![]() In these cases, we often must determine light levels on workplanes that are not horizontal and perpendicular but tilted or even vertical. ![]() Many workplanes are not perpendicular to the direction of light intensity, which is why calculating light level at a point is useful for such applications. Lux = Total Lumens ÷ Area in Square Metersįor planes perpendicular to the direction of candlepower (Inverse Square Law):ĭ = Direct distance between the lamp and the point where light level is calculated Simple Return on Investment (%) = x 100įootcandles (fc) = Total Lumens (lm) ÷ Area in Square Feet Simple Payback on an Investment (Years) = Net Installation Cost ($) ÷ Annual Energy Savings ($)ĥ-Year Cash Flow ($) = 5 Years - Payback (Years) x Annual Energy Savings ($) Voltage (V) = Current in Amperes (A) x Impedance (Ohms) ECONOMIC FORMULAS Watts (W) = Volts (V) x Current in Amperes (A) x Power Factor (PF) Unit Power Density (W/sq.ft.) = Total System Input Wattage (W) ÷ Total Area (Square Feet) Lighting System Efficacy (Lumens per Watt or LPW) = System Lumen Output ÷ Input Wattage Hours of Operation/Year = Operating Hours/Day x Operating Days/Week x Operating Weeks/Year ![]() Demand for Power (kW) = System Input Wattage (W) ÷ 1,000Įnergy Consumption (kWh) = System Input Wattage (kW) x Hours of Operation/Year
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