LED Retrofit Payback Calculator

An LED retrofit payback calculation converts a lighting wattage reduction into annual energy savings, annual utility-cost savings, and a simple payback period in years. It is used during early lighting-upgrade scoping to compare an existing lighting load against a proposed LED fixture load and determine whether the projected savings support the installed project cost.

The calculation applies when the existing fixture wattage, LED fixture wattage, fixture count, operating schedule, electricity rate, and total project cost are known. It assumes every listed fixture operates on the same schedule and uses the same electricity rate.

The primary result is Simple payback, calculated as the total Project cost divided by the projected Annual cost saved.

Lighting Energy Inputs

Enter the existing and proposed connected load on a per-fixture basis, then apply the fixture quantity and annual operating schedule.

Input	Unit	Electrical purpose
Existing fixture watts	W/fixture	Connected wattage of each existing luminaire, ballast-lamp system, or other lighting assembly being replaced
LED fixture watts	W/fixture	Input wattage of each proposed LED fixture
Fixture count	fixtures	Number of fixtures included in the retrofit scope
Operating hours per day	h/day	Average daily energized operating time
Operating days per year	days/year	Number of days the fixtures operate annually
Electricity rate	$/kWh	Energy charge used to convert kWh savings into annual dollar savings
Project cost	$	Total retrofit cost used for the simple-payback calculation

Existing fixture watts and LED fixture watts should represent actual input power rather than nominal lamp wattage alone. For existing fluorescent, HID, or other legacy fixtures, the input wattage should account for the complete operating system where applicable. For LED equipment, use the intended fixture’s listed input wattage.

The calculation is an energy comparison, not a branch-circuit or feeder load calculation. A reduced lighting wattage may affect a load review, panel schedule, feeder demand evaluation, or emergency-power assessment, but those electrical design decisions require the applicable load conditions and project documentation.

Annual Energy and Cost Formula

The calculator applies the same fixture count and operating schedule to the existing and LED lighting systems.

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

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

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

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

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

The division by 1,000 converts watt-hours to kilowatt-hours. The electricity-rate input is then applied only to the calculated kWh savings.

Calculation Example

For a retrofit replacing 50 existing 100 W fixtures with 40 W LED fixtures, operating 10 hours per day for 260 days per year at an electricity rate of $0.12/kWh:

Input	Value
Existing fixture watts	100 W/fixture
LED fixture watts	40 W/fixture
Fixture count	50 fixtures
Operating hours per day	10 h/day
Operating days per year	260 days/year
Electricity rate	$0.12/kWh
Project cost	$6,000

Existing annual energy:

\(\displaystyle \frac{100 \times 50 \times 10 \times 260}{1000} = 13{,}000\text{ kWh/year}\)

LED annual energy:

\(\displaystyle \frac{40 \times 50 \times 10 \times 260}{1000} = 5{,}200\text{ kWh/year}\)

Annual energy saved:

\(\displaystyle 13{,}000 - 5{,}200 = 7{,}800\text{ kWh/year}\)

Annual cost saved:

\(\displaystyle 7{,}800 \times 0.12 = \$936\text{/year}\)

Simple payback:

\(\displaystyle \frac{6{,}000}{936} = 6.4103\text{ years}\)

The calculated results are:

Result	Value
Existing annual energy	13,000 kWh/year
LED annual energy	5,200 kWh/year
Annual energy saved	7,800 kWh/year
Annual cost saved	$936/year
Simple payback	6.4103 years

Electrical Planning Use

The annual energy and cost results support preliminary capital planning and lighting-retrofit comparison. The wattage values can also identify a reduction in connected lighting load for a broader electrical review.

A lower connected lighting load does not automatically establish available capacity or permit a reduction in branch-circuit, feeder, or service equipment ratings. Circuit ampacity, overcurrent protection, conductor AWG or kcmil size, terminal rating, insulation temperature rating, voltage drop, current-carrying conductors, raceway fill, and equipment listing remain separate design and installation determinations.

For a replacement project, verify that the proposed fixture can be supplied from the existing branch circuit and that any changes to controls, dimming, emergency lighting, drivers, disconnecting means, mounting, or wiring methods are addressed in the installation scope.

Field Verification

Use actual operating conditions whenever possible. A lighting schedule based on occupancy, photocell control, timeclock operation, daylight harvesting, or seasonal use may differ substantially from a nominal daily schedule.

Confirm the following outside the payback arithmetic:

• Existing fixture input wattage and proposed LED fixture input wattage
• Actual operating hours and annual operating days
• Utility billing structure and whether the entered Electricity rate reflects the intended energy charge
• Fixture compatibility with the existing wiring method, controls, voltage, and emergency-lighting arrangement
• Required photometric performance, illuminance levels, glare control, and lighting distribution
• Manufacturer installation instructions, fixture listing requirements, and AHJ approval requirements
• Any project costs or savings not included in Project cost or Annual cost saved

This calculation covers LED retrofit energy arithmetic and simple payback only. It does not include rebates, maintenance savings, labor escalation, financing, demand charges, utility tariff changes, fixture compatibility, photometric design, warranty value, or approval outcomes.