Undersizing a shade motor is the most common and most expensive mistake in a motorization project. The motor stalls, overheats, or simply refuses to lift a fully-loaded tube, and the only fix is a new motor. Sizing correctly takes three numbers: the torque the shade demands, the tube it has to fit inside, and the speed you can live with.
1. Work out the torque the shade needs
Torque is rotational force, measured in newton-meters (Nm). The torque a roller shade demands rises with two things: the weight of the fabric assembly, and the radius of the roll it is winding onto.
That second factor is the one people miss. A shade winding onto a 2 in tube needs roughly a quarter more torque than the same fabric on a 1.5 in tube, because the motor is working at a longer lever arm. As fabric layers build on the tube, the effective radius grows through the travel, so peak demand is at the bottom of the shade, fully unrolled, at the moment it starts to lift.
Practical guidance for interior roller shades:
- Small shades up to about 40 in wide in standard solar fabric: 2–4 Nm. The Sonesse 28, 30 and 40 sizes live here.
- Typical residential shades, 40–80 in wide: 6 Nm covers most of this range. This is the busiest size in the catalog — the 506S2 and 506A2 motors.
- Wide or heavy shades, blackout fabric, dual-roller, or anything over 80 in: 10 Nm and up. 510S2, 515S2 and larger.
Exterior work is a different scale entirely, because wind load and fabric weight both climb. Cassette awnings and zip screens typically start at 25 Nm and run to 50 Nm or beyond — see the exterior motor range. Rolling shutters on 60mm tube can need 80 or 100 Nm.
If a shade sits near a boundary, size up. The cost difference between a 6 Nm and a 10 Nm motor is small next to the cost of replacing an undersized one.
2. Check the motor physically fits the tube
Torque is necessary but not sufficient — the motor also has to go inside the tube. Every Somfy motor states a minimum inside tube diameter, and it is a hard limit.
- Sonesse 28 and R28: from roughly 1.13 in (28mm) tube
- Sonesse 30: from roughly 1.13 in, the smallest practical size for a discreet interior shade
- Sonesse 40 / LS40 / LT40: from roughly 37–38mm
- Sonesse 50 / LT50 (the 500 series): 47mm and up — the 2 in and 2.5 in tubes that dominate residential work
- LT60 (600 series): 60mm tube, for rolling shutters and large exterior assemblies
Wall thickness matters as well as diameter. A tube with a thick wall or an internal profile has less usable bore than its nominal outside diameter suggests, and a crown that fits a 2 in round tube will not fit a 2 in profiled tube. Every listing on this site names the tube it fits.
3. Decide how fast it should move
Speed is measured in rpm and trades directly against torque within a motor family. Across the Somfy range, faster motors carry less torque and slower motors carry more:
- 30–38 rpm — fast, for light interior shades where you want the shade down quickly
- 20–24 rpm — the middle ground, and the most common choice for interior work
- 12–14 rpm — slow and strong, for heavy exterior assemblies and large rolling shutters
On a very tall shade, a fast motor is worth having. On a bank of shades that should move together, matching speeds matters more than absolute speed, because mismatched motors finish at visibly different times.
4. Do not forget noise
For bedrooms, media rooms and conference rooms, noise is a specification, not a preference. Standard Sonesse motors run around 44–45 dBA. The Sonesse ULTRA range runs under 38 dBA, which is the difference between a shade you hear and a shade you notice only because the light changed.
Putting it together
Size the torque from fabric weight and tube radius, confirm the motor fits the tube's inside diameter, pick a speed band, and add the ULTRA premium where quiet matters. Then match the crown, drive and bracket to the tube, and choose the control protocol.
Every product listing on this site carries Somfy-verified torque, speed, noise rating and minimum tube diameter, plus links to the official Somfy spec sheet. If you know the part number, searching it will take you straight there.
