Check specific fan power (SFP) against the Approved Document L 2021, Table 6.9 maximum.
SFP = (supply + extract fan input power) ÷ design air-flow, in W/(l·s).
For design guidance only. Always verify with a qualified engineer.
About this specific fan power calculator
This free specific fan power calculator works out the SFP of a mechanical ventilation system and checks it against the maximum permitted by Approved Document L Volume 2 (2021 edition, 2023 amendments), Table 6.9. It is built for building-services engineers and designers who need a quick energy-efficiency check on an air-handling unit or ventilation fan during design or Part L compliance. Everything runs in your browser. Nothing is uploaded.
How specific fan power is calculated
Specific fan power is the total electrical input power of a system's fans, including switchgear and control (inverter) losses, divided by the design air-flow rate: SFP = (Psf + Pef) ÷ q, in W/(l·s), where Psf and Pef are the supply- and extract-fan input powers and q is the design air-flow (for an air-handling unit, the greater of the supply or extract flow). The calculation procedure follows BS EN 16798-3:2017, mandated by ADL 2021 para 6.52. The result is compared against the limit for the system type and building age in ADL 2021 Table 6.9; the limit is a maximum, so a system at or below it is compliant.
Frequently asked questions
What are the units of SFP?
SFP is expressed in watts per litre per second, W/(l·s), which is numerically identical to kW per m³/s. A lower figure means a more efficient system for the air it moves.
Which fan power should I enter?
Enter the total electrical input power of all fans at the design flow rate, including any switchgear and variable-speed-drive (inverter) losses, not the motor shaft power or nameplate rating. For a supply-only system, leave the extract fan power as 0.
What air-flow do I use for an AHU?
Use the greater of the supply or extract design air-flow rate in litres per second. The ADL 2021 Table 6.9 limits already include the heat-recovery and return-filter allowance for balanced systems, so do not add those again.