Formulas

Design formulas and conversions

Footcandle (fc) = total lumens (lm) ÷ area in square feet

Footcandle (fc) = Lux ÷ 1.076

Light to solar heat gain (LSG) = VT/SHGC

Lux = total lumens (lm) ÷ area in square meters

Lux (lx) = footcandle x 10.76

Shading coefficient (SC) = solar heat gain coefficient (SHGC) x 1.15

Watt (W) = Btu/h x 0.2931

Watt per meter squared (W/m2) = watt per square foot x 10.8

Electrical formulas

Demand for power (kW) = system input wattage (W) ÷ 1,000

Energy consumption (kWh) = System input wattage (kW) x hours of operation/year (h)

Hours of operation/year = operating hours/day x operating days/week x operating weeks/year

Lighting system efficacy (lumens per Watt (LPW)) = system lumen output ÷ input wattage

Lighting power density (W/ft2) = total system input wattage (W) ÷ total area (ft2)

Watts (W) = volts (V) x current in amperes (A) x power factor (PF)

Voltage (V) = current in amperes (A) x impedance (Ohms) [Ohm's Law]

Economic formulas

Simple payback on investment (Years) = Net installation costs ($) ÷ Annual energy savings ($)

Simple return on investment (%) = [annual energy savings ($) ÷ net installation cost ($)] x 100