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