Lift stations run around the clock, every day of the year, and most municipal operators have no idea what they actually cost. The pumps turn on, the wet well drops, the pumps turn off. As long as sewage flows downhill to the treatment plant, nobody thinks about the electric bill. Then the budget meeting arrives and the utility director asks why the power line item is 40% over projection, and nobody has an answer.
The reality is that pump efficiency degrades over time, and a pump that was 70% efficient when it was installed five years ago might be running at 50% today. That means you are paying 40% more for the same work. Multiply that across a dozen lift stations running 8-16 hours a day, and the numbers add up fast. This guide walks through the real cost of running lift stations, how to estimate efficiency without expensive testing, and where the biggest savings opportunities are.
Pump Efficiency: What You Are Actually Getting
A new submersible wastewater pump typically runs between 60% and 80% wire-to-water efficiency, depending on the pump type, impeller design, and how well it was selected for the application. Wire-to-water efficiency is the ratio of hydraulic power delivered to the water divided by the electrical power consumed at the motor terminals. It accounts for both motor efficiency and pump hydraulic efficiency.
The wire-to-water formula is: kW = (Q × TDH) / (3960 × η), where Q is flow in gallons per minute, TDH is total dynamic head in feet, and η is the decimal wire-to-water efficiency. A pump moving 500 GPM against 40 feet of head at 65% efficiency draws (500 × 40) / (3960 × 0.65) = 7.8 kW. At $0.10/kWh running 12 hours a day, that is $2,800 per year for one pump at one station.
Worn pumps lose efficiency through several mechanisms. Impeller wear increases the gap between the impeller and the volute, allowing recirculation that wastes energy. Clogged or damaged impellers reduce flow and increase vibration. Worn bearings increase friction losses. Scaled or corroded piping increases system head, pushing the pump off its best efficiency point (BEP). A pump that started at 70% efficiency can easily drop to 45-50% after five to seven years of wastewater service without rebuild.
The problem is that operators rarely know the current efficiency. The pump still moves water. The motor still runs. Nothing looks broken. But the kW meter on the panel tells the real story if anyone bothers to read it. A pump drawing 10 kW to do the same job it used to do at 7 kW is costing you 43% more per gallon pumped. Across a system with 15 lift stations, that efficiency loss can represent $50,000 or more per year in excess energy cost.
kW = (Q × TDH) / (3960 × η)
Q = flow (GPM)
TDH = total dynamic head (feet)
η = wire-to-water efficiency (decimal)
3960 = conversion constant (GPM × ft to horsepower)
Annual cost = kW × $/kWh × hours/year
Pump Energy Cost Calculator
Calculate actual operating cost for any water or wastewater pump based on flow, head, efficiency, and runtime. Includes VFD retrofit savings analysis with simple payback and 10-year projection.
VFD Savings: The Affinity Laws in Your Favor
Variable frequency drives (VFDs) save energy by matching pump speed to actual demand instead of running at full speed and cycling on and off. The savings come from the affinity laws, which govern the relationship between pump speed, flow, and power. Flow is proportional to speed. Head is proportional to speed squared. And power is proportional to speed cubed. That cube relationship is the key: reducing pump speed by 20% reduces power consumption by nearly 50%.
In a typical lift station without a VFD, the pump runs at full speed until the wet well reaches the low-level setpoint, then shuts off until the high-level setpoint triggers it again. During the run cycle, the pump is delivering more flow than the incoming sewer rate, which means it is working harder than necessary. With a VFD, the pump can slow down to match the incoming flow rate, running continuously at lower speed instead of cycling between full speed and off.
Real-world savings from VFDs on wastewater lift stations typically range from 20% to 40% of energy cost, depending on how variable the incoming flow is. Stations with large diurnal flow variation (high flow during morning and evening peaks, low flow overnight) see the biggest savings because the pump can slow way down during off-peak hours. Stations with relatively constant flow see less benefit because the pump is already running near its efficient point most of the time.
A VFD for a 10-15 HP lift station pump costs $3,000 to $6,000 installed, depending on the enclosure rating and integration with existing controls. If the station uses $8,000/year in electricity and the VFD saves 30%, the annual savings are $2,400 and the payback is about two years. For larger stations with 25-50 HP pumps, the absolute savings are bigger and payback periods are often under 18 months. Most utility managers who install one VFD end up retrofitting every station in the system within a few years.
Flow: Q₂ = Q₁ × (N₂ / N₁)
Head: H₂ = H₁ × (N₂ / N₁)²
Power: P₂ = P₁ × (N₂ / N₁)³
80% speed = 51% power. 60% speed = 22% power.
The cube law is why VFDs save so much energy.
The Real Cost of Deferred Pump Maintenance
A pump running at 50% efficiency costs exactly twice as much to operate as one running at the same flow and head at 75% efficiency. The math is direct: if the hydraulic work is the same, the electrical input scales inversely with efficiency. A pump that should draw 7 kW at 70% efficiency draws 9.8 kW at 50% efficiency. Over 4,000 hours of annual runtime at $0.10/kWh, that is $1,120 per year in excess energy cost from one pump at one station.
The decision to rebuild or replace depends on the cost gap. A submersible pump rebuild (new impeller, wear rings, seals, bearings) typically costs $3,000 to $8,000 depending on pump size. A new pump costs $8,000 to $20,000 installed. If the rebuild restores efficiency from 50% back to 70% and saves $1,500/year in energy, the rebuild pays for itself in 2-5 years. If the pump is 15+ years old and the casing is corroded, a new pump with modern hydraulics might deliver 75% efficiency and pay back faster.
The hidden cost of worn pumps goes beyond energy. Low efficiency usually means the pump is operating off its BEP, which increases vibration, accelerates bearing wear, and can cause cavitation damage. A pump running at 50% efficiency is also likely running louder, running hotter, and failing more often. The maintenance calls, overtime labor, and emergency pump-outs during failures add to the true cost of deferral.
A simple screening method: compare the actual power draw (from the kW meter or amp clamp) to the expected power draw from the pump curve at the current flow and head. If the actual power is more than 120% of the curve value, the pump has lost significant efficiency and is a candidate for rebuild or replacement. You do not need a formal pump test to make this comparison. A flow measurement from the SCADA system, a pressure reading at the discharge, and an amp clamp are enough to get within 10-15% of the true efficiency.
Backup Power: Generator Sizing and Fuel Cost
Every lift station needs a backup power plan. When the grid goes down, sewage does not stop flowing. Without pumps, the wet well fills, the manhole overflows, and raw sewage enters the environment. Depending on the station, you have 30 minutes to several hours before overflow, based on wet well storage volume and incoming flow rate.
Generator sizing starts with the pump motor: a 15 HP pump draws about 12 kW at full load, but starting current can be 5-7 times running current. A generator must handle the inrush without excessive voltage dip. Rule of thumb: size the generator at 2-3 times the running kW for across-the-line motor starting. A 15 HP pump needs a 25-35 kW generator. If the station has two pumps and both might run simultaneously, size for both. VFDs eliminate the inrush problem because they soft-start the motor, allowing a smaller generator.
Fuel consumption for a diesel generator is approximately 0.07 gallons per kWh at 75% load. A 30 kW generator running at 75% load burns about 1.6 gallons per hour. A 200-gallon tank provides roughly 125 hours of runtime, or about 5 days. At $4.00/gallon for off-road diesel, that is $6.40/hour or $154/day. During extended outages, fuel delivery logistics become the limiting factor, not the generator itself.
Bypass pumping is the expensive alternative. If the generator fails or the station is being rebuilt, portable diesel bypass pumps can keep sewage moving. Rental costs for a 6-inch diesel bypass pump run $500 to $1,500 per day plus fuel and operator labor. A week of bypass pumping can easily cost $10,000 to $15,000. This is why preventive generator maintenance (load testing, fuel quality, transfer switch testing) is critical. A $500 annual service contract on the generator prevents a $15,000 emergency bypass.
15 HP pump: 25-35 kW generator (across-the-line start)
25 HP pump: 40-60 kW generator
With VFD: generator can be sized at 1.25× running kW
Diesel fuel consumption: ~0.07 gal/kWh at 75% load
Run time = tank gallons ÷ (kW × 0.07)
Lift Station Runtime & Backup Power Calculator
Calculate pump cycles, runtime hours, backup generator sizing, and time to overflow for any lift station. Size generators for emergency power and estimate fuel consumption during outages.
Monitoring: Catching Problems Before They Cost You
SCADA and runtime monitoring turn your lift stations from black boxes into managed assets. At the most basic level, tracking pump runtime hours tells you when a pump is running longer than it should. If a pump that historically runs 8 hours a day starts running 12 hours a day with no change in wet weather or customer flow, something has changed: the pump is losing capacity, a check valve is leaking, or inflow and infiltration (I&I) has increased.
The next level is tracking flow versus runtime. A healthy pump moves a predictable volume per cycle. If the pump runs for 15 minutes per cycle but the wet well drawdown decreases from 4 feet to 3 feet over a period of months, the pump is losing capacity. Plot gallons per pump cycle over time and you will see the degradation trend. This is the earliest warning sign of impeller wear, and it appears months before the pump fails completely.
Power monitoring is the most valuable data point for energy management. A current transducer on each pump leg costs $100-200 and provides continuous kW data. Combined with flow, you can calculate real-time efficiency. When efficiency drops below a threshold (say, 55%), the system flags the pump for inspection. This data-driven approach replaces the old method of waiting until the pump fails and then spending $15,000 on an emergency replacement.
The payoff from monitoring is difficult to see in any single month but dramatic over years. Systems with active monitoring programs report 15-25% lower energy costs, 30-40% fewer emergency callouts, and 20-30% longer pump life compared to run-to-failure operations. For a utility with 20 lift stations spending $200,000/year on pump energy and maintenance, that translates to $40,000-$60,000 in annual savings. The SCADA upgrade to add monitoring to all stations might cost $50,000-$80,000, paying for itself in 1-2 years.
1. Runtime hours per day (trending up = problem)
2. Gallons per pump cycle (trending down = wear)
3. kW draw at operating point (trending up = efficiency loss)
4. Starts per hour (excessive cycling = control issue)
Any sustained change in these trends warrants investigation.