A cordless window cleaning robot in 2026 typically runs 25-45 minutes per charge, cleaning 15-35 m² of glass in two passes. The battery also has to hold the vacuum if power fails, which is why runtime claims are optimistic. Above about 40 m² of glass per session, a corded unit remains the better tool.
A cordless window cleaning robot sounds like the obvious upgrade. No cable dragging across a wet roof, no socket hunt, no cable-management risk on a 30 m drop. All real advantages. The trade-off is the one number sellers round up.
Why is stated runtime shorter in practice?
Because the same pack does two jobs. It drives the tracks, runs the pump, and holds enough reserve to keep the vacuum alive for several minutes if the machine loses its footing logic or a connection. That reserve is not optional, and it is drawn from the same cells.
Manufacturers quote runtime on a clean, dry, vertical pane at moderate temperature. Add a wet surface with more track slip and more pump work, add 5 degrees of extra incline, add a cold morning where lithium cells lose a chunk of their capacity, and 45 minutes becomes 28. That is normal, not a defect.
How much glass will one charge cover?
| Battery class | Stated runtime | Real, wet + incline | Glass per charge (2 passes) |
|---|---|---|---|
| Entry (~2,000 mAh) | 20-25 min | 14-18 min | 8-12 m² |
| Mid (~4,000 mAh) | 30-40 min | 22-30 min | 15-22 m² |
| Large (~6,000 mAh) | 45-60 min | 32-45 min | 22-35 m² |
Two observations. First, the glass-per-charge figure assumes the robot can complete a bay without repositioning; in practice you lose time to carrying it between panes. Second, battery capacity degrades. Expect 70-80% of original capacity after two years of weekly commercial use, so spec the machine for the area you will have in year three, not year one.
Is cordless safer, or does it just feel safer?
Mixed. Removing a trailing cable removes a genuine trip and snag hazard, especially on a roof where someone is walking behind the machine. That is a real safety gain and it is the strongest argument for cordless.
But a corded unit can draw unlimited hold power from the mains, and it fails to a powered state rather than a battery state. There is also a maintenance angle nobody mentions: a corded machine has no pack to age, so its hold reserve in year three is the same as in year one. A cordless machine slowly loses the one safety margin that keeps it on the glass. A cordless machine that dies mid-climb depends entirely on its reserve and its tether. So cordless shifts risk from “cable management” to “battery state management”. Whether that is better depends on whether your team actually checks charge level before every climb. Most do not.
When is a cord the better choice?
Long runs on fixed glass. An atrium with 300 m² of skylight and a socket at the plant room is a corded job, no question. Same for a ground-floor shopfront rotation where the robot works for hours. The cable is annoying once and then it is just part of the routine.
Cordless wins when the glass is scattered: six dormers, a stairwell window, a roof light you reach from a ladder. Setup time dominates there, and not running a cable between stops saves more minutes than the shorter runtime costs.
What should you check before buying cordless?
- Number of packs included – one pack is a half-product on anything above 20 m²
- Charge time – 3-4 hours is typical; swapping is faster than waiting
- Hold reserve spec – minutes of vacuum hold at empty, not “safety battery included”
- Cell chemistry and replacement price – a 60-120 USD pack every two years is a running cost
That last item is where cheap units fall apart. If the battery is not user-replaceable, the machine has a fixed service life of about three years regardless of how well the rest is built.
Who should skip cordless entirely?
Anyone cleaning more than 50 m² per session, anyone working a pitched glass roof where repositioning is slow and expensive, and anyone whose crew will not reliably swap and charge packs. In all three cases the cable is less trouble than the battery discipline.
For spec comparisons across both types, our skylight robot review archive lists runtime as measured rather than claimed, and the ECOVACS WINBOT W3 review covers the corded-versus-cordless trade-off on a popular model in detail.
Why does a cordless machine fail at the worst moment?
Because lithium packs do not fail linearly. The voltage curve stays flat for most of the discharge and then drops sharply in the last few minutes. A machine showing three bars can go from comfortable to reserve-only in under two minutes, and if that happens halfway up a 6 m pane, the reserve is what is holding it. This is why the honest advice for cordless work is to treat the last quarter of the pack as unavailable and plan the bay pattern around it.
Two habits fix most of the problem. Swap packs at 50% rather than waiting for the warning, and always start a climb from the bottom of the pane so a low-battery finish happens near the ground rather than at the top.
Key Takeaways
- Expect 25-45 minutes real runtime and 15-35 m² per charge, not the brochure figure.
- Cold weather and wet glass can cut runtime by a third.
- Cordless removes cable hazards but adds battery-state risk; check charge before every climb.
- Over 40-50 m² per session, use a corded unit and stop fighting the pack.