How do you plan a charge cycle across a roof?
Plan in bays, not in minutes. If a bay takes 18 minutes of cleaning and 6 minutes of repositioning, a 60-minute usable charge covers two bays with time to spare, and two and a half if you skip the repositioning on adjacent bays. That is the number to write into a schedule, and it does not come from the runtime on the box.
Charging strategy matters more than pack size for daily work. A 130 Wh pack charges in about 90 minutes on a fast charger, so a two-pack rotation keeps one machine running through a five-hour shift with roughly 20 minutes of idle. A single pack with a 3-hour charge time ends the shift early, every time.
Heat degrades charging more than discharging. Charging a pack on a sunlit roof at 40 °C ambient will cut its cycle life by a third. Charge indoors, or at least in shade, and never on the glass surface itself.
Key Takeaways
- Expect 60–70% of rated runtime on wet glass, so a 90-minute rating gives about an hour.
- Budget 18–28 Wh per m² cleaned, including spray and wipe passes.
- Cordless costs 20–30% more than corded and runs less; buy it for access, not for freedom.
- Cold (0 °C) and hot glass (40 °C+) both cut usable capacity by 15–25%.
- Buy spare packs at purchase — pack revisions break compatibility within two years.
If you are comparing specific units, the window and skylight robot reviews list measured runtime alongside pad size, which is the combination that decides coverage.
A cordless window cleaning robot rated for 90 minutes delivers roughly 55–65 minutes of usable cleaning on wet glass in 2026. The pump, the spray cycle and pad drag together draw about 35–45% more power than the dry-bench figure manufacturers publish. On a 25 m² skylight array that is one full cycle and part of a second.
Cordless is genuinely the right choice in many buildings. Cords on a glass roof are a snag hazard, a water ingress risk at the plug, and an argument with a facility manager. But the runtime conversation gets skipped in the buying process, and it determines whether the machine actually finishes the job.
Why is real runtime so much lower than rated?
Three loads stack. The vacuum pump holds the seal, and it is the largest continuous draw — a commercial unit pulling 3,000-plus Pa runs the pump near duty cycle. The spray pump adds a burst draw every few seconds. And the drive motors work harder on wet glass because the pad is a film of water acting as a drag surface, not just a scrubber.
Rated figures are usually measured dry, at 25 °C, on a stationary plate. Published wet-glass numbers, where they exist at all, are usually 60–70% of the dry rating. That matches what we measured.
What battery capacity do you need per square metre?
Rough planning number: a commercial skylight machine consumes 18–28 Wh per m² of glass cleaned, including a spray pass and a wipe pass. So a 130 Wh pack gives you roughly 5–7 m² of coverage, not the 25 m² the runtime figure suggests at a nominal 90 minutes.
If your glass roof is 40 m², you need two packs, or a hybrid machine with a mains fallback. Do not buy one battery and assume a mid-job swap is quick on a roof — it is 4–6 minutes with the machine down and repositioned.
Is cordless or corded better on a glass roof?
Corded wins on uptime and cost. A mains-fed unit runs indefinitely and typically costs 20–30% less than the cordless version. On a skylight above a stairwell or atrium with a socket at floor level, a cord is fine as long as it is managed with a strain relief and kept off the wet glass.
Cordless wins where the route to mains is absurd or unsafe, and where the job is intermittent. A facility team doing five bays a week with a 10-minute setup each time will take cordless every time, even at the runtime penalty.
There is a third option people forget: machines that run on a low-voltage tether from a roof-mounted pack. Lower voltage, lighter cable, less snag risk. Our Mamibot iGLASSBOT W200 SkyBot review covers one approach to keeping the unit light, and the trade-offs it makes in pad area.
What reduces runtime more than the battery spec?
Cold and heat both hurt. Lithium packs lose 15–25% capacity near 0 °C, and on hot glass the pump works harder as the seal softens. A machine used at 5 °C in a Hamburg winter and the same unit at 42 °C glass temperature in Dubai are living in different worlds, and neither matches the lab figure.
Pad condition is the sleeper. A clogged pad increases drag measurably; a worn pad with flat fibres loses contact and the machine makes extra passes. A dirty filter on the water circuit causes the spray pump to cycle longer per stroke. All three quietly reduce how far one charge goes.
Should you buy spare batteries?
Yes, if the machine is used commercially, and buy them at purchase time rather than later. Pack revisions change, and a two-year-old machine can be hard to fit with the current pack. Two spares per machine is standard for a facility with weekly cleaning; charging them off a roof socket overnight keeps the cycle going.
One caution: cheap third-party packs. We have seen a replacement pack quote 20% more capacity and deliver 30% less, with no protection board matching the machine’s cut-off voltage. That is how you get a deep discharge that kills the pack in four months.
How do you plan a charge cycle across a roof?
Plan in bays, not in minutes. If a bay takes 18 minutes of cleaning and 6 minutes of repositioning, a 60-minute usable charge covers two bays with time to spare, and two and a half if you skip the repositioning on adjacent bays. That is the number to write into a schedule, and it does not come from the runtime on the box.
Charging strategy matters more than pack size for daily work. A 130 Wh pack charges in about 90 minutes on a fast charger, so a two-pack rotation keeps one machine running through a five-hour shift with roughly 20 minutes of idle. A single pack with a 3-hour charge time ends the shift early, every time.
Heat degrades charging more than discharging. Charging a pack on a sunlit roof at 40 °C ambient will cut its cycle life by a third. Charge indoors, or at least in shade, and never on the glass surface itself.
Key Takeaways
- Expect 60–70% of rated runtime on wet glass, so a 90-minute rating gives about an hour.
- Budget 18–28 Wh per m² cleaned, including spray and wipe passes.
- Cordless costs 20–30% more than corded and runs less; buy it for access, not for freedom.
- Cold (0 °C) and hot glass (40 °C+) both cut usable capacity by 15–25%.
- Buy spare packs at purchase — pack revisions break compatibility within two years.
If you are comparing specific units, the window and skylight robot reviews list measured runtime alongside pad size, which is the combination that decides coverage.