Solar Tube vs. Skylight Daylighting Calculator
Fill in the room and product details to compare daylight, energy impacts, and payback.
| Metric | Solar Tubes | Skylights |
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Natural Light Choices: Solar Tubes Versus Skylights
Homeowners looking to brighten interior rooms often debate between installing tubular daylighting devices (solar tubes) and traditional framed skylights. Both deliver free daylight that can boost mood, productivity, and property value, but they differ in cost, installation complexity, thermal performance, and daylight distribution. Solar tubes use reflective tunnels to bring sunlight through roof decks and attic spaces into ceiling diffusers, typically with diameters between 10 and 22 inches. Skylights cut larger openings in the roof to install insulated glazing units that act like roof windows. This calculator gives you a comprehensive comparison by estimating light levels, energy savings from reduced electric lighting, and heating/cooling penalties.
Daylighting investments increasingly appear in sustainability retrofits because they reduce reliance on electric lights during the day, lower utility bills, and highlight architectural features. At the same time, poorly chosen skylights can introduce unwanted heat gain in summer or heat loss in winter. Solar tubes, with their smaller penetrations and sealed tunnels, often outperform in energy efficiency but deliver smaller pools of light. The right choice depends on your room layout, climate, and aesthetic priorities. By quantifying lumen output, annual energy impacts, and net present value, this calculator turns subjective preferences into data-informed decisions.
Input Parameters Explained
Room area and ceiling height determine the volume you want to illuminate. Larger spaces may require multiple devices to achieve target illuminance levels. Solar tube count and diameter define the capture area for tubular daylighting devices; manufacturers publish lumen output curves by diameter that we approximate in the calculation. Skylight count and glazed area determine direct aperture area. Costs should include flashing, drywall repair, and labor.
Skylight U-value and solar heat gain coefficient (SHGC) affect thermal performance. Lower U-values reduce heat loss, while lower SHGC reduces summer heat gain. Solar tubes generally have higher effective R-values because the tunnels are insulated; we model their U-value implicitly in the energy calculations. Lighting hours replaced per day indicates how much artificial lighting you expect to turn off thanks to daylight. Existing lighting load captures the wattage of the fixtures being replaced. Electricity price converts that reduction into monetary savings.
Heating and cooling degree days quantify climate severity. The HVAC efficiency input allows the calculator to translate thermal gains or losses into energy costs. A gas furnace with 95 percent efficiency would use 0.95, while a heat pump with a seasonal COP of 3 would enter 0.88 (representing 1/3). Adjust analysis horizon and discount rate to reflect your financial planning horizon.
Daylight and Energy Modeling
The calculator estimates daylight contribution by converting aperture area into lumens delivered to the room. Solar tube lumen output is approximated using the formula L = A · T · D, where A is capture area, T is optical transmission (assumed at 0.7 for high-quality tubes), and D is average daylight availability (in lumens per square foot). Skylights use a similar approach but factor in a daylight distribution multiplier since direct shafts can create brighter hotspots. We normalize results to produce an estimated illuminance in footcandles, which you can compare to recommended levels (20-30 fc for hallways, 50-70 fc for kitchens).
Energy savings stem from reduced electric lighting: wattage multiplied by hours replaced and electricity price. Thermal impacts consider heat loss through glazing and heat gain from solar radiation. Skylights have higher U-values and SHGC, so they may lose more heat in winter but can provide useful passive solar gain. Solar tubes have negligible solar gain and minimal heat loss. The calculator balances these effects by computing seasonal loads based on degree days and subtracting or adding them to the lighting savings. The MathML expression below captures the net annual savings for each option.
Here, EL is electric lighting savings, HW is additional heating demand caused by heat loss, and CS is extra cooling load from solar gain. For solar tubes, HW and CS are small. The calculator compares annual savings against installed costs to produce net present value and payback for each option.
Worked Example: Windowless Hallway Upgrade
Consider a 140-square-foot hallway with a 9-foot ceiling in Atlanta. The homeowner wants to brighten it during daytime hours without dramatically increasing cooling loads. They evaluate two 14-inch solar tubes versus a single 10-square-foot low-E skylight. Installed costs are $1,700 per solar tube (including flexible tubing through the attic) and $3,400 for the skylight (including drywall finishing). Lighting currently relies on recessed fixtures totaling 180 watts, running eight hours per day. Electricity costs $0.16 per kWh. Heating degree days are 2,800 and cooling degree days 1,500. The furnace is 95 percent efficient, and the air conditioner has a seasonal COP equivalent of 0.85 in the input field.
Running these inputs shows solar tubes delivering roughly 32 footcandles, while the skylight reaches 45 footcandles. Electric lighting savings are identical because both reduce artificial lighting for the same hours. However, the skylight incurs $82 per year in additional cooling load due to solar heat gain, partially offset by $34 of winter heating benefit. Solar tubes avoid these penalties, so their net annual savings are $210 compared to $162 for the skylight. Net present value over fifteen years at a 3 percent discount rate is $1,350 for the solar tubes and $340 for the skylight. Simple payback is eight years for the tubes and twenty-one for the skylight.
Scenario Table
| Scenario | Solar Tube NPV | Skylight NPV | Notes |
|---|---|---|---|
| Base Case | $1,350 | $340 | Atlanta climate, 8 hr/day usage |
| High Solar Gain (SHGC 0.55) | $1,350 | -$420 | Skylight introduces more cooling load |
| Passive Solar Winter (HDD 4500) | $1,120 | $780 | Colder climate favors skylight heat gain |
| LED Lighting 80 W | $520 | $-280 | Lower lighting load reduces savings |
The scenarios demonstrate how climate and lighting load shape outcomes. In cold climates, skylights’ passive solar gain can make them financially competitive. In hot climates, solar tubes shine by avoiding extra cooling demand. Upgrading to efficient LED lighting reduces the value of both options, so daylit aesthetics and well-being become the primary drivers.
Interpreting Results and Next Steps
Use the illuminance estimates to verify that either option meets your target light levels. Offices typically require 50 footcandles, while hallways need less. Compare net present values and paybacks to your financial threshold. If the skylight delivers the desired architectural effect but has a weaker financial case, consider energy-saving measures like shades or exterior baffles to reduce cooling penalties. The CSV export lists annual energy flows, making it easy to share with contractors or include in green building certifications.
Beyond dollars, daylight quality matters. Skylights provide views of the sky, which many homeowners value for circadian health. Solar tubes provide diffuse light that minimizes glare and maintain roof integrity. Some installations use both: tubes for interior rooms and skylights for living areas. Use the calculator iteratively to size combinations—adjust counts and areas until you meet illuminance goals at acceptable cost.
Limitations and Assumptions
The model uses average daylight availability and does not account for roof orientation, shading from trees, or local cloud patterns. Thermal calculations assume consistent indoor setpoints and do not model ventilation associated with operable skylights. Solar tubes with integrated LEDs or fans may have different costs and benefits. Maintenance, such as cleaning skylight glass or replacing tube diffusers, is not included but can be added to annual costs if desired. Despite these simplifications, the calculator offers a grounded comparison that blends energy performance with daylight design, helping you choose the option that best fits your home.