Lighting and Motors/Drives
Core concepts
For lighting, understand lumens per watt (efficacy), color rendering index (CRI), correlated color temperature (CCT), and lighting power density (LPD) in W/ft². Know LED versus fluorescent and HID sources, ballast types, occupancy and daylight sensors, and lighting controls. For motors, understand NEMA Premium efficiency, load factor, rewinding versus replacement, and variable frequency drives (VFDs). The affinity laws are essential: flow varies with speed, while power varies with the cube of speed.
Traps to watch
A classic trap is applying the cube law to a system with static head, where the savings are less than predicted. Another is assuming oversized motors are efficient - they run at poor load factor and low power factor. Questions often confuse efficacy (lumens/W) with efficiency. Watch for lighting questions that ignore lumen depreciation and maintenance factors, or that compare lamp wattage without including ballast losses. Also beware of assuming VFDs save energy on all loads - they are ineffective on constant-torque loads that always run at full speed.
Worked example
A 100 hp fan motor draws 75 kW at full speed. Reducing speed to 80% with a VFD, assuming a pure variable-torque load, gives power ≈ 75 × 0.8³ = 75 × 0.512 ≈ 38.4 kW. Over 4,000 hours, savings ≈ (75 − 38.4) × 4,000 ≈ 146,400 kWh. At $0.10/kWh, that is about $14,640 per year.
Practical guidance
Measure actual motor load, verify operating hours, and confirm system curves before claiming affinity-law savings.
Industrial Process Energy and Combined Heat and Power
Core concepts
Industrial systems dominate energy use in many facilities. Understand process heating (furnaces, ovens, dryers), process cooling, pumping, compressed air, and fans. For compressed air, know that leaks, artificial demand, pressure setpoints, and heat recovery are major opportunities; compressed air is one of the most expensive utilities per unit of work. For pumping and fan systems, understand system curves, affinity laws, and trimming impellers.
Combined heat and power
CHP, or cogeneration, produces electricity and useful thermal energy from one fuel source. Know the metrics: power-to-heat ratio, electrical efficiency, thermal efficiency, total efficiency, and effective electrical efficiency. Understand topping versus bottoming cycles, gas turbines, reciprocating engines, steam turbines, and organic Rankine cycles. Know that CHP economics depend on the spark spread (electricity price minus fuel price) and on matching thermal load.
Traps to watch
A common trap is comparing CHP total efficiency to central station generation efficiency without accounting for the fact that central stations discard heat. Another is assuming CHP always saves energy - if the thermal output is not used, efficiency collapses. Watch for confusion between thermal load following and electric load following. Also beware of ignoring standby charges and interconnection costs in CHP economics.
Worked example
A 5 MW gas turbine at 35% electrical efficiency and 50% thermal efficiency has 85% total efficiency. Fuel input = 5 MW / 0.35 ≈ 14.3 MW. Useful heat = 14.3 × 0.50 ≈ 7.1 MW thermal. If that heat displaces an 80%-efficient boiler, fuel savings are substantial.
Practical guidance
Measure compressed air leaks with ultrasonic detectors, reduce pressure, and recover waste heat from compressors and processes.
Renewable Energy
Core concepts
Understand solar photovoltaics (PV): irradiance, peak sun hours, capacity factor, performance ratio, tilt and azimuth, shading, and inverter losses. Know solar thermal for hot water and process heat, wind power (with the cube of wind speed relationship), biomass, geothermal, and hydro. Candidates should understand net metering, interconnection, renewable energy credits (RECs), and power purchase agreements.
Traps to watch
A frequent trap is confusing kW (capacity) with kWh (generation). Another is assuming a system produces at nameplate all day - capacity factor for PV is typically 15-25% depending on location. Watch for confusion between DC and AC ratings; inverter and wiring losses reduce delivered energy. Questions may ask about tilt angle - a common rule is latitude for fixed tilt, though it varies with load shape. Also beware of ignoring soiling, degradation (often about 0.5% per year), and shading losses.
Worked example
A 100 kW PV array in a location with 1,600 peak sun hours per year, with a performance ratio of 0.80, generates 100 × 1,600 × 0.80 = 128,000 kWh per year. At $0.12/kWh, that is $15,360 per year. If installed cost is $250,000, simple payback is about 16 years before incentives.
Practical guidance
Screen sites for shading, roof condition, structural capacity, and interconnection limits. Model production with a recognized tool and validate assumptions about degradation and soiling.
Sustainability and Emissions
Core concepts
Understand greenhouse gas (GHG) accounting under the GHG Protocol: Scope 1 (direct combustion and fugitive), Scope 2 (purchased electricity, steam, heat), and Scope 3 (value chain). Know that emissions are calculated as activity data × emission factor. Candidates should understand carbon dioxide equivalent (CO₂e) and global warming potential (GWP) - methane has a much higher GWP than CO₂. Familiarity with renewable energy credits, carbon offsets, science-based targets, and reporting frameworks such as CDP and GRI is expected.
Traps to watch
A common trap is classifying purchased electricity as Scope 1 - it is Scope 2 (or Scope 3 under the market-based/ location-based distinction depending on framework). Another is confusing location-based and market-based Scope 2 methods; market-based reflects contractual instruments like RECs. Watch for questions that treat all GHGs as equivalent without applying GWP. Also beware of double counting offsets and RECs, and of assuming a reduction target is science-based without validation.
Worked example
A facility uses 1,000,000 kWh of electricity per year. With a grid emission factor of 0.40 kg CO₂e per kWh, Scope 2 emissions = 1,000,000 × 0.40 = 400,000 kg, or 400 metric tons CO₂e. If the facility purchases RECs matched to 25% of consumption, market-based Scope 2 emissions fall by 100 metric tons.
Practical guidance
Set a base year, choose consistent emission factors, document methodology, and report both location- and market-based Scope 2 figures where frameworks require.