Disclaimer - The following "Rules of Thumb" have been created so industries can compare their energy usage and costs with parameters resultant of PWI engineering studies and analyses thereof. These rules are based on standard practices and should be used as guidelines and/or general ideas to which to compare your facility's energy usage data. They will not apply to all facilities or all projects. (All costs are in $US.)

1. Energy Costs

• If natural gas costs more that $3.50/MMBtu including Local Distribution Company (LDC) charges, review buying strategies.
• If local gas distribution charges exceed $.50 per MMBtu, look for leverage to negotiate lower rates.
• By-pass natural gas pipelines cost approximately $100,000/mile plus $300,000 - $500,000 for the take-station connection cost.
• Average maintenance charges for a natural gas bypass are 4 cents/MMBtu.
• Electricity should not cost more than 3 cents over the wholesale price.
• The generation component of electricity prices should be in the 2-3 cent range.
• The cost of oil does not always track the cost of gas. (See chart for cost comparison)
• Electric resistance heat is typically more expensive than natural gas or oil heat.

2. Lighting Systems

• Lighting energy is wasted when there are no local switches.
• Activities requiring high visibility or color resolution require task lights.
• Excess lighting levels are counter-productive, waste energy, and can harm eyesight.
• Day-lighting is better than artificial lighting in that it is less expensive and emits less heat.
• Lighter, reflective ceiling, floor and wall colors require less lighting.
• Multiple lighting levels (ambient and task) save energy.
• Examples of ambient lighting levels

• Office - 30-50 FC (300-500 Lux)
• Laboratory - 30-50 FC (300-500 Lux)
• Production - 50 - 75 FC (500-750 Lux)

3. Control Systems

• The simplest control systems are the most efficient, effective, and reliable control systems.
• Time clocks, occupancy sensors, and programmable timers usually save energy.
• Production activities, clean-up activities, and off-hour conditions require different environmental criteria and different control cycles.

4. Boiler Systems

• At peak capacity, boiler systems should operate at 80-85% efficiency.
• Boilers operate off-season at an efficiency as low as 60% due to stand-by losses.
• Maintaining combustion efficiency requires consistent monitoring.
• Excess boiler steam pressure promotes waste.
• The de-aeration temperature is critical to the removal of oxygen in condensate feed systems.
• Boiler de-aeration systems should be equipped with air vent valves to minimize the loss of steam.
• Steam-traps do leak and will continue to leak if not monitored and maintained.
• For modulating burners, inlet vane damper control on forced draft fans or fan speed control is much more efficient than a discharge damper.
• Standby steam losses due to leakage and heat loss should never exceed 5% of boiler production.
• Unless contaminated, steam condensate should be returned to the boiler.
• Hot water heating provides more precise temperature control than steam heating.
• Use hot water heat to distribute heat for a distance less than 2,000 ft. (609.7 meters) Over 2000 ft. (609.7 meters) use steam or high temperature water (300⁰ F - 400⁰ F) for cost- effectiveness.
• Recover high pressure flash steam and re-use in low pressure steam systems.

5. Refrigeration Systems

• Use central chillers for loads greater than 100 tons (350 kWh)
• If criteria will permit, chilled water temperature should be raised as high as possible.
• Condensing water temperature for chillers should be no less than 65⁰ F (18⁰ C) and no greater than 85⁰ F (29⁰ C).
• Packaged, direct expansion (DX) units operate at 1.25 times the energy requirement of central electrical chillers
• Central electrical chillers without a capacitor bank will have the single worst impact on reducing power factor. The compressors are often the largest motors in plants.
• Chillers should always be started with demand limiters set at no greater than 50%. The start up of chillers will demand full load until the entire chilled water loop is cooled to the setpoint. Take more time with demand limiting and save on demand charges.
• Chilled water pumping should normally be sized at a 15⁰ F - 17⁰ F temperature differential.
• Condenser water pumping should be at a 10⁰ F temperature differential.
• Cooling towers should be engineered for a 3⁰ F - 5⁰ F wet bulb temperature approach.
• Chilled water pumping stations should be equipped with a 2-way modulating valve, not 3-way by-pass valves.
• Chilled water pumping stations should be equipped with variable frequency drive pumps. Primary and secondary (and in some cases tertiary) piping arrangements are optimum.

6.   Air Conditioning

• 400 cfm per ton (10-15% outside air)

• 200 cfm per ton (100% outside air) typical occupied space

• 80-150 cfm per ton (100% outside air) humidity limited space

• For humidity controlled space, use ASHRAE 1-5% dew-point conditions with Mean Coincident Dry Bulb for design purposes, depending on allowable moisture range.

• Reciprocating engine driven generators produce hot water and low-pressure steam only.
• For every 100 BTU's of fuel input

• 25-30 Btu's of electricity are produced.
• 30-50 Btu’s of hot water or low-pressure steam can be recovered from a reciprocating engine
• 25-30 Btu’s of high pressure steam can be recovered from a turbine

• Turbine generators require a minimum of 15-20 minutes for start time
• Cogeneration costs between $800-$1200/kW installed.
• Back-up generators cost between $300-$800/kW installed.
• Gas fired generators cost 2-3 times oil fired generators. (installation costs are the same.)

  VIII.  Compressed Air Service

• Compressors over 100 HP should have a compressed air storage tank
• Water-cooled compressors over 100 HP can include heat recovery for pre-heating domestic water or boiler feed water.
• Excess air pressure wastes energy
• High demand devices should include a local storage tank to absorb peak air demands
• Compressed air distribution systems will always leak, requiring consistent monitoring and repair (most leakage occurs at regulators and metal to plastic connectors)

9. Electrical Service

• The higher voltage of service, the cheaper the power.
• Industrial Distribution Voltage = 13.2 kV - 34.5 kV
• Sub-transmission Voltage = 34.5 kV- 115 kV
• Bulk transmission service = 115 kV & higher
• Site Distribution Voltage = 13.2 kV, 4160V,480V
• The closer the power factor is to 1.00, the cheaper the power.
• The higher the load factor, the cheaper the deregulated generation.

Link to Energy Unit Conversion Chart