Pump Energy Cost Calculator - Operating-Cost and VFD Review Prompts

How It Works

1. Enter Pump Operating Parameters

   Input flow rate (GPM or MGD), total dynamic head (feet), pump efficiency, and motor efficiency. Replace any default efficiency prompt with measured data, current pump curves, or source-reviewed project assumptions.

2. Set Runtime and Electric Rate

   Enter daily runtime hours and electric rate in $/kWh. Include demand charges if your utility bills them separately; actual tariffs, riders, taxes, time-of-use periods, and demand ratchets need bill or utility review.

3. Review Energy Cost Prompts

   See local kW demand, daily/monthly/annual energy cost, cost per million gallons pumped, and a wire-to-water efficiency prompt that still needs measured field data.

4. Check VFD Assumptions (Optional)

   Toggle the VFD section to review a local friction-head cube-law prompt. Static head, minimum speed, wastewater velocity, pump/system curves, drive losses, tariffs, and installed cost remain outside the local model.

Built For

• Public works directors screening annual pump station energy-cost assumptions
• Operators flagging pumps that need measured flow, head, power, and curve review
• Engineers collecting first-pass VFD retrofit assumptions before source review
• Utility managers comparing local cost-per-MG prompts across pump stations
• Energy reviewers checking municipal pumping source gaps before audit work

Assumptions

• Water horsepower formula: WHP = (GPM x TDH) / 3960, which assumes water at standard temperature and density
• Pump efficiency is user-entered or defaults to a local prompt that must be replaced with measured data, current pump curves, or source-reviewed project assumptions
• Motor efficiency is user-entered or defaults to a local prompt that must be replaced with nameplate, measured, or manufacturer data
• VFD prompts use pump affinity-law arithmetic only for the entered friction-head portion: flow varies linearly with speed, head varies with speed squared, and power varies with speed cubed
• Electric rates and demand charges are treated as constant local prompts and do not include time-of-use periods, tiered rates, riders, taxes, or demand ratchets
• Pump operates at a single duty point for the entire runtime period (no variable flow conditions modeled in the base calculation)

Limitations

• Affinity-law prompts do not replace a pump curve and system curve review; actual savings depend on the ratio of static to friction head in the specific system
• Systems with high static head (e.g., pumping to an elevated tank) may realize significantly less VFD savings than a friction-only cube-law prompt suggests
• Does not account for VFD efficiency losses (typically 2-5%) or harmonic distortion effects on motor performance
• Pump efficiency changes with speed - operating far from the BEP reduces efficiency and may offset some VFD energy savings
• Does not model demand ratchet clauses where the utility charges peak demand for 12 months after a single high-demand event
• Does not include maintenance cost savings from reduced mechanical stress at lower speeds or increased seal and bearing life

References

• US DOE / Hydraulic Institute - Improving Pumping System Performance: A Sourcebook for Industry
• US DOE - Variable Speed Pumping: A Guide to Successful Applications
• US DOE - Continuous Energy Improvement in Motor Driven Systems
• Hydraulic Institute standards and pump-system resources
• EIA Electric Power Monthly Table 5.3 for broad electricity-price context
• OSHA 1910.147, OSHA 1910.333, and NFPA 70E for hazardous-energy and electrical safe-work context

Frequently Asked Questions