Solar Tube vs. Skylight Daylighting Calculator

Introduction: Choosing Solar Tubes or Skylights for Daylighting

Solar tubes and framed skylights solve the same basic problem—bringing daylight into rooms that need it—but they do so through very different roof openings. A solar tube directs light from a small roof collector through a reflective tube to a ceiling diffuser. A skylight uses a larger glazed opening and can provide a view as well as direct daylight. This calculator compares their estimated room illuminance, lighting-related savings, thermal effects, installed cost, payback, and net present value using the values you enter.

For a daylighting project, the visual character of the light is as important as the financial result. Tubes generally spread diffuse light from a ceiling diffuser and require a smaller penetration through the roof. Skylights can create a stronger, more directional source of daylight and change the appearance of a room, but their glazing is also included in the heating and cooling portion of this model. Use the comparison as a planning aid alongside product specifications, roof details, and contractor advice.

Solar Tube and Skylight Input Parameters Explained

The solar tube versus skylight model uses room area to convert estimated delivered lumens into footcandles. Ceiling height is collected as a room-description input, although the current calculation does not use room volume in the displayed results. Solar tube count and diameter determine the circular collector area used for the tube estimate. Skylight count and glazed area determine the total glazed area used for the skylight estimate and its thermal calculations.

Enter installed costs on a per-unit basis. The calculator multiplies each unit cost by the corresponding count before calculating simple payback and net present value. Lighting hours replaced per day, existing lighting wattage, and electricity price determine the annual lighting savings assigned to both alternatives. This means the model assumes either option replaces the same entered lighting use; it does not reduce lighting savings when one option produces a lower illuminance estimate.

For the skylight side, U-value, SHGC, heating degree days, cooling degree days, and HVAC efficiency determine the modeled thermal penalty. The script treats both conductive heat loss and solar heat gain as costs, converts their Btu values to kWh using 3,412 Btu per kWh, adjusts by the entered HVAC efficiency, and values them at the electricity price. The HVAC field is used as a divisor, so enter the factor that matches the model’s intended convention. Analysis horizon and discount rate determine the present value of the recurring annual result.

Formula: Solar Tube and Skylight Daylighting Model

For solar tubes, the calculator finds total circular collector area from the entered diameter, converts that area to estimated lumens using 10,000 lumens per square foot and a 0.7 transmission factor, and divides by room area to estimate illuminance. For skylights, it multiplies total glazed area by 10,000 lumens per square foot and a 0.8 factor, then divides by room area. These are comparative estimates rather than measured photometric predictions for a particular product or roof orientation.

Annual lighting savings equal the existing lighting load in kilowatts times the entered daily hours, 365 days, and the electricity price. Solar tubes receive that lighting-savings amount without a modeled thermal penalty. For skylights, the calculator subtracts the heating-loss and cooling-gain penalties from the same lighting-savings amount. The net annual relationship used for the skylight comparison is:

Here, E_L is annual electric-lighting savings, H_W is the modeled skylight heating-loss cost, and C_S is the modeled skylight solar-gain cooling cost. The calculator then discounts the same net annual savings for every year in the selected horizon and subtracts installed cost. Simple payback is installed cost divided by positive annual savings; the result is shown as “Not reached” when annual savings are not positive.

Worked Example: Reading a Solar Tube Versus Skylight Comparison

Start with the room dimensions, product counts, product sizes, costs, lighting use, utility price, and climate inputs that describe the actual project. After selecting Compare Options, read the installed-cost row before comparing the estimated illuminance row. A higher footcandle estimate does not by itself mean that an option will have a shorter payback, because the calculation also applies the full entered cost and, for skylights, the modeled heating and cooling penalties.

For a solar tube installation, increasing tube count or diameter increases the circular area used in the light estimate and also increases installed cost when the per-tube cost is unchanged. For a skylight installation, increasing count or glazed area raises estimated illuminance, installed cost, heat loss, and solar heat gain in this model. Check the result after changing one design assumption at a time so that the effect of size, quantity, glazing performance, or lighting use remains clear.

Scenario Table: Inputs That Change the Daylighting Comparison

This solar tube and skylight sensitivity view is directional rather than a substitute for rerunning the calculation. Tube size and quantity primarily affect the tube illuminance estimate and installed cost. Skylight area and glazing inputs affect more of the model because they also drive the thermal terms. Efficient existing lighting can lower the value of the lighting savings for both options, making design preferences and the amount of daylight more influential in the decision.

Interpreting Solar Tube and Skylight Results

Use the daylighting results to compare the two estimated illuminance values with the level of light you want in the room, then review installed cost, net annual savings, simple payback, and net present value together. The figures are model outputs based on the entered assumptions, not a guarantee of on-site light levels or utility savings. If a skylight is preferred for its view or architectural effect, the comparison can still help show the cost associated with that preference.

The results table can be downloaded as a CSV after a calculation, which can be useful when discussing alternatives with a contractor or keeping a record of assumptions. Run a separate solar tube versus skylight scenario when a meaningful input changes, such as a different tube diameter, glazing area, SHGC, electricity price, or planned lighting schedule. Avoid combining changes in several fields if you need to identify which assumption is driving the result.

Limitations and Assumptions for Solar Tube and Skylight Daylighting

This solar tube versus skylight calculator uses fixed lumen factors and does not account for roof orientation, roof pitch, tube length, bends, interior finishes, obstructions, tree shading, nearby buildings, seasonal sun angle, or local cloud patterns. It therefore cannot replace a product-specific daylight simulation or a site assessment. The tube calculation also does not apply a separate thermal penalty, while the skylight calculation applies only the heat-loss and solar-gain penalties defined in the script.

Thermal results assume the entered degree days, electricity price, and HVAC-efficiency factor are appropriate for the project. The model does not account for ventilation from operable skylights, maintenance, water-management details, electric lights integrated with daylight controls, or changes in lighting behavior beyond the entered hours. Verify product ratings, roof suitability, code requirements, and installation details before committing to a solar tube or skylight installation.

How to Use This Solar Tube vs. Skylight Calculator

1. Enter the room floor area and the ceiling height for the space being evaluated.
2. Enter the solar tube and skylight quantities, then provide the tube diameter and skylight glazed area for the alternatives you want to compare.
3. Add per-unit installed costs, skylight U-value and SHGC, expected lighting hours replaced, existing lighting load, utility price, climate values, HVAC factor, analysis horizon, and discount rate.
4. Select Compare Options, review the daylighting and financial rows, then change one solar tube or skylight assumption and calculate again to test a second design scenario.