Lighting Energy Savings Calculator

A lighting retrofit changes the connected load of a lighting system and produces a direct annual operating-cost comparison. The Lighting Energy Savings Calculator calculates Existing annual energy, Replacement annual energy, Annual energy saved, Annual cost saved, and Simple payback for a defined group of fixtures.

The result is used to evaluate whether replacement luminaires, LED retrofit kits, or lower-wattage fixtures reduce enough kWh to justify a project cost. It supports preliminary retrofit pricing, facility energy reviews, utility-cost estimates, and load planning for lighting systems.

Energy savings do not automatically establish branch-circuit ampacity, feeder ampacity, overcurrent protection, raceway fill, voltage drop, or NEC load-calculation compliance. Those electrical design decisions require the actual installation details, including fixture nameplate data, circuit arrangement, conductor sizes in AWG or kcmil, terminal ratings, continuous-load treatment, voltage, and the requirements accepted by the AHJ.

Lighting Energy Inputs

The calculation uses the connected wattage of each fixture and the expected annual operating schedule.

Input	Electrical meaning
Existing fixture watts	Input wattage for one currently installed fixture, in W
Replacement fixture watts	Input wattage for one proposed replacement fixture, in W
Fixture count	Number of fixtures included in the comparison
Operating hours per day	Average energized hours per fixture each day
Operating days per year	Number of days per year the fixtures operate
Electricity rate	Energy charge used for the estimate, in $/kWh
Project cost	Optional installed cost used to calculate simple payback

Fixture wattage should represent the actual input wattage of the complete operating fixture or retrofit assembly. For example, a replacement LED fixture may have a nominal lamp wattage that differs from its listed driver-and-fixture input watts. Using the listed input wattage produces a more reliable energy estimate.

The operating schedule should reflect actual controls and occupancy. Photocells, occupancy sensors, time clocks, dimming, daylight harvesting, and emergency operation can materially change annual hours. If a system operates at different schedules across zones, calculate each zone separately rather than applying one average schedule to the entire building.

Annual Energy Calculation

The calculator converts fixture watts to kilowatt-hours by multiplying fixture wattage, Fixture count, Operating hours per day, and Operating days per year, then dividing by 1,000.

\(\displaystyle \text{Existing annual energy} = \frac{\text{Existing fixture watts} \times \text{Fixture count} \times \text{Operating hours per day} \times \text{Operating days per year}}{1000}\)

\(\displaystyle \text{Replacement annual energy} = \frac{\text{Replacement fixture watts} \times \text{Fixture count} \times \text{Operating hours per day} \times \text{Operating days per year}}{1000}\)

\(\displaystyle \text{Annual energy saved} = \text{Existing annual energy} - \text{Replacement annual energy}\)

Annual energy is expressed in kWh/year. The calculation compares energy use, not instantaneous circuit current. A fixture replacement may lower branch-circuit current and reduce a lighting panel’s connected load, but those effects must be evaluated separately using the applicable fixture voltage and nameplate electrical characteristics.

Annual Cost Savings and Payback

The calculator applies Electricity rate to the calculated annual kWh reduction:

\(\displaystyle \text{Annual cost saved} = \text{Annual energy saved} \times \text{Electricity rate}\)

When a nonzero Project cost is entered, simple payback is:

\(\displaystyle \text{Simple payback} = \frac{\text{Project cost}}{\text{Annual cost saved}}\)

Simple payback is expressed in years. It measures how long estimated energy-cost savings take to equal the entered installed project cost. It does not include financing, maintenance savings, lamp replacement labor, demand charges, rebates, rate escalation, tax treatment, cooling-load effects, or changes in lighting quality.

Calculation Example

Using the values shown in the calculator:

• Existing fixture watts: 100 W
• Replacement fixture watts: 60 W
• Fixture count: 10 fixtures
• Operating hours per day: 8 h/day
• Operating days per year: 365 days/year
• Electricity rate: $0.15/kWh
• Project cost: $700

Existing annual energy:

\(\displaystyle \frac{100 \times 10 \times 8 \times 365}{1000} = 2920\ \text{kWh/year}\)

Replacement annual energy:

\(\displaystyle \frac{60 \times 10 \times 8 \times 365}{1000} = 1752\ \text{kWh/year}\)

Annual energy saved:

\(\displaystyle 2920 - 1752 = 1168\ \text{kWh/year}\)

Annual cost saved:

\(\displaystyle 1168 \times 0.15 = $175.20/\text{year}\)

Result	Value
Existing annual energy	2,920 kWh/year
Replacement annual energy	1,752 kWh/year
Annual energy saved	1,168 kWh/year
Annual cost saved	$175.20/year
Simple payback	4.0 years at a Project cost of $700

With a Project cost of $700 and the same annual cost savings, the simple payback is:

\(\displaystyle \frac{700}{175.20} \approx 4.0\ \text{years}\)

Electrical Field Limits

The calculator assumes every compared fixture operates at its entered wattage for the same schedule. It does not model ballast losses separately, LED driver losses separately, dimming profiles, standby power, power factor, harmonics, demand charges, or utility tariff tiers.

For electrical design work, verify the following separately:

• Fixture input watts, voltage, and current from the fixture listing or manufacturer data.
• Existing and proposed branch-circuit loading, including continuous-load treatment where applicable.
• Panelboard and feeder load calculations using the governing code method and the installation’s actual connected loads.
• Conductor ampacity after any applicable ambient-temperature correction factor and adjustment factor for current-carrying conductors.
• Conductor terminal rating and insulation temperature rating when selecting ampacity.
• Voltage-drop performance where long lighting branch circuits, feeders, or low-voltage lighting systems are involved.
• Raceway fill, box fill, grounding and bonding, control wiring, emergency-lighting requirements, and any local amendments enforced by the AHJ.

A lower fixture wattage can reduce operating energy and connected lighting load, but it does not by itself authorize a change to conductor size, breaker size, feeder size, or raceway layout.

Related calculations: Electricity Cost Calculator, Lumen Calculator.