 1,000 W/m² irradiance and 25 °C cell temperature, the reference used in IEC photovoltaic module testing. Real panels rarely see those conditions for long, which is why harvest is calculated from peak sun hours and a loss factor rather than from the Wp figure directly. The LED figures are power out of the battery, at one chosen setting.
The controller rating and the package rating are capacities. They describe what the hardware is designed to handle, and a well-designed light often runs its LEDs below the package maximum, because lower drive current means lower junction temperature and a slightly higher luminous efficacy. So a figure that describes capacity and a figure that describes operation can both be correct for the same light.
The lamp comparison is useful shorthand when replacing an existing installation, but it rests on the optic and the lumens delivered, so pair it with measured lumens and a layout. The practical rule: whatever figure appears in the title, find out which one it is, and ask for the others.
Average power across a night is a separate number
Even when the watt figure is clearly the LED power at full output, it rarely describes the whole night. Most solar street lights run a dimming profile: full output in the busy evening hours, a lower step after midnight, and sometimes a motion-sensor boost. The watt figure normally describes the top step. The average power over twelve hours, which is what the battery actually has to supply, is lower by design. The complete solar street light guide explains why profiles are built this way; the point here is that the average and the headline are two different numbers, and you need both.
The Energy Balance of a Solar Street Light in One Picture
Every solar street light, whatever its wattage label, has to satisfy the same chain:
Panel harvest per day → usable battery energy → LED power × hours at each dimming step
If the worst-month harvest is below what the profile spends, the battery drains a little each night until the light shuts down early. If the battery is small relative to the nightly spend, there is no reserve for cloudy days. The watt figure sits at the far right of the chain, so it cannot show whether the chain balances.
Worked example: building the chain forwards
The inputs below are round, hypothetical numbers chosen to show the arithmetic. They do not describe any particular product. The method, including the 0.7 loss factor and 80% usable depth of discharge, follows our solar sizing article, which also covers temperature derating and panel tilt.
Inputs:
| Input | Value |
|---|---|
| LED power at full output | 40 W |
| Dimming profile | 100% for 5 h, then 50% for 7 h (12 h night) |
| Luminaire efficacy | 150 lm/W (assumed for the example) |
| Worst-month peak sun hours | 4.0 |
| Combined losses (controller, temperature, wiring, dust) | factor 0.7 |
| Autonomy required | 3 nights |
| Usable depth of discharge | 80% |
Step 1: nightly energy. 40 W × 5 h = 200 Wh, plus 20 W × 7 h = 140 Wh, giving 340 Wh per night. The average power across the night is 340 ÷ 12 ≈ 28 W, against a 40 W headline.
Step 2: battery. 340 Wh × 3 nights ÷ 0.8 ≈ 1,275 Wh.
Step 3: panel. 340 Wh ÷ (4.0 × 0.7) ≈ 121 Wp, so the next standard panel size up.
Step 4: light delivered. At 150 lm/W, the top step gives about 6,000 lm and the 50% step about 3,000 lm. Across the night that is 6,000 × 5 + 3,000 × 7 = 51,000 lumen-hours.
Worked example: reading the chain backwards
Buyers more often work in the other direction: a listing gives a panel and a battery, and the question is what night they can support. Take a hypothetical configuration with a 100 Wp panel and a 600 Wh battery at a site with 3.0 worst-month peak sun hours.
- Daily harvest: 100 Wp × 3.0 × 0.7 = 210 Wh. That is the nightly budget the panel can replace in the worst month.
- Average power over a 12 h night: 210 ÷ 12 = 17.5 W.
- Reserve: 600 Wh × 0.8 = 480 Wh usable, about 2.3 nights at that budget.
This configuration could legitimately run a 40 W top step for 3 hours (120 Wh) and then 10 W for 9 hours (90 Wh), using exactly 210 Wh. Its LED power at full output is 40 W, and that statement is accurate. The night it delivers is simply shaped differently from the forward example above, and whether it suits a given road depends on the brief.
Two site-specific details change these numbers more than anything else. First, peak sun hours in the worst month can be far below the annual average; public tools such as the EU Joint Research Centre’s PVGIS and NREL’s PVWatts give monthly solar radiation for a location. Second, architecture matters: an all-in-one solar street light carries a near-horizontal panel whose worst-month yield falls at higher latitudes, while a split solar street light can tilt its panel to the latitude.
How to Read a Solar Street Light Listing: The Five Numbers to Ask For
A listing or catalogue page has limited space, so it usually shows a wattage class and a run time. For a project, five numbers let you rebuild the energy balance yourself.
| # | Number | Unit | Why it matters | What to confirm |
|---|---|---|---|---|
| 1 | Panel rated power | Wp | Sets the daily harvest at your sun hours | Panel type, and whether it is integrated (fixed angle) or separate (tiltable) |
| 2 | Battery capacity and chemistry | Wh, plus chemistry | Sets the nightly budget and the cloudy-day reserve | Nominal voltage, usable depth of discharge, cycle rating and temperature range |
| 3 | LED power at each dimming step | W per step | Converts the profile into nightly watt-hours | Which step the headline figure describes |
| 4 | Measured lumens | lm per step | Decides how much light reaches the road | Measured on the complete luminaire, with an IES file for the layout |
| 5 | Run profile and autonomy | hours per step, nights | Shows how the budget is spent and how long it lasts without sun | The site and sun hours the profile was set for |
Battery capacity: convert amp-hours to watt-hours
Battery capacity is often given in amp-hours, and amp-hours alone are not comparable across products, because pack voltage varies. Energy is voltage × amp-hours:
| Stated capacity | Nominal pack voltage | Energy |
|---|---|---|
| 30 Ah | 3.2 V (single LiFePO4 cell group) | 96 Wh |
| 30 Ah | 12.8 V (4-cell LiFePO4 pack) | 384 Wh |
| 30 Ah | 25.6 V (8-cell LiFePO4 pack) | 768 Wh |
The same “30 Ah” can mean an eightfold difference in stored energy. Ask for watt-hours, or for the nominal voltage so you can calculate them. Chemistry matters as much as capacity for service life; the battery section of the complete guide linked above compares the common chemistries.
Lumens: the number that reaches the road
Watts describe energy; lumens describe light. Two LED heads at the same power can emit different lumens depending on the LED packages, the drive current and the optic. For a project, ask for lumens measured on the complete luminaire (the LM-79 method from the Illuminating Engineering Society), at each dimming step, plus the IES file. Where the light lands is then a layout question, covered in our EN 13201 roadway lighting design guide. If you want a line-by-line walk through the rest of a specification sheet, see how to read an LED lighting datasheet.
Run time: what “12 hours” describes
A run-time figure gives the length of night the configuration is set up for. Pair it with the profile (how the hours split between steps) and the autonomy (how many sunless nights the battery covers); both depend on the site.
Comparing Two Solar Street Light Quotes on Equal Terms
When two offers carry the same watt figure, put them in one table and normalise everything to your site. Copy the template below and fill in the blanks.
The template
| Field | Quote A | Quote B | How to normalise |
|---|---|---|---|
| Headline watt figure, and what it refers to | Confirm it is LED power at full output | ||
| Panel rated power (Wp) | |||
| Panel mounting (integrated / tilted) | Apply a lower yield to a flat panel at higher latitudes | ||
| Battery energy (Wh) and chemistry | Convert Ah × nominal V | ||
| LED power at each step (W × h) | List every step | ||
| Nightly energy (Wh) | Sum of W × h across steps | ||
| Daily harvest at your worst-month PSH (Wh) | Wp × PSH × 0.7 | ||
| Harvest ÷ nightly energy | Should be at least 1.0 | ||
| Usable battery Wh ÷ nightly energy | Autonomy in nights | ||
| Lumens at each step | Measured, complete luminaire | ||
| Lumen-hours per night | Sum of lm × h across steps | ||
| IES file provided | Needed for the layout |
A filled-in example
Both configurations below are hypothetical, share a 40 W headline and a 6,000 lm top step, and are evaluated at a site with 4.0 worst-month peak sun hours.
| Field | Configuration A | Configuration B |
|---|---|---|
| Panel | 130 Wp | 90 Wp |
| Battery | 1,280 Wh (100 Ah × 12.8 V) | 640 Wh (50 Ah × 12.8 V) |
| Profile | 40 W × 5 h, then 20 W × 7 h | 40 W × 3 h, then 12 W × 9 h |
| Nightly energy | 340 Wh | 228 Wh |
| Daily harvest (Wp × 4.0 × 0.7) | 364 Wh | 252 Wh |
| Harvest ÷ nightly energy | 1.07 | 1.11 |
| Autonomy (usable Wh ÷ nightly) | 3.0 nights | 2.2 nights |
| Lumen-hours per night | 51,000 | 34,200 |
Lumens at the dimmed steps are taken as proportional to power, which is slightly conservative since LEDs tend to run a little more efficiently at lower current.
Both configurations balance at this site. They are built for different briefs. A delivers about half as much light again across the night and covers three sunless nights, which suits a road with traffic after midnight or a cloudier climate. B spends less energy after the evening peak, which can suit a quiet footpath where a low late-night level is acceptable. The watt figure is identical and accurate in both cases; the table shows what it could not.
Now move the same two configurations to a site with 2.5 worst-month peak sun hours. A harvests 130 × 2.5 × 0.7 ≈ 228 Wh against a 340 Wh night; B harvests about 158 Wh against 228 Wh. Neither balances any longer, and each would need a larger panel, a gentler profile or both. The configuration belongs to the site, not to the watt figure. For how an unbalanced chain shows up in the field, see why solar street lights fail.
Questions to Ask a Solar Street Light Supplier
- Which quantity does the headline watt figure describe? LED power at full output, package rating, controller rating, panel power, or a lamp comparison.
- What is the LED power at each dimming step, and how many hours does each step run?
- What is the battery energy in watt-hours, its chemistry, its nominal voltage and its usable depth of discharge?
- What is the panel’s rated power, and is it fixed in the housing or tiltable?
- What are the measured luminaire lumens at each step, and is an IES file available?
- Which peak sun hours was the configuration sized for, and for which month?
- How many autonomy nights does the configuration give at my profile?
- Can the profile be adjusted after installation, and does that change the autonomy figure?
Questions 2, 3, 4 and 6 rebuild the energy balance; question 5 lets a designer check the road against its lighting class.
The Bottom Line
The watt figure on a solar street light is a useful starting point that describes one operating point of a system. Whether that system suits your road depends on the panel, the battery and the profile balancing at your site in its worst month, and on the lumens delivered at each step. Compare offers on nightly watt-hours, lumen-hours and autonomy, and the headline wattage takes its proper place as one line in the table.
That balance is what we calculate with every quotation for our all-in-one and split solar street lights: the panel, battery and run profile are sized to your site’s worst-month sun hours rather than taken from a fixed menu. Send us the site location and the hours of light you need, and the configuration comes back with the arithmetic shown.
FAQ
What does the wattage of a solar street light mean? Most often it is the LED’s power at its brightest dimming step, but it can also be the LED package rating, the controller’s maximum output, the panel’s rated power or a lamp comparison. Ask which it is, then ask for panel watts, battery watt-hours and the dimming profile.
Is a higher-wattage solar street light always brighter? Not necessarily across the whole night. A higher LED power at full output means more light at the top step, but brightness through the night depends on the lumens at each dimming step and on how long each step runs. A light with a lower headline and a gentler profile can deliver more lumen-hours than one with a higher headline and a steep midnight step-down.
What is the difference between watts and lumens on a solar street light? Watts measure electrical power; lumens measure light output. Their ratio is luminous efficacy, which varies with LED packages, drive current and optic. On a solar light, watts show how fast the battery is used and lumens show how much light results, so compare both at each dimming step.
How do I check that the panel and battery match the LED? Work out the nightly energy by multiplying the LED power at each step by the hours at that step and adding them up. Then multiply the panel’s rated watts by your site’s worst-month peak sun hours and a loss factor of about 0.7. The daily harvest should at least equal the nightly energy, and the usable battery watt-hours divided by the nightly energy gives the autonomy in nights.
Why can two solar street lights with the same wattage have different panels and batteries? Because the watt figure usually describes only the top dimming step. One light may be configured for a long full-output evening and several nights of reserve, another for a short peak and a low late-night level. Both carry the same accurate headline while spending different amounts of energy.
What battery and panel size does a solar street light need? It depends on the nightly energy of the dimming profile, the worst-month peak sun hours at the site, and the number of autonomy nights required. The battery is roughly nightly energy × autonomy nights ÷ usable depth of discharge, and the panel is nightly energy ÷ (peak sun hours × 0.7). Our solar sizing guide works through the full method with climate examples.