Photocell Load Calculator

A photocell load calculator determines the connected wattage, annual energy use, and estimated electricity cost of repeated loads controlled by a photocell. It is commonly used for exterior lighting groups such as pole lights, wall packs, canopy fixtures, sign lighting, landscape lighting, and other loads that energize automatically as ambient light decreases.

The primary electrical result is Total photocell-controlled load in watts. That value establishes the connected load represented by the fixture group and can be carried into a broader branch-circuit, feeder, panel-load, controls, or voltage-drop review. The annual energy and cost outputs then translate the entered operating schedule into consumption and budget figures.

A photocell does not change the connected wattage of the fixtures. It changes when the controlled load is energized. A group of twenty 60 W fixtures remains a 1,200 W connected lighting load whether the photocell operates it for 8 hours or 12 hours per day; the duty schedule changes annual kWh and cost, not fixture connected load.

Connected Load Calculation

The calculator uses these input fields:

Input	Unit	Electrical use
Connected watts per load unit	W	Connected wattage of one fixture or repeated load unit
Fixture or load count	count	Quantity of identical connected load units
Duty hours per day	h/day	Average daily energized time under photocell control
Operating days per year	days/year	Number of days the load operates during the year
Electricity rate	$/kWh	Utility or internal rate used for the cost estimate

The connected load is calculated as:

\(\displaystyle \text{Total photocell-controlled load (W)} = \text{Connected watts per load unit (W)} \times \text{Fixture or load count}\)

For electrical load review, convert the result to kilowatts where needed:

\(\displaystyle \text{Connected load (kW)} = \frac{\text{Total photocell-controlled load (W)}}{1000}\)

The calculator does not calculate circuit current. Current must be determined separately from the system voltage, phase arrangement, power factor where applicable, driver characteristics, and the actual electrical distribution design. A 1,200 W lighting group may produce materially different circuit current on a 120 V branch circuit, a 277 V lighting circuit, or another supply arrangement.

Annual Energy and Cost

Annual energy is based on the total connected load and the entered schedule:

\(\displaystyle \text{Annual energy (kWh/year)} = \frac{ \text{Total photocell-controlled load (W)} \times \text{Duty hours per day (h/day)} \times \text{Operating days per year (days/year)} }{1000}\)

Annual electricity cost is then:

\(\displaystyle \text{Annual electricity cost (\$/year)} = \text{Annual energy (kWh/year)} \times \text{Electricity rate (\$/kWh)}\)

The duty schedule should represent the average hours the photocell-controlled circuit is actually energized. For exterior lighting, that may differ by season, geographic location, photocell setpoint, fixture orientation, shading, time-clock overrides, occupancy controls, commissioning settings, and any control-system logic installed with the photocell.

Calculation Example

Use the following entered values:

Input	Entered value
Connected watts per load unit	60 W
Fixture or load count	20 count
Duty hours per day	12 h/day
Operating days per year	365 days/year
Electricity rate	$0.15/kWh

First, calculate the connected lighting load:

\(\displaystyle 60\text{ W} \times 20 = 1200\text{ W}\)

Total photocell-controlled load = 1,200 W, or 1.2 kW.

Next, calculate annual energy:

\(\displaystyle \frac{1200\text{ W} \times 12\text{ h/day} \times 365\text{ days/year}}{1000} = 5256\text{ kWh/year}\)

Finally, calculate estimated annual electricity cost:

\(\displaystyle 5256\text{ kWh/year} \times \$0.15/\text{kWh} = \$788.40/\text{year}\)

The calculator result is:

Result	Value
Connected watts per unit used	60 W
Fixture or load count used	20 count
Total photocell-controlled load	1,200 W
Duty hours used	12 h/day
Operating days used	365 days/year
Annual energy	5,256 kWh/year
Annual electricity cost	$788.40/year

Electrical Design Use

The total photocell-controlled load is useful as an input to a larger electrical review, including:

• Branch-circuit and feeder load schedules.
• Lighting-panel connected-load summaries.
• Preliminary circuit grouping and circuit-directory planning.
• Voltage-drop calculations after the actual circuit current, conductor length, conductor material, AWG or kcmil size, and system voltage are known.
• Raceway routing and conduit layout when the controlled fixtures require a common branch circuit or feeder path.
• Lighting retrofit comparisons where fixture wattage, fixture count, annual kWh, and operating cost are being evaluated.
• Controls planning where photocells, contactors, lighting control panels, or relay packs serve a known lighting load.

For example, reducing the connected watts per load unit from 60 W to 40 W while keeping twenty fixtures, 12 h/day, and 365 days/year would reduce the connected load from 1,200 W to 800 W. The annual energy reduction would be:

\(\displaystyle \frac{(1200 - 800)\text{ W} \times 12 \times 365}{1000} = 1752\text{ kWh/year}\)

At $0.15/kWh, that represents an estimated annual energy-cost reduction of $262.80.

Field Verification

This calculation provides photocell load and energy arithmetic only. It does not determine photocell rating, relay or contactor rating, switching capability, LED driver inrush, branch-circuit ampacity, overcurrent protection, conductor AWG or kcmil size, terminal rating, insulation temperature rating, adjustment factor, correction factor, voltage drop, raceway fill, product listing, control commissioning, utility tariff treatment, or code compliance.

Verify the following separately before installation or modification:

• Actual fixture input watts from the installed luminaire or driver data, rather than assumed lamp-equivalent wattage.
• Supply voltage, circuit current, and branch-circuit or feeder ampacity.
• Photocell, relay, contactor, or lighting-control equipment ratings for the actual connected load and applicable inrush characteristics.
• Control wiring, line-voltage switching arrangement, and installation instructions.
• Seasonal operating behavior and any time-clock, astronomical-clock, occupancy, dimming, or networked-control overrides.
• Local electrical requirements and the AHJ’s applicable inspection and approval requirements.