Do any of these window robots have a backup battery that holds the unit on glass if the cord is unplugged?

Yes, all four. Each record lists a 20-minute backup battery. The Mamibot iGLASSBOT W120-T's 600 mAh backup cell holds it on the glass if power is cut, and independent testing confirmed suction held through a simulated cut. The HOBOT 388 Ultrasonic stayed in place when its plug was pulled but did not resume on its own. The HUTT A1 pairs its 650 mAh pack with an audible and visual alarm, and the HOBOT-S6 Pro's battery holds it until someone reaches it.

What actually happens when suction fails on these robots?

On power loss, each robot's backup battery keeps suction for about 20 minutes so someone can reach it; the HOBOT 388 Ultrasonic stopped and stayed in place but did not resume. When the seal itself breaks, the HOBOT 388 record cites owner reports of mid-pane detachment caught by the safety rope. The Mamibot iGLASSBOT W120-T record cites owner reports of suction loss on textured, frosted or curved glass. No record describes a robot re-gripping on its own.

Is the safety rope truly optional, or is it required?

The records treat it as required. The Mamibot iGLASSBOT W120-T record lists a mandatory safety rope anchored to an indoor fixture before each session. HOBOT specifies tying the HOBOT 388 Ultrasonic's 4.5 m rope, rated to 441 lb, to an indoor fixture before each run. The HUTT A1 record calls its 326 lb rope the fall protection and requires inspection before every run. The HOBOT-S6 Pro record lists a safety rope rated to 507 lb.

Which unit has the most suction headroom above the minimum grip threshold?

Only the HUTT A1 record lists a suction figure, 6000 Pa. None of these records states a suction figure for the Mamibot iGLASSBOT W120-T, HOBOT 388 Ultrasonic or HOBOT-S6 Pro, or a minimum grip threshold for any model, so headroom cannot be compared from the records. What they do describe is where grip is lost: textured, frosted, curved or divided glass, and residue left on the pane or the suction ring.

Does the glass size or exposure change the risk profile before the tether is all that is left?

On these records, surface condition is the documented risk more than pane size. The HOBOT 388 Ultrasonic record cites owner reports of mid-pane detachment from residue or small surface irregularities, caught by the safety rope. The Mamibot iGLASSBOT W120-T record cites suction loss on textured, frosted or curved glass, and its manual prohibits frameless glass. For exterior panes, independent testing of the HOBOT-S6 Pro found strong wind gusts affect its sprayed liquid, and the W120-T routine includes cordoning off the ground below.

Comparisons · Robot Window Cleaners3 robots · checked 16 Oct 2026

Claims against testing

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

A corded window robot holds to the glass until power or suction fails. This page lists what each maker prints about the backup battery, the alarm, suction and the safety rope.

This page checks

Backup when power is cutMinutes the robot stays on the glass
What it signalsAlarm or lights during the backup
Suction and ropeFigures the maker prints
Where grip can failGlass the maker warns about

Mamibot

iGLASSBOT W120-T

IIAssistedLimited

$179list

Where to buy

Amazon

Mar 2024W120-T edge-detection false stops documented across Amazon owner reviews

HOBOT

388 Ultrasonic

IIAssistedStrong

$299list

Where to buy

Amazon

Sep 2023HOBOT 388 app firmware addresses 5 GHz Wi-Fi dropout in owner reports

HUTT

IIIConditionalCapable

$270list

Where to buy

Amazon

Apr 2025Owner reports document HUTT A1 edge detection limits on frameless glass

Full specifications14 figures on record · 10 differ

Figure

iGLASSBOT W120-T

388 Ultrasonic

List price

iGLASSBOT W120-T$179

388 Ultrasonic$299

A1$270

Released

iGLASSBOT W120-TSep 2022

388 UltrasonicNov 2021

A1Oct 2024

Power

iGLASSBOT W120-TMains-corded with backup battery

388 UltrasonicMains-corded, 5 m cord

A1Mains-corded with backup battery

Smallest pane

iGLASSBOT W120-T39.4 in

388 Ultrasonic13 in

A1Not on record

Safety rope

iGLASSBOT W120-TNot on record

388 Ultrasonic441 lb

A1326 lb

Weight

iGLASSBOT W120-T2.98 lb

388 Ultrasonic2.02 lb

A14.2 lb

Charge time

iGLASSBOT W120-T60 min

388 UltrasonicNot on record

A1Not on record

Battery type

iGLASSBOT W120-TNot on record

388 UltrasonicNot on record

A1Li-ion

Adhesion

iGLASSBOT W120-T2,800 Pa

388 UltrasonicNot on record

A16,000 Pa

Noise

iGLASSBOT W120-T65 dB

388 UltrasonicNot on record

A165 dB

Navigation

iGLASSBOT W120-TStrip-pattern auto-path with edge-detection sensors; RF remote manual mode.

388 UltrasonicSystematic path algorithm with edge detection; no LiDAR or camera mapping

A1Laser edge-detection pathing

Connectivity

iGLASSBOT W120-TBluetooth

388 UltrasonicWi-Fi, Bluetooth

A1Not on record

Size

iGLASSBOT W120-T9.8 × 9.8 × 3.9 in

388 Ultrasonic5.83 × 11.6 × 3.74 in

A110.3 × 10.3 in

The same on all three

Backup hold

iGLASSBOT W120-T20 min

388 Ultrasonic20 min

A120 min

Add a robot20 more robot window cleaners on record + Add a robot20 more robot window cleaners on record

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If the grip goes

What each maker says holds the robot up

Two makers print a 20-minute backup, and a hands-on review gives the same for the Mamibot. No test with the plug pulled was found for any of the three. Only the HUTT A1 page prints a suction figure (6,000 Pa peak) and a rope rating (326 lb).

MamibotHOBOTHUTT

iGLASSBOT W120-T

388 Ultrasonic

Backup when power is cut

Minutes it stays on the glass, as printed

iGLASSBOT W120-T: 20minA review says the built-in battery holds suction for up to 20 minutes. Its manual prints 600 mAh and no minutes.Hands-on review, Jan 2023
388 Ultrasonic: 20minProduct page: embedded UPS keeps it in position for 20 minutes.HOBOT US product page
A1: 20minPage lists "20-min Power-off Protection". Backup battery size not on the page.HUTT A1 product page

What it signals

Alarm or lights while on backup

iGLASSBOT W120-T: BeepsA review describes an audible beeping to warn of the power loss.Hands-on review, Jan 2023
388 Ultrasonic: Alert soundProduct page says the unit stays in place "with alerting sound".HOBOT US product page
A1: Alarm listedPage lists an alarm in emergencies. It does not say the alarm sounds during backup.HUTT A1 product page

Suction and rope figures

What the maker's own page or manual prints

iGLASSBOT W120-T: Not foundManual gives no suction figure and no rope rating. It says to tie the rope to an indoor fixture.Maker manual
388 Ultrasonic: Not foundProduct page gives no suction or rope figure. The maker's Taiwan page, which may hold them, did not load.HOBOT US product page
A1: 6,000 Papeak suctionRope rated to withstand 326 lb of pull. Page also lists 0.03 s real-time pressure compensation.HUTT A1 product page

iGLASSBOT W120-T Hands-on review, Jan 2023 · Maker manual388 Ultrasonic HOBOT US product pageA1 HUTT A1 product page

Same on all three: no hands-on test with the plug pulled was found, so no independent figure exists here for how long any of them stays on the glass.

Inside a living room, a black oval window-cleaning robot holds still on the glass on its backup battery while its mains plug hangs half out of the wall socket beside the window board.

Your home

How each one fits a home like yours

Set the conditions that describe your home. Each robot answers from its own Fit Check; nothing is scored until you choose.

Fit check

Describe your home

Pick what applies. Each robot answers from its own record.

Window style

Frame style

Building height

Across what you setEach robot’s own Fit Check

iGLASSBOT W120-TSet a condition

388 UltrasonicSet a condition

A1Set a condition

Window styleNot set

iGLASSBOT W120-TNot set

388 UltrasonicNot set

A1Not set

Frame styleNot set

iGLASSBOT W120-TNot set

388 UltrasonicNot set

A1Not set

Building heightNot set

iGLASSBOT W120-TNot set

388 UltrasonicNot set

A1Not set

Everything else

Where they land among 27 robot window cleaners

Only the measures on which these three differ. The bar is the whole category; the block is the typical middle half; the tick is the median.

WeightThe robot alone

iGLASSBOT W120-T

3 lb

388 Ultrasonic

2 lb

4.2 lb

2typical 2.8–4 · median 3.44.8

List priceThe maker's list

iGLASSBOT W120-T

$179

388 Ultrasonic

$299

$270

$150typical $249–$500 · median $399$850

Sources

Where every figure comes from

iGLASSBOT W120-T · Mamibot W120-T manual (manualslib.com); mamibotusa.com/w120-t; Cleanup Expert review (updated 7 Jan 2023)
388 Ultrasonic · hobot.us/pages/hobot-388 (hobot.com.tw did not load)
A1 · huttwisdom.com/pages/hutt-a1; Window Robot Lab review (read for a power-cut test, none found)

Every figure was read on a maker page or manual on 2 Oct 2026, or on a dated review where marked. A blank means the page opened did not print the figure.

Questions

What readers ask

Does a backup battery mean the robot can run on it?

No. The pages and manuals describe the battery as a hold for about 20 minutes with an alert, long enough to remove the unit or restore power.

Why do some cells show Not found?

The maker pages and manuals opened for these models did not print a suction figure, rope rating or glass warning in those cells. The HOBOT Taiwan site did not load.

Is the safety rope optional?

The Mamibot manual says to fix the rope and tie it to an indoor fixture before use. A review of the HUTT A1 says to use it every time above the first floor.

Questions

What readers ask about this comparison.

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

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