VFD Energy Savings Calculator (Fans & Pumps)

How it works

Affinity laws: flow ∝ speed, head ∝ speed², power ∝ speed³ (with static head: P ∝ Q × (static + (1 − static) Q²)); damper/valve power ≈ 0.55–0.6 + 0.4–0.45 × flow fraction; drive + motor derating ≈ 3–6%; savings = Σ hours × (P_current − P_VFD)

A centrifugal fan or pump obeys the affinity laws: flow is proportional to speed, pressure to speed squared and power to speed cubed, so running at 80% speed takes about half the power and at 60% roughly a fifth. Traditional control wastes that potential — a damper or throttling valve leaves the machine at full speed and burns the difference as pressure drop, so power falls only modestly with flow, and bypass control saves nothing. A variable-frequency drive slows the motor to deliver just the flow required, and because most systems spend most hours well below design flow, savings of 30–50% of the energy bill are common with paybacks of one to three years. Systems with significant static head (pumping uphill, pressure set-points) gain less, since speed cannot drop below what the static head demands.

Worked example

A 50 hp fan at 90% load (36 kW input) running 6,000 hours a year, 20/40/30/10% of the time at 100/80/60/40% flow: an outlet damper uses about 189,000 kWh; a VFD about 112,000 — saving 78,000 kWh (41%), $8,500 a year at 11¢, paying back a $9,000 drive in just over a year.

Frequently asked questions