Energy Audit Calculations


Energy Calculations requires familiarity with a few basic equations. Equipment is typically rated in watts,  amps, or horse power for electricity and BTUs for natural gas. Apply run time to equipment ratings, estimate a load factor, apply energy costs, and the cost to operate the equipment can be reasonably known.

Electric Equipment

RULEOFTHUMB1 watt = 3.413 BTUs 1 kW = 1,000 watts = 3,413 BTUs 1 MW = 1,000 kW = 1,000,000 watts = 3,413,000 BTUs 95 to 100% efficient at THE POINT OF USE. 746 watts or 0.746 kW = 1 Horse Power Mechanical 3 Phase Factor is 1.73 Watts = Volts x Amps Resistance Equipment Resistance equipment – heating equipment and anything designed primarily to heat, is typically rated in watts.

Watts x Load Factor x Hours of Use = Watt-Hours / 1,000 = kWh

For example: a 2,400 watt tank heater is used to heat a small parts washer. 2,400 watts x 60% Load Factor x 8 Hours = 11,520 watts / 1,000 = 11.5 kWh per 8 hour shift Motor Loads Motors are typically rated in Horse Power. Motors may be single or 3 phase and operate on a variety of voltages. The only time it’s necessary to know the phase and voltage is if amp readings are taken.

Horse Power x 0.746 / Motor Efficiency = kW

kW x Load Factor x Run Hours = kWh

For example: a 50 horse power air compressor runs continuously 50 hp x 0.746 kW/hp   /.95 efficiency = 39.2 kW 39.2 kW x 80% Load Factor x 24 hours = 753.8 kWh per 24 hour day If Amp Readings are taken:

Amps x Voltage x Phase Factor = Watts

For example: a motor running on a 440 volt 3 Phase system is measured with an amp meter and is drawing 25 amps. 440 volts x 25 amps x 1.73 3Phase Factor = 19,030 watts 19,030 watts / 746 watts per horse power = 25.5 horse power 19,030 watts / 1,000 = 19 kW 19 kW x 24 hours = 456 kWh per 24 hour day   Lighting, Office Equipment, Hand Tools Most all small electric equipment is rated in watts. Use the same formula for resistive equipment. For example: a small office has 2 copiers that are rated at 750 watts each. 750 watts x 2 units x 10 hours / 1,000 = 15 kWh per day (a little less if they have a power saver mode)

Natural Gas Equipment

RULEOFTHUMB 1 Cubic Foot = CF = 1,000 BTUs Gas Quality Spec allows 950 – 1,100 BTUs/CF 1 Hundred Cubic Feet = CCF = 100,000 BTUs 1 Therm = EXACTLY 100,000 BTUs of ANY Fuel 1 CCF is about equal to 1 Therm 1 MCF = 10 CCF = 1,000 CF = 1,000,000 BTUs All natural gas equipment is rated in BTUs per Hour. For older and custom equipment, the biggest challenge is generally trying to find an Input Rating Label. See Estimating BTU Ratings. Always be sure to get ‘INPUT’ Rating as Output Rating will generally be about 80% of Input.   Boilers Boilers are generally rated in Boiler Horse Power and very large boilers rated in Pounds per Hour of Steam.

  • 1 boiler horse power is about 42,000 BTUs of INPUT fuel
  • 1 pound of steam is about 1,200 BTUs of INPUT fuel, and about 1,000 BTUs at the point of use, depending on the pressure of the steam

Higher and Lower Heating Values If working with international equipment and power generation equipment, the terms HHV and LHV will come up. The difference is about 10%. The Higher Heating Value is TOTAL BTUs in the fuel, and the Lower Heating Value considers that about 10% of the heat in the fuel will be lost to water vapor so it discounts its value at the start of the equation. Natural Gas utilities sell only HHV gas. In my opinion, the main reason LHV was invented is to make some equipment look more efficient, and to create confusion among gas people.                   Source: Text Bob Fegan 1/2009

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