Lighting Cost Calculator — Free Online Calculator
Calculate the total monthly electricity cost of all lighting fixtures in your home or building.
How to Use This Calculator
Enter total bulb count, average wattage, daily hours, and electricity rate.
The Formula Explained
Monthly cost = (Bulbs × Watts × Hours × 30) / 1000 × Rate.
Lighting Cost Analysis
Lighting accounts for 5-15% of residential electricity use and 20-40% of commercial electricity use in buildings without heavy HVAC or process loads. Calculating the cost reveals surprising findings: a few high-use, high-wattage fixtures can dominate lighting bills, while dozens of rarely-used closet bulbs add up to very little. Understanding where the lighting energy actually goes lets you target retrofits and efficiency upgrades where they pay back fastest, rather than spreading effort across fixtures where savings would be negligible.
The core formula is universal: annual kWh = watts × hours per day × 365 / 1000. Multiply by rate per kWh for cost. For a proper retrofit analysis, compare the annual cost of the existing fixture to the replacement, factor in any bulb or fixture replacement costs (including labor for commercial), and consider additional benefits like reduced HVAC load in summer (lights produce heat that AC must remove). Properly analyzed lighting retrofits are among the most reliably profitable energy efficiency investments.
Worked Example: Commercial Office Lighting Audit
A 10,000 sq ft office building has 200 linear fluorescent fixtures (4-lamp T8, 64W total per fixture including ballast losses). Operating 10 hours per day, 5 days per week, 50 weeks per year = 2,500 hours per year.
Annual energy: 200 × 64 × 2,500 / 1000 = 32,000 kWh. Cost at 11 cents commercial rate: 3,520 USD per year. LPD: 200 × 64 / 10,000 = 1.28 W/sq ft, which exceeds California Title 24 limit for offices (0.69 W/sq ft new construction, 0.75 W/sq ft existing) — would require retrofit for any major renovation.
LED replacement option: 2x4 LED panels at 40W each, same 200 count. Annual energy: 200 × 40 × 2,500 / 1000 = 20,000 kWh. Cost: 2,200 USD per year. Annual savings: 1,320 USD. LED cost: 200 × 120 USD (fixture + install) = 24,000 USD. Payback: 18 years — too long without other factors.
Better retrofit: 2x4 LED panels with occupancy sensors (cut runtime 30% during off-peak), target 20W each instead of 40W. Annual energy: 200 × 20 × 2,500 × 0.70 / 1000 = 7,000 kWh. Cost: 770 USD. Savings: 2,750 USD per year. Payback: 24,000 / 2,750 = 8.7 years. Add utility rebates (common in CA, MA, NY) of 20-30 USD per fixture = 4,000-6,000 USD off, dropping payback to 6-7 years. Plus 15+ year life produces additional savings after payback.
Worked Example: Residential Whole-House LED Retrofit
A home with 40 incandescent and halogen bulbs averaging 60W each. Daily use averaging 4 hours per day across all bulbs. Annual energy: 40 × 60 × 4 × 365 / 1000 = 3,504 kWh. Cost at 16 cents: 561 USD per year.
LED replacement: 40 × 10W LED (same brightness). Annual energy: 40 × 10 × 4 × 365 / 1000 = 584 kWh. Cost: 93 USD per year. Annual savings: 468 USD.
LED bulb cost: 40 × 5 USD = 200 USD. Payback: 200 / 468 = 5 months. Over the 15+ year LED life, savings total 7,000+ USD. This is why whole-house LED retrofit is one of the most recommended energy upgrades — the economics are consistently excellent.
Additional benefits: reduced summer AC load (incandescent bulbs produce 95% heat, LEDs only 20% heat). For a home using 2,000 kWh per summer for cooling, reducing lighting heat can save another 50-100 kWh per summer worth 8-16 USD. Small but free.
Lighting Cost Mistakes
1. Using total building cost instead of target fixture cost. If only 10 of 40 fixtures are high-use, focus the retrofit on those 10. The other 30 might not pay back.
2. Forgetting labor costs on commercial retrofits. Installing a commercial LED panel can cost 50-100 USD in labor per fixture. This doubles the effective retrofit cost.
3. Ignoring dimming losses. Dimmed LED fixtures do not save power proportionally. At 50% brightness, LED power may only drop 40%. Factor in actual dimming efficiency.
4. Not accounting for ambient heat in cooling-dominated buildings. In Florida or Arizona, the HVAC savings from LED retrofit can equal 15-25% of the direct lighting savings.
5. Choosing bulbs by wattage instead of lumens. LED wattages are small compared to incandescent. Match lumens (the actual light output), not watts. 60W incandescent = 800 lumens = 8-10W LED.
Typical Fixture Energy Profiles
Residential (at 16 cents/kWh, 3 hr/day):
60W incandescent: 26 USD per year per bulb.
14W CFL (equivalent): 6 USD per year.
10W LED (equivalent): 4.40 USD per year.
Savings 60W to 10W LED: 21.60 USD per year per bulb.
Commercial (at 11 cents/kWh, 10 hr/day, 250 days/year):
65W T12 fluorescent fixture: 18 USD per year.
64W T8 fluorescent fixture: 17.60 USD per year.
40W T8 LED retrofit: 11 USD per year.
Savings T12 to LED: 7 USD per year per fixture, plus maintenance savings.
Commercial high-bay (at 11 cents/kWh, 12 hr/day, 300 days/year):
400W metal halide: 158 USD per year per fixture.
150W LED high-bay replacement: 59 USD per year.
Savings: 99 USD per year per fixture. Big opportunity in warehouses and manufacturing.
Energy Code Context
ASHRAE 90.1 is the model energy code adopted by most US states for commercial buildings. It specifies maximum LPD by occupancy type. California Title 24 is stricter, particularly for new construction. IECC (International Energy Conservation Code) covers residential energy efficiency including lighting requirements. ENERGY STAR certifies bulbs and fixtures that meet minimum efficacy and quality standards.
Lighting cost: from per-bulb pennies to commercial whole-building bills
Lighting cost is straightforward: wattage times runtime times rate, summed across all fixtures. The reason it matters is scale. A 50-bulb home might burn $200/year. A 500-fixture office burns 0,000/year and is a prime target for retrofit. Commercial lighting is governed by ASHRAE 90.1 lighting power density (W/sq-ft) limits, which the calculator can check.
The formula and what it does
Sum across all bulbs or fixtures. For commercial buildings, also useful to express as W/sq-ft (Lighting Power Density, LPD) to compare against ASHRAE 90.1 limits (typically 0.7-1.0 W/sq-ft for office, 0.9-1.4 for retail).
Worked example
Scenario: 25,000 sq-ft office building. Original lighting: 250 four-lamp T8 fluorescent fixtures at 128 W each (32,000 W total). Operating 12 hr/day x 250 work days/yr.
Original LPD: 32,000 / 25,000 = 1.28 W/sq-ft (exceeds ASHRAE 90.1 limit for new construction). Annual kWh: 32,000 x 12 x 250 / 1000 = 96,000 kWh. At 13 cents commercial rate: 2,480/year. LED retrofit at 56 W per fixture: 14,000 W total, LPD 0.56 W/sq-ft. Annual kWh: 42,000. Cost: $5,460/year. Savings: $7,020/year. Retrofit cost at $200/fixture installed: $50,000. Payback: 7.1 years before utility rebates. Most commercial retrofits beat 5-year payback after rebates.
Common mistakes to avoid
undefinedFrequently asked questions
What is ASHRAE 90.1?
The commercial building energy standard adopted into most state building codes. Section 9 governs lighting, with LPD limits by building type. New construction must demonstrate compliance.
Do residential homes have LPD requirements?
No. IECC Residential covers other aspects (insulation, windows, HVAC) but not lighting power density. Residential lighting choice is free; most homes are well under what would be a commercial limit anyway.
What is the cheapest way to cut commercial lighting cost?
Occupancy sensors and dimming controls before retrofit. Cuts runtime 30-50 percent at near-zero hardware cost. Then LED retrofit on top. The two together can cut commercial lighting cost by 70 percent.
Are LEDs subject to rebates?
Most utilities offer commercial LED retrofit rebates: $20-100 per fixture. Some residential rebates too but smaller (often $3-5 per bulb). Check DSIRE database or your utility rebate page.
What about daylight harvesting?
Sensor-based dimming based on natural daylight contribution. Common in offices with windows. Saves 20-40 percent additional on perimeter zones beyond just LED retrofit.
How much does outdoor lighting add?
A typical home with two 60 W floodlights running dusk-to-dawn (12 hr/day): 525 kWh/year, $84 at 16 cents. LED replacement (12 W each): 105 kWh/year, 7. Outdoor LED is a quick win.