From SFP Class to Euros — Annual Fan Energy Cost and Payback

A building owner has never once asked for 690 W/(m³/s). They ask what the unit costs to run. SFP is the right engineering metric and the wrong sales metric, and the translation between them is three lines of arithmetic that most quotes never carry out — which is why the cheaper unit usually wins the tender and the expensive one wins the twenty years that follow.

The identity worth memorising

Fan shaft power is airflow times pressure rise, divided by efficiency. Divide both sides by airflow and the flow term cancels:

SFP = Δp / η_total

That is the whole story. Specific fan power is internal pressure drop divided by total fan efficiency — motor, drive and impeller together. It does not depend on airflow at all. A unit at 700 Pa internal with a fan running at 60 % total efficiency sits at 1 167 W/(m³/s), whether it moves 3 000 or 30 000 m³/h.

For a double-deck unit, EN 13053 defines SFP_int over the whole unit — both fans' absorbed power divided by the larger of the two airflows — so with balanced flows you simply add the two sides: SFP_int ≈ Δp_sup/η_sup + Δp_ext/η_ext.

From W/(m³/s) to kWh to euros

Three steps, no modelling software required:

1. Power. P [kW] = SFP × q_v ÷ 1000, with q_v in m³/s (divide m³/h by 3 600).
2. Energy. E [kWh/yr] = P × annual operating hours.
3. Cost. €/yr = E × tariff.

The operating-hours figure is the single largest lever and the one you must get from the client rather than assume. An office on a 12 h × 250 d schedule runs about 3 000 h/yr. A hospital, data hall or food plant runs 8 760 h/yr — nearly three times the energy for the identical unit.

Worked at 5 000 m³/h (q_v = 1.39 m³/s), 3 000 h/yr, €0.22/kWh:

SFP_int W/(m³/s)	Fan power	kWh/yr	€/yr	€/20 yr
350 (SFP 1, generous casing)	0.49 kW	1 460	€321	€6 420
500 (SFP 1 limit)	0.69 kW	2 080	€458	€9 160
750 (SFP 2/3 boundary)	1.04 kW	3 130	€688	€13 760
1 000 (SFP 3)	1.39 kW	4 170	€917	€18 340
1 250 (SFP 3/4 boundary)	1.74 kW	5 210	€1 146	€22 920

Same airflow, same duty, same building. The spread between the top and bottom row is €16 500 over the unit's life on one 5 000 m³/h AHU — undiscounted, at today's tariff, on a site that shuts down every evening.

The payback the owner actually wants

Now put a capex number against it. The most common energy decision in an AHU quote is whether to go one casing size up:

• Unit A — face velocity 2.5 m/s, internal Δp ≈ 690 Pa, η_total 0.60 → SFP_int ≈ 1 150
• Unit B — one casing size up, 1.8 m/s, internal Δp ≈ 420 Pa, η_total 0.61 → SFP_int ≈ 690

Δ SFP = 460 W/(m³/s) → 0.64 kW saved → 1 920 kWh/yr → €422/yr at 3 000 h. If the larger casing plus the correspondingly larger coils and filters adds €1 800 to the price, simple payback is 4.3 years, and the unit returns roughly €8 400 against that €1 800 over a 20-year life.

Run the same unit continuously and it becomes unarguable: 5 600 kWh/yr, €1 230/yr, payback under 18 months.

One caution on the pressure side of that comparison: component dP does not all scale with velocity squared. Coils and heat exchangers land near v^1.8–2.0, but filter media is closer to v^1.0–1.3, so the pure square-law estimate overstates the saving on filter-heavy units. Take the dP from the manufacturer's own selection data at the actual face velocity, not from a scaling rule — see face velocity and casing sizing.

Three things that make the number defensible

Quote SFP_int, and say so. If the schedule caps "SFP ≤ 1 500" it usually means the installed system figure including ductwork, not your unit's internal share. Mixing the two invalidates the whole cost calculation.

State the tariff and the hours as assumptions, on the page. €0.22/kWh and 3 000 h are inputs, not facts. Print them next to the result so the owner can substitute their own contract price — and so nobody can accuse the calculation of being tuned.

Treat the ErP SFP_int cap as a floor, not a target. Regulation 1253/2014 sets the maximum a unit may legally exceed; it says nothing about what is economic. The table above shows a fully compliant SFP 3 unit costing nearly €12 000 more to run than a compliant SFP 1 unit over the same period.

Fan energy typically accounts for a third to a half of an AHU's lifecycle cost, and it is decided in about ninety seconds — during casing sizing, long before anyone opens a spreadsheet. The engineer who can put a euro figure on that ninety seconds while the client is still in the room wins arguments the datasheet cannot.

In AirSelect3D, SFP_int recalculates on every component drop and casing resize, sourced from the real fan engines rather than curve fits, so the euro figure behind a casing decision is available at the moment the decision is being made — not three PDFs later.

Size a unit and watch SFP move in real time →