Energy Units

Energy is what you actually pay for. Power × Time = Energy. This is the most important unit for understanding compressed air costs.

Power vs Energy

Critical Difference

• Power = Consumption rate (kW) - how fast you use energy
• Energy = Total consumption (kWh) - how much energy you used

The fundamental relationship:

$$E = P \times t$$

Where:

• $E$ = Energy (kWh)
• $P$ = Power (kW)
• $t$ = Time (hours)

Water analogy:

• Power = Flow rate (liters/minute)
• Energy = Total volume (liters)

Energy Units

Unit	Symbol	Equivalence
Kilowatt-hour	kWh	Billing unit
Megawatt-hour	MWh	1,000 kWh
Joule	J	1 Ws (watt-second)
Kilojoule	kJ	1,000 J
BTU	BTU	1,055 J
Calorie	cal	4.186 J

Key Conversions

$$1 \text{ kWh} = 3{,}600 \text{ kJ} = 3{,}412 \text{ BTU}$$ $$1 \text{ HP-hour} = 0.746 \text{ kWh}$$

The kWh: Your Enemy on the Bill

The kilowatt-hour (kWh) is what appears on your electric bill:

$$E = P \times t = 37.3 \text{ kW} \times 24 \text{ h} = 895.2 \text{ kWh/day}$$

For a 50 HP compressor running 24/7:

$$E_{monthly} = 895.2 \times 30 = 26{,}856 \text{ kWh}$$

Energy Cost

Basic Calculation

$$\text{Cost} = E \times \text{Rate} = 26{,}856 \text{ kWh} \times \$0.10/\text{kWh} = \$2{,}685.60 \text{ USD/month}$$

Annualized:

$$\text{Annual Cost} = \$2{,}685.60 \times 12 = \$32{,}227 \text{ USD/year}$$

Rate Structure

Most industrial rates include:

Component	Description
Energy charge	$/kWh consumed
Demand charge	$/kW of peak maximum
Power factor	Penalty if PF < 0.9
Time of use	Peak, intermediate, off-peak

Savings

Operating during off-peak hours can reduce costs 20-40%.

Specific Energy of Compressed Air

The key efficiency metric:

$$\text{Specific Energy} = \frac{\text{kW}}{100 \text{ cfm}}$$

Scenario	kWh/100 cfm	Annual Cost*
Efficient system	18	$28,000
Average system	22	$34,200
Poor system	28	$43,500

*For 500 cfm, 8,000 hours/year, $0.08/kWh

The 76% That Matters

Total lifetime cost of a compressor:

Component	Percentage
ENERGY	76%
Equipment	12%
Maintenance	12%

Reality

For every $1 you spend buying a compressor, you'll spend $6+ on energy during its lifetime.

Energy ROI Calculation

Example: Leak Repair

1/8" leak @ 100 PSI = 26 cfm lost

Wasted energy:

$$P_{lost} = 26 \text{ cfm} \times \frac{22 \text{ kW}}{100 \text{ cfm}} = 5.72 \text{ kW}$$

Annual cost:

$$\text{Cost} = 5.72 \text{ kW} \times 8{,}760 \text{ h} \times \$0.08/\text{kWh} = \$4{,}008 \text{ USD/year}$$

ROI:

$$\text{ROI} = \frac{\$4{,}008}{\$50} \times 100\% = 8{,}000\%$$

Example: VSD vs Load/Unload

100 HP compressor @ 70% average load:

Without VSD:

$$E_{without} = 74.6 \text{ kW} \times 8{,}760 \text{ h} = 653{,}496 \text{ kWh/year}$$

With VSD:

$$E_{with} = 74.6 \text{ kW} \times 0.70 \times 8{,}760 \text{ h} = 457{,}447 \text{ kWh/year}$$

Savings:

$$\text{Savings} = (653{,}496 - 457{,}447) \times \$0.08 = \$15{,}684 \text{ USD/year}$$

Energy Monitoring

Essential metrics to track:

Metric	Formula	Goal
kWh/unit produced	Total kWh ÷ Production	Minimize
Cost/1000 scf	$ ÷ (cfm × 60 × h / 1000)	< $0.25
Specific energy	kW ÷ 100 cfm	< 22

Measure to Improve

What doesn't get measured doesn't get improved. Install energy meters on your compressors.