Calculation sequence
Step-by-step method
- Subtract proposed connected power from existing power.
- Multiply kW reduction by equivalent annual hours.
- Apply the user-entered energy rate.
ElectricalBeginnerLighting and Energy
Lighting Retrofit Energy Savings Calculator
Estimate annual lighting demand, energy, and energy-only cost reduction from existing and proposed connected power.
Why it matters: A transparent screen separates connected-power change from hours and user-entered energy rate.
Calculated result
Engineering evaluation
Annual lighting energy savings (ΔE)12,000 kWh/year
- Connected-demand reduction (ΔP)
- 4 kW
- Energy-only cost savings (ΔC)
- 1,440 currency/year
- Connected-power reduction (r)
- 40 %
Engineering interpretation
The screen yields 12000 kWh/year and 1440 currency units/year energy-only savings, with 40% connected-power reduction.
Important limitations
- Demand charges, HVAC interaction, maintenance, rebates, degradation, controls, schedules, capital cost, and lighting quality require separate review.
Engineering knowledge center
Understand the calculation—not just the answer
Governing relationships
ΔP = (P₁−P₂)/1000
ΔE = ΔP·t
ΔC = ΔE·Rᵉ
r = 100(P₁−P₂)/P₁
Current substitution
ΔP = (10000−6000)/1000 = 4 kW; ΔE = 4×3000 = 12000 kWh; ΔC = 1440; r = 40%.
Equation legend
Inputs and calculated quantities
- P₁
- Existing lighting power — W
- P₂
- Proposed lighting power — W
- t
- Equivalent annual operating hours — h/year
- Rᵉ
- Blended energy rate — currency/kWh
- ΔE
- Annual lighting energy savings — kWh/year
- ΔP
- Connected-demand reduction — kW
- ΔC
- Energy-only cost savings — currency/year
- r
- Connected-power reduction — %
Applications
How this calculation is used
Building design
Screen a proposed lighting power reduction.
Construction
Check fixture counts, wattages, and control assumptions.
Commissioning
Compare verified existing and proposed power with equivalent annual hours.
Quality controls
Assumptions and limitations
Assumptions
- Entered values describe one stable operating condition and use a consistent electrical or metering boundary.
- Measured quantities are simultaneous, representative, and obtained with appropriately rated instruments.
Limitations
- Energy-only screen; demand, HVAC interaction, maintenance, rebates, degradation, and capital cost are omitted.
- Lighting quality, controls, schedules, and adopted-code requirements require project review.
Field use
Verification procedure
- Follow the approved electrical-safety program and use correctly rated instruments.
- Confirm wiring, ratios, units, operating mode, and measurement boundaries.
- Document simultaneous readings and compare with approved project and manufacturer data.
Common engineering mistakes
- Combining values from different time intervals, phases, or measurement boundaries.
- Treating a screening result as equipment selection, protective-device coordination, or code compliance.
Technical basis
References and source standards
- U.S. Department of Energy—Energy Saver and Better BuildingsFederal energy-management, lighting, motor, and building-energy guidance.
- ANSI/ASHRAE/IES Standard 90.1Lighting-power and energy-efficiency context; the adopted edition and local code remain controlling.
Engineering calculation disclaimer
These engineering tools are provided for educational, preliminary evaluation, field verification, and commissioning support. Results depend on the accuracy of user-entered information and the assumptions stated for each calculation. Every colored status and comparison is a screening indicator only; project criteria, contract requirements, applicable codes, manufacturer instructions, measurement uncertainty, and authorized engineering judgment govern. The tools do not replace project-specific engineering analysis or the judgment of a licensed professional engineer.
Read the full use and limitation statement