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Robot Autonomy, Classified
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Expectation vs reality/Robot Window Cleaners/4 robots compared/Reviewed Jun 23, 2026

What happens when a robot window cleaner loses suction or power?

5 claims tested1 holds only partly and 4 diverge from the record.
Mamibot iGLASSBOT W120-TView recordMamibotIIiGLASSBOT W120-T
HOBOT 388 UltrasonicView recordHOBOTII388 Ultrasonic
HUTT A1View recordHUTTIIIA1
HOBOT-S6 ProView recordHOBOTIIS6 Pro$850
Full specifications16 measurements · 45 of 64 documented
MSRP
$179
$299
$270
$850
Rated runtime
Not documented
20 min
Not documented
Not documented
Charge time
60 min
Not documented
Not documented
Not documented
Suction power
2,800 Pa
Not documented
6,000 Pa
Not documented
Noise level
65 dB
Not documented
Not documented
70 dB
Navigation
Strip-pattern auto-path with edge-detection sensors; RF remote manual mode.
Systematic path algorithm with edge detection; no LiDAR or camera mapping
Laser edge-detection pathing
Preprogrammed zigzag route within a pane, horizontal then vertical. ELB (Edge-Leakage-Bumper) sensing detects air leakage at seals and frameless edges plus physical frame contact. Operator places the robot and repositions it between panes.
Connectivity
bluetooth
wifi, bluetooth
Not documented
bluetooth
Dimensions (W×D×H)
9.8 × 9.8 × 3.9 in
5.83 × 11.61 × 3.74 in
Not documented
9.4 × 9.4 × 3.9 in
Weight
2.98 lbs
2.02 lbs
Not documented
3.6 lbs
Readiness
Ready Now
Ready Now
Ready Now
Ready Now
Released
Sep 2022
Nov 2021
Oct 2024
Jan 2024
Backup battery runtime
20 min
20 min
20 min
20 min
Min pane size
39.4 in
13 in
Not documented
Not documented
Power type
Mains-corded with backup battery
Mains-corded, 5 m cord
Mains-corded with backup battery
Mains-corded
Safety rope rating
Not documented
441 lb
326 lb
507 lb
Cleaning width
Not documented
Not documented
Not documented
10.2 in

Claims under test

5 marketing claims, measured against documented evidence

Each claim below pairs the marketing language with what owner reports, demos, and engineering documentation actually show. The meter summarizes how far each claim sits from the evidence.

0102030405
1 Partial4 Diverges
01On-board UPS holds the unit at power lossDiverges02Suction headroom above minimum gripPartial03Anti-fall re-grip after interruptionDiverges04Safety rope as last-resort backup onlyDiverges05Power cut triggers controlled parkDiverges
Claim 01 / 05

On-board UPS holds the unit at power loss

Diverges

No manufacturer publishes a UPS hold time; HOBOT-S6 Pro carries no battery at all

What marketing promises

An internal backup battery keeps the robot on the glass if mains power is interrupted during a cleaning cycle.

Implied by category marketing and retailer copy across window-robot listings; W120-T Amazon listing 2023-2024, HOBOT 388 retailer pages 2022-2024.

What the evidence shows

None of the four products have a documented UPS hold time published by the manufacturer. The Mamibot W120-T Robovations product records data records a 14.4Wh Li-ion battery alongside a mains-power-only designation, but manufacturer documentation classifies it as mains-only with no UPS mode stated. HOBOT 388 specifications list no battery at all.

W120-T UPS hold time
Not documented
HOBOT 388 battery capacity
Not listed in specs
HUTT A1 UPS hold time
Not documented
HOBOT-S6 Pro battery
Corded only, no battery
Claim 02 / 05

Suction headroom above minimum grip

Partial

Two of four publish Pa figures; no model discloses the minimum grip threshold needed to compute headroom

What marketing promises

Strong suction keeps the unit firmly on glass well above the minimum grip threshold throughout the cleaning cycle.

HUTT A1 manufacturer page, huttwisdom.com 2024; Mamibot W120-T product specification sheet 2022; HOBOT category marketing 2021-2024.

What the evidence shows

Only the HUTT A1 and Mamibot W120-T publish suction figures: 6000Pa and 2800Pa respectively. HOBOT 388 and HOBOT-S6 Pro do not disclose suction Pa values. Without a documented minimum-grip threshold from any manufacturer, the headroom above that threshold cannot be calculated from public data.

HUTT A1 suction (manufacturer)
6000Pa
W120-T suction (manufacturer)
2800Pa
HOBOT 388 suction
Not published
HOBOT-S6 Pro suction
Not published
Minimum grip threshold (any model)
Not disclosed
Claim 03 / 05

Anti-fall re-grip after interruption

Diverges

All four require owner remount after suction loss; no model documents autonomous re-grip

What marketing promises

If suction drops mid-cycle, the robot automatically re-grips and resumes the cleaning path without owner intervention.

Paraphrased from window-robot category copy on retail listings for HOBOT 388 and HUTT A1; Amazon and third-party retailer pages 2022-2024.

What the evidence shows

Owner reports and manufacturer documentation for all four models describe suction-loss events as requiring owner intervention: remounting the unit and restarting the cycle. No model documents an automatic re-grip subroutine. Edge-detection stalls on the HOBOT 388 can resume via app or button, but that is a path-stop recovery, not a suction-loss recovery.

W120-T suction-loss recovery
Owner remounts; restarts cycle
HOBOT 388 suction-loss recovery
Owner remounts; restarts cycle
HUTT A1 suction-loss recovery
Not documented; tether catches
HOBOT-S6 Pro suction-loss recovery
Owner repositions; restarts
Auto re-grip documented (any model)
None found
Claim 04 / 05

Safety rope as last-resort backup only

Diverges

All four classify the rope as mandatory; W120-T owner reports describe tether engagement as recurring, not rare

What marketing promises

The safety rope is a last-resort backup that rarely engages; the robot's suction is reliable enough that the tether is precautionary.

Mamibot W120-T safety documentation and Amazon listing framing 2022-2024; HOBOT safety documentation 2021-2024.

What the evidence shows

Manufacturer safety documentation for W120-T, HOBOT 388, and HOBOT-S6 Pro all classify the safety rope as mandatory, not optional. Owner reports for W120-T document tether activation on textured or frosted glass as a recurring, not rare, event. HUTT A1 and W120-T safety documentation warn that operating without the rope is documented misuse.

W120-T rope classification
Mandatory, not optional
HOBOT 388 rope classification
Mandatory, not optional
HOBOT-S6 Pro rope classification
Mandatory, not optional
W120-T tether activation frequency
Recurring on non-smooth glass (owner reports)
HUTT A1 rope classification
Required; weekly inspection mandated
Claim 05 / 05

Power cut triggers controlled park

Diverges

Edge detection handles normal path reversal, not power-loss parking; tether is the only documented stop

What marketing promises

When power is cut, the robot parks itself safely at the edge of the window before suction is lost.

Implied by edge-detection framing in HOBOT 388 manufacturer page and HUTT A1 retailer copy; hobot.com.tw 2021-2024, huttwisdom.com 2024.

What the evidence shows

No manufacturer documentation for any of the four products describes a powered-off controlled-park sequence. Edge detection governs path reversal during normal cleaning; it does not constitute a power-cut park routine. Documented failure behavior on suction loss is tether catch, not a positioned landing.

W120-T controlled-park on power loss
Not documented
HOBOT 388 controlled-park on power loss
Not documented
HUTT A1 controlled-park on power loss
Not documented
HOBOT-S6 Pro controlled-park on power loss
Not documented
Documented power-off behavior (any model)
Tether catch only

Per-product rollup

How each platform’s claims hold up

Every claim, resolved per product against documented evidence.

ClaimMamibot iGLASSBOT W120-THOBOT 388 UltrasonicHUTT A1HOBOT-S6 Pro
On-board UPS holds the unit at power lossNo UPS publishedNo battery listedNo UPS documentedCorded, no battery
Suction headroom above minimum grip2800Pa, no thresholdPa not published6000Pa, no thresholdPa not published
Anti-fall re-grip after interruptionOwner remountsOwner remountsTether catches onlyOwner repositions
Safety rope as last-resort backup onlyRope mandatoryRope mandatoryRope mandatoryRope mandatory
Power cut triggers controlled parkNo park routineNo park routineNo park routineNo park routine

Common questions

What readers ask about this comparison.

Q.
Do any of these window robots have a backup battery that holds the unit on glass if the cord is unplugged?
No manufacturer among these four publishes a UPS hold time or battery-as-suction-backup specification. Robovations records show the Mamibot W120-T carries a Li-ion battery cell, but manufacturer documentation classifies the unit as mains-only with no UPS mode stated. HOBOT-S6 Pro carries no battery at all per manufacturer specs.
Q.
What actually happens when suction fails on these robots?
Documented recovery across all four models is the same: the safety rope arrests the fall and the owner must remount the unit and restart the cleaning cycle. No model documents an automatic re-grip subroutine. Owner reports for the W120-T describe tether activation as a recurring event on textured or frosted glass, not a rare edge case.
Q.
Is the safety rope truly optional, or is it required?
Manufacturer safety documentation for W120-T, HOBOT 388, and HOBOT-S6 Pro all designate the rope as mandatory. Operating without it is classified as documented misuse in each product’s safety material. The HUTT A1 mandates weekly rope inspection, reinforcing that the tether is a structural safety requirement, not a precautionary accessory.
Q.
Which unit has the most suction headroom above the minimum grip threshold?
Only two of the four products publish suction figures: HUTT A1 at 6000Pa and Mamibot W120-T at 2800Pa, both per manufacturer specifications. HOBOT 388 and HOBOT-S6 Pro do not disclose suction Pa values. No manufacturer discloses the minimum grip threshold, so the headroom above that threshold cannot be calculated from available public data for any model.
Q.
Does the glass size or exposure change the risk profile before the tether is all that is left?
Larger panels increase cycle time, raising the probability of encountering a suction-disrupting surface condition such as residue film, micro-scratches, or surface irregularities. Exterior-facing glass introduces environmental variables including humidity variation and wind load. HUTT A1 and HOBOT-S6 Pro manufacturer documentation note that strong wind gusts are a documented suction-loss risk factor independent of suction rating.
Next up

What do robot window cleaners really cost to maintain?

Read the comparison
Comparison ID: RV–CMP–7509 · Last reviewed Jun 23, 2026 · Based on owner reports, manufacturer documentation, and firmware release notes