SFP Calculator — Specific Fan Power

Specific Fan Power (SFP) is the total electrical power consumed by a ventilation fan system divided by the airflow rate it delivers, measured in watts per litre per second — W/(L/s). It is the primary metric for assessing ventilation energy efficiency under UK Building Regulations. Lower SFP means more efficient use of electricity to move air.

For new dwellings the SFP limits come from the Domestic Building Services Compliance Guide (published under Approved Document L, the energy-efficiency part), not Approved Document F. Part F sets minimum ventilation rates in litres per second; Part L sets fan-power efficiency in watts per litre per second. The limits are: intermittent extract fans less than 0.5 W/(L/s), continuous supply 0.5 W/(L/s), continuous extract (MEV/dMEV) not worse than 0.7 W/(L/s), and continuous balanced supply and extract with heat recovery (MVHR) 1.5 W/(L/s). These apply at the design flow rate.

For balanced systems (MVHR), add the power of the supply fan and extract fan together, then divide by the maximum airflow of either the supply or extract side (whichever is greater, as they should be balanced). SFP = (supply fan W + extract fan W) / max(supply L/s, extract L/s). The limit is 1.5 W/(L/s).

The ventilation system will not comply with Building Regulations and will fail commissioning. Options include: selecting a more efficient fan motor (EC motors achieve lower SFP than AC motors), reducing duct system resistance (larger ducts, fewer bends), or changing the fan type. SFP is checked during commissioning and must be documented.

Replace AC motors with EC (electronically commutated) motors — this alone can halve the SFP. Reduce system resistance by replacing flexible duct with rigid, removing unnecessary bends, increasing duct diameters, or cleaning dirty filters. Ensure dampers are fully open and not restricting flow unnecessarily. Re-commission after changes.

SFP stands for Specific Fan Power. A fan's SFP is how many watts of electricity it draws to move one litre of air per second, so it is the standard way to express how efficient a fan or ventilation system is. It is written in W/(L/s) — watts per litre per second. A fan with a lower SFP moves the same amount of air for less electricity, which is why UK Building Regulations cap SFP rather than just the fan wattage.

Specific Fan Power is measured in watts per litre per second, written W/(L/s) or W/(l·s). It is the fan's electrical input power in watts divided by the airflow it delivers in litres per second. Some manufacturers and the SAP methodology also quote it as W/(l·s); both are the same unit. To convert from watts per cubic metre per second, divide by 1000, because 1 m³/s equals 1000 L/s.

Lower is better. For new dwellings the Domestic Building Services Compliance Guide (under Approved Document L) caps SFP at less than 0.5 W/(L/s) for intermittent extract fans, 0.5 for continuous supply, not worse than 0.7 for continuous extract (MEV/dMEV) and 1.5 for balanced MVHR with heat recovery. A modern EC-motor extract fan can sit well under 0.4 W/(L/s); anything markedly above the limit for its system type signals an inefficient motor or an over-restrictive duct run.

Wattage alone tells you nothing about value for energy — a powerful fan moving a lot of air can be efficient, while a small fan choked by poor ductwork can be wasteful. SFP normalises power against the air actually delivered (W per L/s), so two fans can be compared fairly and a single regulatory limit can apply regardless of size. It is the metric used in Part L compliance, in SAP/EPC calculations for dwellings, and in BS EN 16798-3 for non-domestic systems.

Non-domestic limits come from Table 6.9 of Approved Document L Volume 2, 2021 edition (incorporating 2023 amendments). For new buildings the maximum SFP at design flow is 2.0 W/(L/s) for a central balanced system with heating and cooling, 1.9 for heating only, 1.5 for all other central balanced systems, 1.1 for a zonal supply system with the fan remote from the zone, 0.5 for a zonal extract, 0.3 for a local supply or extract unit (e.g. toilet extract) and 1.0 for kitchen extract with the fan remote and a grease filter. Existing buildings get slightly higher allowances. SFP must be calculated to BS EN 16798-3 at full design load (BS 8850 for fan coil units).