I started in window coverings in 2006, on the shop floor of a small factory in Suzhou making roller shades for the European market. I cut fabric, machined aluminum tube and assembled motors. In 2008 I joined Xiamen Goodwood Industry Co., Ltd., the company behind Shadesmart, and I now run production. With a 16-person QC team and a 17-person R&D team behind our motorized roller shades, one pattern still repeats: most motor burnouts are sizing mistakes, not motor defects.
If you install motorized roller shades, or source them from motorized blinds manufacturers, this guide gives you the load formula we use, two worked examples, and the safety margin rule we apply before a motor ships. (In sourcing conversations "blinds" is often a catch-all term. Here we mean roller shades specifically.)
Why do roller shade motors burn out, and is it really the motor's fault?
Motor overload is the most common cause, and it usually starts at selection: the motor was chosen by window size instead of by fabric weight. In our after-sales records from 2019 to 2024, motor overload was the number one complaint category in every year.
The typical story goes like this. An installer measures a 2.4 m (94.5 in) wide window, finds a motor rated for that width, and orders it. What they didn't add up was the fabric, the bottom rail and the friction inside the system. Two windows with identical dimensions can need different motors, because a 280 g/m² blackout fabric and a 450 g/m² blackout fabric weigh very different amounts once they hang on the same tube.
The motor doesn't fail on day one. It runs hot, strains a little on every cycle, and gives out months later. By then nobody connects the failure to a sizing decision made at quoting. Limit-setting errors are the second most common reason for warranty motor replacement, and I'll come back to that at the end.
How do you calculate the real load on a roller shade motor?
Actual load (kg) = fabric area (m²) × fabric weight (g/m²) ÷ 1,000 + bottom rail weight (kg) + 0.3 kg mechanical resistance. Each term matters, and the last two are the ones people skip.
| Variable | How to get it | What people usually forget |
|---|---|---|
| Fabric area (m²) | Width × drop, in meters | Using visible window size instead of the ordered fabric size |
| Fabric weight (g/m²) | From the fabric spec sheet | Assuming a "standard" 250 g/m² when the project fabric is heavier |
| Bottom rail weight (kg) | Rail weight per meter × width | Leaving it out, or assuming a light rail on a wide shade |
| Mechanical resistance | Fixed 0.3 kg | Treating friction as zero |
The bottom rail isn't a free variable. Wider shades need heavier rails to hang straight and resist lifting in a draft, and that extra weight is load the motor has to carry. Size the motor with the rail you are actually shipping.
The 0.3 kg is an empirical constant we use for gear, bearing and tube friction. It comes from years of production data. It is not a textbook engineering formula.
Which motor does a 2,400 × 2,200 mm roller shade actually need?
The same window can need a different motor depending on the fabric alone. Here is one 2,400 × 2,200 mm (about 94.5 × 86.6 in) shade with a 180 g/m bottom rail, calculated twice.
| Example A: 280 g/m² fabric | Example B: 450 g/m² fabric | |
|---|---|---|
| Fabric area | 5.28 m² | 5.28 m² |
| Fabric weight | 1.478 kg | 2.376 kg |
| Bottom rail (2.4 m × 0.18 kg/m) | 0.432 kg | 0.432 kg |
| Mechanical resistance | 0.300 kg | 0.300 kg |
| Total load | 2.21 kg | 3.11 kg |
| 1.0 Nm class (3 kg max) | 74% | 104%: overloaded |
| 1.5 Nm class (6 kg max) | 37%: comfortable | 52%: comfortable |
In Example A, the 1.5 Nm class runs at about 37% of its rated load, which is a healthy margin. In Example B, someone who sized for the 280 g/m² fabric and later switched to 450 g/m² (a common request on Northern European projects) would put a 1.0 Nm motor at 104% of its rated maximum. It would be overloaded from the first cycle.
Example A on the 1.0 Nm class isn't safe either. 2.21 kg is already 74% of that motor's 3 kg rating, and the window is wider than that class's recommended maximum. That brings us to the rule that decides most of our selections.
How much safety margin should you leave when choosing motor torque?
Our rule: actual load should not exceed 70% of the motor's rated maximum load. This is our internal production standard, not an industry-mandated figure. We use it because a motor running near its rated limit has no room for heat, wear or a slightly stiff mechanism.
| Motor torque class | Rated max load | 70% ceiling | Recommended max width |
|---|---|---|---|
| 1.0 Nm | 3 kg | 2.1 kg | 1,600 mm (63 in) |
| 1.5 Nm | 6 kg | 4.2 kg | 2,400 mm (94.5 in) |
| 2.0 Nm | 10 kg | 7.0 kg | 3,000 mm (118 in) |
| 3.0 Nm | 16 kg | 11.2 kg | 4,000 mm (157 in) |
| 5.0 Nm | 28 kg | 19.6 kg | 6,000 mm (236 in) |
Run three checks before you confirm a motor:
- Load check: total load from the formula is at or below the 70% ceiling.
- Width check: the shade width is within the class's recommended maximum.
- Tube check: the fabric weight is within what the tube can carry. In our tube standard, a 50 mm tube is limited to fabrics up to 350 g/m², so Example B's 450 g/m² fabric belongs on a 63 mm tube. Tube diameter follows the same "weight, not window size" logic as the motor. We cover it in why the wrong roller shade tube diameter costs you returns.
If any one of the three fails, go up a class or change the configuration. Don't argue with the numbers.
Where the formula stops being enough. The 0.3 kg resistance is a fixed empirical value. It doesn't scale with tube diameter or drop, so on very tall or unusual builds it can be off in either direction. We don't tell customers to trust the formula blindly at the edges. If your project sits near a class boundary (say 65–75% utilization) or uses an unusual fabric, send us the window dimensions and fabric specs and we'll run the numbers ourselves. We would rather do that than have you find out in the field. A much larger motor isn't automatically the answer either. We pick the smallest class that passes all three checks with margin.
What should you check after picking the right motor?
Correct torque solves half the problem, and limit settings cause much of the rest. In our warranty records, incorrect limit setting is the second most common reason for motor replacement.
The mechanism is simple. If the upper limit is set so the shade sits hard against the tube, the motor overloads for a fraction of a second at the end of every upward run. In our experience, the current peak at that moment is 2.5 to 3.5 times the rated value. One cycle does no damage. A thousand cycles will burn the winding. Our installation guidance sets the upper limit so the shade stops 8–12 mm short of the tube's outer diameter.
Limit setting deserves its own walkthrough, so I've kept it short here. The full step-by-step is in our guide to limit-setting mistakes that quietly burn out motors.
Key takeaways
- Size motors by total load, not by window width.
- Use: fabric area × fabric weight ÷ 1,000 + bottom rail weight + 0.3 kg.
- Keep actual load at or below 70% of the motor's rated maximum.
- Check load, width and tube capacity together.
- Recheck the calculation whenever the fabric changes.
Frequently asked questions
What size motor do I need for a 2.4 m wide roller shade?
It depends on total load, not width alone. Add fabric weight, bottom rail weight and 0.3 kg of mechanical resistance, then keep the result at or below 70% of the motor's rated load. A 2,400 mm shade with 280 g/m² fabric and a 180 g/m bottom rail comes to about 2.21 kg, which suits the 1.5 Nm class with a comfortable margin.
Why does my motorized roller shade motor keep burning out?
The two most common causes in our records are overload from sizing by window dimensions instead of fabric weight, and an upper limit set too tight against the tube. Both are setup errors rather than motor defects. Check the load calculation first, then the limit positions.
Can I use a 1.0 Nm motor on a 2.4 m wide shade?
Not under our sizing rules. The 1.0 Nm class is recommended up to 1,600 mm wide, and its 70% load ceiling is 2.1 kg. Even the lighter fabric in our example, at 2.21 kg, exceeds that ceiling on a 2.4 m shade.
What happens if I switch to a heavier fabric after the motor is chosen?
The load changes, so the selection must be rechecked. In our example, changing from 280 g/m² to 450 g/m² fabric raises the total load from 2.21 kg to 3.11 kg, which overloads a 1.0 Nm motor. Heavier fabric may also need a larger tube.
Building a smart-home project around motorized roller shades? Send us your window dimensions and fabric specs and we'll run the load calculation with you before you order. If you'd like to see the shades themselves, start with our motorized shades and roller shades pages.
