The average commercial building wastes 20-30% of its lighting electricity through mistakes that are easy to fix once you know what to look for. Across five common lighting mistakes, I have calculated the actual energy and dollar costs per fixture below, along with specific fixes and product recommendations from KASTLITE. For more information, see the U.S. Department of Energy.
Mistake 1: Still Running Incandescent or Old Fluorescent Bulbs
This is the single biggest energy waste in any building that has not upgraded to LED. The numbers are stark: For more information, see the ENERGY STAR.
| Bulb Type | Watts (for ~1,600 lumens) | Lifespan (hours) | Annual Cost per Bulb (12 hrs/day, $0.14/kWh) |
|---|---|---|---|
| Incandescent | 100W | 1,000 | $61.32 |
| CFL | 23W | 10,000 | $14.10 |
| T8 Fluorescent | 32W | 20,000 | $19.62 |
| LED | 15-18W | 50,000 | $9.20-$11.04 |
Switching one 100W incandescent bulb to a 15W LED saves $50.12 per year. In a building with 50 incandescent fixtures, that is $2,506 per year in energy savings alone, before counting reduced replacement labor.
Even replacing fluorescent T8 tubes with LED equivalents saves $8.58-10.42 per tube per year. For a 100-fixture office, that is $858-1,042 annually. The U.S. Department of Energy reports that LED equivalents use 44% less wattage than standard T8 fluorescents while producing the same lumens.
How to Fix It
- For incandescent and CFL fixtures: Swap to LED bulbs with the same base (E26 for standard, E12 for candelabra). No rewiring needed. KASTLITE carries LED replacement bulbs for common fixtures
- For fluorescent tube fixtures: Install LED T8 tubes. Type A (plug-and-play) tubes work with existing ballasts. Type B (ballast bypass) tubes wire directly for maximum efficiency. Browse KASTLITE LED products
- For complete fixture upgrades: Replace old housings with integrated LED fixtures. See the KASTLITE LED fixture collection
Mistake 2: Leaving Lights On in Unoccupied Rooms
A 2014 study by the Lawrence Berkeley National Laboratory found that occupancy sensors reduce lighting energy in commercial buildings by an average of 24%. In rooms with intermittent use (restrooms, conference rooms, storage areas), the savings jump to 40-60%.
Here is what the waste looks like in dollar terms: A conference room with six 32W fluorescent fixtures that sits empty 60% of the workday wastes $430 per year. An occupancy sensor costing $30-50 would eliminate that waste, paying for itself in less than a month.
How to Fix It
- Install occupancy sensors in rooms with intermittent use: restrooms, break rooms, copy rooms, conference rooms, and storage areas
- Use vacancy sensors (manual-on, auto-off) in offices where occupants may want lights off even when present
- For outdoor areas: Use motion-activated fixtures like the KASTLITE LED Solar Powered Flood Light, which combines motion detection with solar power for zero electricity cost
- For parking lots and building exteriors: LED wall packs with built-in photocells turn on at dusk and off at dawn automatically
Mistake 3: Over-Lighting Spaces (More Fixtures Than Needed)
Over-lighting is surprisingly common, especially in offices and retail spaces that were designed before LED. Older lighting plans compensated for fluorescent lumen depreciation (light output drops 20-30% over the tube's life) by installing extra fixtures. When you replace those old tubes with LEDs that maintain 90%+ output for 50,000 hours, the room becomes over-lit.
The IES recommends 30-50 foot-candles for general office work. Many over-lit offices measure 70-100 foot-candles, double what is needed. That excess light is pure waste.
How to Fix It
- Measure your current light levels with a light meter (available for $20-40) or a free smartphone app. If you are above 50 foot-candles in a standard office, you have room to reduce
- Decommission excess fixtures: Remove tubes from every other fixture, or disconnect every third fixture. This is free and immediately cuts energy use
- Replace multiple dim fixtures with fewer bright ones: A single high-output LED panel can replace two older fluorescent troffers
- Upgrade to better diffusion: Proper light distribution from a quality diffuser panel means fewer fixtures cover the same area. KASTLITE Acrylic Prismatic Panels spread light evenly to reduce hotspots and dark zones, so each fixture covers more usable area
Mistake 4: Poor Light Diffusion and Glare
Bare bulbs and clear lenses create hotspots (too bright directly below the fixture) and shadows (too dark between fixtures). The instinctive response is to add more fixtures, which doubles the energy cost. The better solution is to diffuse the light you already have.
How Diffusers Save Energy
A prismatic diffuser redirects light at controlled angles, spreading it over a wider area. This means a single diffused fixture can cover 30-40% more floor area than the same fixture with a clear lens. In practical terms, a room that needs 8 bare-bulb fixtures might need only 5-6 fixtures with proper prismatic diffusers.
Types of Diffuser Patterns
- Prismatic: Small pyramid shapes molded into the surface. Best for offices, classrooms, and retail. Redirects light predictably while maintaining high transmission. KASTLITE offers both acrylic prismatic panels and polycarbonate prismatic panels
- Frosted/Opal: Smooth translucent white. Best for healthcare, waiting rooms, and anywhere glare causes discomfort. Softer light, slightly lower transmission than prismatic
- Egg crate: Grid-style louver that controls glare while allowing airflow. Used in spaces with computer screens where reflected glare is a problem. See the KASTLITE egg crate collection
When to Replace Old Diffusers
If your existing diffuser panels are yellowed, cracked, or sagging, they are reducing light output by 20-30%. Replacing a yellowed diffuser with a new clear prismatic panel can restore enough brightness to eliminate the need for supplemental fixtures. See the KASTLITE guide to replacing yellowed light covers for more on this.
Mistake 5: Ignoring Free Natural Light
Many buildings run electric lights at full output even when windows and skylights provide more than enough ambient light. A south-facing window in a sunny climate can deliver 100+ foot-candles at the perimeter, well above the 30-50 foot-candles needed for office work. Running electric lights in that zone wastes every watt.
How to Fix It
- Daylight harvesting: Install photosensors that automatically dim electric lights when natural light is sufficient. The New Buildings Institute reports savings of 20-40% in perimeter zones (within 15 feet of windows)
- Manual zoning: Wire perimeter lights on separate switches from interior lights. This costs nothing if done during construction, and allows manual shutoff of window-side lights on bright days
- For outdoor lighting: Solar-powered fixtures harvest daylight by design. The KASTLITE 45W Solar Barn Light charges all day and runs all night with zero grid electricity. The full KASTLITE solar collection includes pathway bollards, flood lights, and barn lights
- Dusk-to-dawn photocells: Add a photocell to any outdoor fixture so it operates only when natural light drops below the threshold. The KASTLITE photocell guide covers compatibility and installation
Total Savings Potential: A Real Example
Here is what a 5,000 sq ft small business (office/retail hybrid) can expect by fixing all five mistakes:
| Fix | Estimated Annual Savings | Upfront Cost | Payback |
|---|---|---|---|
| LED retrofit (60 tubes) | $515-626 | $480-720 | 10-14 months |
| Occupancy sensors (6 rooms) | $180-360 | $180-300 | 6-12 months |
| Decommission 10 excess fixtures | $193 | $0 | Immediate |
| Replace 20 yellowed diffusers | $100-150 (reduced fixture count) | $200-400 | 18-30 months |
| Daylight dimming (perimeter zone) | $120-240 | $300-600 | 18-36 months |
| Total | $1,108-1,569 | $1,160-2,020 | 9-16 months avg |
Combined savings of $1,100-1,570 per year from a one-time investment of $1,160-2,020. After payback, that money goes straight to the bottom line every year. Over 10 years (the typical LED lifespan), cumulative savings exceed $10,000 for this small example. Larger facilities see proportionally larger returns.
Frequently Asked Questions
What is the single biggest lighting energy waste?
Running incandescent bulbs instead of LEDs. A single 100W incandescent costs $61.32 per year to operate (at 12 hrs/day, $0.14/kWh), while an equivalent 15W LED costs $9.20. That is an 85% reduction per bulb.
How much do occupancy sensors actually save?
The Lawrence Berkeley National Laboratory found an average of 24% energy reduction in commercial buildings. In spaces with intermittent use (conference rooms, restrooms), savings reach 40-60%.
Is it worth upgrading from fluorescent to LED?
Yes. LED T8 tubes use 44% less wattage, last 2.5 times longer (50,000 vs 20,000 hours), and maintain brightness throughout their lifespan. Fluorescent tubes lose 20-30% of their output before burning out. The payback on a fluorescent-to-LED retrofit is typically 10-18 months.
For more energy-saving guides and products, explore:
- Smart Lighting for Small Businesses: Affordable Automation Upgrades
- Best Commercial Lighting Upgrades for 2026
- Eco-Friendly Lighting Materials
- Replacing Yellowed Light Covers
- KASTLITE Light Covers and Diffusers
Related: Near the coast, one major lighting mistake is ignoring sea turtle rules. See turtle-friendly lighting.