Occupancy Sensor Savings Calculator

An occupancy sensor savings calculation estimates how much lighting energy and electricity cost can be reduced when a sensor lowers the average number of hours that lighting operates each day. The primary result is Annual energy saved in kWh/year, along with the corresponding Annual cost saved in $/year.

This calculation supports preliminary lighting-control evaluations for spaces where luminaires are routinely left energized after occupants leave. Typical applications include offices, corridors, storage rooms, restrooms, classrooms, break rooms, utility spaces, and similar areas with intermittent occupancy.

The calculation changes operating time only. It uses the same connected lighting load in both cases, so it does not estimate savings from LED retrofits, fixture wattage reductions, daylight harvesting, dimming levels, occupancy sensor coverage, sensor timeout settings, or demand charges.

Lighting Operating-Hour Reduction

The calculator compares a baseline lighting schedule against a reduced schedule assumed after occupancy-sensor control is applied.

Input	Unit	Electrical Purpose
Baseline hours per day	h/day	Average daily lighting operating time before the occupancy-sensor assumption
Reduced hours per day	h/day	Average daily lighting operating time after the occupancy-sensor assumption
Lighting watts	W	Total connected lighting wattage covered by the occupancy-sensor assumption
Operating days per year	days/year	Number of annual days the lighting is expected to operate
Electricity rate	$/kWh	Energy rate used to convert saved kWh into an estimated annual cost reduction

Lighting watts should represent the actual load affected by the control strategy. For example, if a sensor controls four 200 W lighting loads, enter 800 W. Do not include lighting that remains on a separate circuit, emergency lighting that must remain energized, exterior lighting governed by photocell control, or fixtures not switched by the sensor assumption.

Baseline hours per day and Reduced hours per day are average operating values, not necessarily scheduled building hours. A space open for 12 hours may have less than 12 hours of actual energized lighting if existing switching practices already reduce runtime.

Annual Energy Calculation

The calculator converts lighting wattage to kilowatts by dividing by 1,000, then multiplies that load by daily operating hours and annual operating days.

\(\displaystyle \text{Baseline annual kWh} = \frac{\text{Lighting watts} \times \text{Baseline hours per day} \times \text{Operating days per year}}{1000}\)

\(\displaystyle \text{Sensor-case annual kWh} = \frac{\text{Lighting watts} \times \text{Reduced hours per day} \times \text{Operating days per year}}{1000}\)

\(\displaystyle \text{Annual energy saved} = \text{Baseline annual kWh} - \text{Sensor-case annual kWh}\)

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

The calculation also reports Hours reduced per day:

\(\displaystyle \text{Hours reduced per day} = \text{Baseline hours per day} - \text{Reduced hours per day}\)

A larger connected lighting wattage, more operating days, or more daily hours reduced will increase the estimated kWh savings proportionally.

Calculation Example

Assume an occupancy sensor is proposed for lighting with the following operating assumptions:

Input	Value
Baseline hours per day	12 h/day
Reduced hours per day	8 h/day
Lighting watts	800 W
Operating days per year	365 days/year
Electricity rate	$0.15/kWh

Baseline annual energy:

\(\displaystyle \frac{800 \times 12 \times 365}{1000} = 3504 \text{kWh/year}\)

Sensor-case annual energy:

\(\displaystyle \frac{800 \times 8 \times 365}{1000} = 2336 \text{kWh/year}\)

Annual energy saved:

\(\displaystyle 3504 - 2336 = \mathbf{1168 \text{kWh/year}}\)

Annual cost saved:

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

The daily operating-time reduction is:

\(\displaystyle 12 - 8 = \mathbf{4 \text{h/day}}\)

For these assumptions, the occupancy-sensor case reduces lighting energy from 3,504 kWh/year to 2,336 kWh/year, producing an estimated annual reduction of 1,168 kWh/year and $175.20/year in energy charges.

Electrical Design Use

Occupancy sensor savings estimates are commonly used during lighting-control planning, retrofit scoping, energy-audit screening, and preliminary project economics. The result can help compare control opportunities across rooms or areas when the connected lighting watts and expected runtime reduction are known.

The calculation may also support broader lighting design work:

• Compare expected operating cost between branch circuits or controlled lighting zones.
• Prioritize high-wattage areas where reducing energized hours produces greater kWh savings.
• Estimate the effect of separating lighting zones instead of controlling an entire floor or tenant area from one switch.
• Evaluate whether a sensor-control scope justifies further site investigation, fixture inventory, control design, commissioning planning, or utility-rebate review.
• Develop preliminary annual energy inputs for a project budget or life-cycle cost comparison.

The result does not change the branch-circuit ampacity, feeder ampacity, conductor AWG or kcmil selection, raceway fill, overcurrent protection, or voltage-drop calculation. Those electrical design values are based on the connected load and installation conditions, not the assumed reduction in annual operating hours.

Field Verification

The energy estimate depends primarily on whether the entered daily hours reflect actual lighting operation. Verify the affected lighting load and control boundary before using the result for project pricing or savings commitments.

Confirm the following conditions in the field:

• The entered Lighting watts matches the fixtures or drivers actually controlled by the occupancy sensor.
• The proposed sensor controls the intended luminaires rather than only part of the lighting zone.
• Existing manual switching, time scheduling, lighting control panels, photocells, or building automation do not already reduce the baseline operating hours.
• Emergency egress lighting, required illumination, and other lighting that must remain energized are excluded unless the control arrangement permits their operation to change.
• Sensor placement, field of view, room partitions, ceiling height, obstructions, and occupant activity support the assumed reduction in operating hours.
• Sensor timeout, manual-on or automatic-on behavior, override provisions, and commissioning settings are reviewed as part of the actual control design.
• The electricity rate represents the applicable energy charge for the project, recognizing that tariff structure, demand charges, taxes, riders, and future rate changes may not be represented by a single $/kWh value.

Code and Project Limits

The worksheet performs occupancy-hour energy arithmetic only. It does not determine whether occupancy sensors are required, permitted, properly located, or compliant with adopted energy codes, building codes, electrical codes, manufacturer instructions, project specifications, or AHJ requirements.

It also does not determine sensor coverage, control-zone suitability, code-required lighting controls, emergency-lighting treatment, commissioning acceptance, utility-rebate eligibility, installed cost, maintenance cost, payback period, or project approval. Those decisions require the actual building use, lighting layout, control sequence, adopted code edition, applicable energy standard, equipment listing, and site conditions.