How to Size Commercial Solar Lighting: Fixture Selection by Climate, Load, and Autonomy

A solar lighting system is a balance of three interdependent variables: how much energy the fixture draws each night, how much the panel can harvest given local conditions, and how much battery capacity is available to carry the system through nights when charging falls short. Get those variables right and the system performs reliably through winter. Get them wrong and the battery gradually depletes over successive short-day periods, reducing runtime and accelerating battery wear.

The balance shifts with geography. The same fixture that recharges fully each night in Atlanta may run a chronic energy deficit in Chicago, where December delivers half the peak sun hours. Panel sizing, battery autonomy, and working mode all need to be matched to the specific climate of the installation site -- not just the application. A fixture well suited for Phoenix may provide only one to two nights of autonomy in Seattle during December.

Most commercial solar lighting fixtures come pre-configured with a fixed panel, battery, and controller sized as a package. The manufacturer has done much of the sizing work for you. But those configurations are optimized for a specific load and climate range. When a fixture is well matched to an installation, the panel recovers at least a full night's energy draw on an average winter day and the battery carries the system through multiple days of poor charging. When components are undersized, the battery gradually runs down over successive short-day periods.

Understanding how to calculate ideal component sizing for your location lets you compare models with confidence and make the right selection for your specific application. For a broader introduction to solar lighting system types and applications, see our Solar Lighting Buyer's Guide.

How to Calculate Your Solar Panel & Battery Requirements

Step 1: Identify the Nightly Energy Load

The starting point is how much energy the fixture consumes each night, measured in watt-hours (Wh).

For a fixture running at a single output level:

Nightly load (Wh) = Fixture wattage (W) x Hours of operation

For fixtures using timed multi-step dimming -- the most common working mode in commercial solar area lights -- each stage is calculated separately and summed:

Nightly load = (W1 x T1) + (W2 x T2) + (W3 x T3)

Most fixtures in LED Living Technology's SL Series ship with a default working mode already programmed. If it matches the application's requirements, no reprogramming is needed and the fixture's published autonomy ratings apply directly.

For application-specific working mode guidance, see our guides to Solar LED Pathway and Walkway Lighting, Solar Perimeter and Security Lighting, and Solar Lighting for Parking Lots and Parking Structures.

Step 2: Apply a System Efficiency Factor

Real-world charging and discharge cycles introduce losses. A system efficiency factor of 90% is a reasonable baseline for LiFePO4 systems with MPPT controllers -- the majority of current commercial solar fixtures.

Adjusted daily load = Nightly load / 0.90

This adjusted figure is what the panel must actually deliver, not just what the fixture consumes, and is the number to compare against the panel's expected output at the installation location.

Step 3: Determine Peak Sun Hours for the Location

Peak sun hours (PSH) expresses total daily solar energy as equivalent hours of full-rated panel output at 1,000 W/m². A 40W panel in a location with 3.5 PSH delivers approximately 140 Wh per day.

PSH is not the same as daylight hours. A location may have 10 hours of daylight but only 3.5 PSH in December due to low sun angles, cloud cover, haze, and site obstructions.

For year-round installations, size based on December PSH values rather than annual averages. A system sized on annual averages will recharge reliably in spring and fall but deplete battery reserve through winter.

Representative December PSH ranges by U.S. region:

PSH varies significantly within regions. Before finalizing a selection, look up December PSH for the specific installation zip code using a solar resource database or PSH calculator. The figures above are a starting point.

Step 4: Calculate the Required Panel Wattage

Minimum panel wattage = Adjusted daily load (Wh) / Peak sun hours

Apply a 20% safety margin for panel degradation, soiling, non-optimal tilt, and partial shading:

Recommended panel wattage = Minimum panel wattage x 1.20

Compare the result against the fixture’s integrated panel wattage. If the fixture’s panel wattage is close to, at, or above your calculated target, the panel is appropriately sized for solar recovery at your location. If it falls short, see When a Fixture's Panel or Battery Falls Short below.

Step 5: Calculate the Required Battery Capacity

Battery capacity must cover the target number of nights without solar recharging -- a parameter called battery autonomy.

Required battery capacity (Wh) = Nightly load x Autonomy (nights) / Usable DoD

LiFePO4 batteries, the standard in current commercial solar fixtures, can typically be discharged to 80-90% of rated capacity without accelerating degradation. A DoD factor of 0.90 is used in these calculations.

Autonomy guidelines by application:

It often makes sense to purchase a fixture with battery capacity above the calculated requirement, as this supports long-term battery health by reducing average depth of discharge. For more on battery technologies and autonomy, see Solar Lighting Battery Systems.

Once you have panel and battery requirements in hand, the next step is finding the right fixture from an existing lineup–and knowing what to do when the specs fall short of your location's demands.

When a Fixture's Panel or Battery Falls Short: The Step-Up Strategy

Because integrated fixtures come with fixed panel and battery configurations, the response to a sizing shortfall is to select a different model, not add separate components.

The step-up approach: choose the next larger model in the same series and program it to run at a lower output level. The result is a fixture that delivers the same lumen output as the original target while carrying proportionally more panel and battery capacity relative to the nightly load. This is especially relevant for northern and low-PSH climates -- Chicago, Seattle, Minneapolis -- where a fixture sized for a sunbelt installation will routinely fall short on panel capacity. For remote or off-grid installations where resupply is difficult, the step-up approach is also worth considering as a baseline precaution regardless of climate. See solar off-grid lighting for more.

No field modifications, additional wiring, or separate panel hardware required.

For solar sign lighting, LED Living Technology's SB Series supports an auxiliary add-on panel as an alternative to stepping up. This is useful when lumen output and battery capacity are already correctly sized but panel harvest needs to increase for a lower-PSH location.

The SL Series in Atlanta vs. Chicago: A Worked Example

This example uses LED Living Technology's 60W Solar LED Area Light with Tiltable Lenses -- one of ELEDLights' most popular commercial solar area lights -- and its 100W counterpart to show how the same lumen target can require a different fixture depending on location.

Fixture specs (60W model):

• Maximum output: 60W / 12,000 lumens
• Solar panel: 60W mono-crystalline
• Battery: LiFePO4, 460.8 Wh
• Controller: MPPT
• Default mode: 5-stage intelligent mode with motion sensing; rated 3-4 days autonomy

Because the default mode includes motion-sensing stages with variable output, exact nightly load depends on site traffic. This example assumes moderate traffic and a nightly energy consumption of 120 Wh.

Atlanta installation (December PSH ~4.0):

Adjusted daily load: 120 / 0.90 = ~133 Wh Minimum panel: 133 / 4.0 = ~33W. With 20% margin: ~40W recommended. Battery for 3-night autonomy: 120 x 3 / 0.90 = ~400 Wh recommended.

The 60W panel clears the requirement comfortably. The 460.8 Wh battery meets the 3-night target with headroom. The 60W fixture is well matched for Atlanta.

Chicago installation (December PSH ~2.5):

Adjusted daily load: 120 / 0.90 = ~133 Wh Minimum panel: 133 / 2.5 = ~53W. With 20% margin: ~64W recommended. Battery for 3-night autonomy: ~400 Wh recommended (same as above).

The 60W panel falls short of the ~64W requirement. The battery is close but offers limited reserve for extended overcast. The 60W fixture is a marginal fit for Chicago.

Applying the step-up strategy for Chicago:

The 100W model carries a 90W panel and a 922 Wh battery. Programmed to 60% output, it delivers 60W of effective draw and approximately 12,000 lumens -- matching the 60W model's full output while providing the panel capacity needed for reliable winter recovery. The 922 Wh battery against a ~400 Wh 3-night requirement provides substantial reserve for multi-day overcast stretches common in the upper Midwest. The additional capacity also serves as a buffer during cold stretches when the battery's charging threshold prevents daytime recharge entirely.

The step-up requires no field modifications. The larger panel and battery are already integrated; programming the output level down is the only adjustment needed at installation.

Climate Factors That Affect Fixture Selection

The steps above give you a reliable framework, but the inputs vary with real-world site conditions. Several location-specific factors are worth accounting for before finalizing a selection.

Winter sun angle. At latitudes above 40°N, the sun tracks lower in winter, reducing effective irradiance on a flat or shallowly tilted panel. For northern installations, verify the panel tilt assumed in the fixture's published ratings.

Site shading. Trees and structures that cast no shadow in summer may significantly shade a panel at low winter sun angles. Even partial shading can reduce output substantially. A shading check during winter conditions is worthwhile before finalizing fixture selection.

Cold-weather battery derating. LiFePO4 batteries retain roughly 80% of rated capacity at 14°F (-10°C). In cold climates, use derated battery capacity in sizing calculations rather than the rated spec-sheet figure. If a fixture's battery is marginally sized at standard conditions, cold-weather derating may push it below the autonomy requirement -- another situation where the step-up approach adds useful reserve.

Extended overcast in maritime and northern climates. The Pacific Northwest, Great Lakes region, and northern tier states regularly see multi-day overcast in winter. Portland and Seattle regularly see December PSH below 2.0. Three-day autonomy may prove marginal in these climates. Fixtures with adaptive output control, which scales brightness down as battery reserve approaches depletion, reduce the risk of complete shutoff during extended cloudy periods.

To learn more about the impacts of climate and geography on system sizing, see our guide to Solar Lighting Performance by Climate and Geography.

Common Mistakes in Solar Fixture Selection

• Sizing for average annual PSH instead of December worst case.

Systems sized on annual averages will deplete battery reserve through short winter days.

• Applying the same fixture to all regions.

A fixture right for Dallas may have half the panel capacity needed in Chicago or Minneapolis.

• Ignoring panel tilt and shading.

Suboptimal tilt or partial shading can significantly undercut published PSH performance. Check site conditions before finalizing.

• Skipping temperature derating on battery capacity.

A 300 Wh LiFePO4 battery delivers closer to 240 Wh at 14°F (-10°C). Derate for cold-climate installations.

• Undersizing autonomy for the local climate.

Three nights is the baseline for most of the country. The Pacific Northwest, upper Midwest, and high elevations warrant additional reserve.

Next Steps: Selecting the Right Fixture

ELEDLights carries solar lighting fixtures in configurations suited to a wide range of climate conditions and commercial applications. The step-up strategy is a reliable way to meet sizing requirements across challenging installations. Our experts are here to help you identify the light that works best for project. 

Get expert assistance:

• Request a free lighting layout for your project
• Know what you want? Get a price quote
• Questions about solar lighting? Call or text our team at 858.650.9400

Browse products:

• Solar Area Lights - For parking lots, campuses, and open spaces
• Solar Pathway Lights - For walkways and pedestrian areas
• Solar Wall Lights - For perimeters and access points
• Solar Sign & Billboard Lights - For signs, billboards, bulletins, and more
• Full Solar Lighting Category

Frequently Asked Questions about Sizing a Commercial Solar Lighting Solution

What are peak sun hours and why are they different from daylight hours?

Daylight hours count all the time the sun is above the horizon. Peak sun hours measure usable solar energy -- equivalent hours of full-rated panel output at 1,000 W/m². A December day in Chicago might offer 9 hours of daylight but only 2.5 peak sun hours due to low sun angles and cloud cover. PSH is the number that drives panel sizing.

How do I find the peak sun hours for my installation location?

Use a solar resource database or online PSH calculator to look up December plane-of-array irradiance for your specific zip code. The regional figures in this guide are a starting point; site-specific data should drive final fixture selection.

How many days of battery autonomy do I need?

For most commercial installations: 2 nights for sign lighting, 3 nights for area and pathway lighting, and 3 to 5 nights for security and perimeter lighting. Add 1 to 2 nights for the Pacific Northwest, upper Midwest, or high elevations.

What should I do if a fixture's panel is undersized for my location?

Step up to the next model in the same series and program it for a lower output level. The larger fixture's panel and battery will provide the additional harvest and reserve the location demands, while the programmed output level keeps light levels matched to the application.

Does cold weather affect solar lighting performance?

Yes, in two ways. LiFePO4 batteries retain roughly 80% of rated capacity at 14°F (-10°C), so usable capacity is lower than the spec-sheet figure in cold climates. Charging is also restricted below freezing on some systems. Apply a temperature derating factor to battery capacity when sizing for cold-winter installations.