Battery-powered skylight robots run 3-5 hours per charge and suit roofs under 2,000 m2; tethered units run continuously and suit large flat roofs, but the cable limits turning radius and adds a trip-hazard check on glass.
This choice gets buried under suction and speed talk, yet it decides how your crew’s day is shaped. A battery machine means carrying spare packs and a charging station near the roof access. A tether means one long power line and a second person watching the cable.
How long does a skylight robot battery actually last?
Real numbers, not brochure numbers. The Lingyun Y3 runs about 4 hours at mixed speed with the pump active, covering 700-900 m2 before a swap. The heavier Lingkong K3 stretches to 5 hours on a 48 V pack because its drive motors are more efficient on steep pitches. Cold weather cuts both figures by 20-25 percent below 5 degrees Celsius.
| Power type | Runtime | Coverage per charge/day | Best for |
|---|---|---|---|
| Battery (Y3) | 3.5-4 h | 700-900 m2 | Roofs under 2,000 m2, awkward shapes |
| Battery (K3) | 4.5-5 h | 900-1,100 m2 | Steep pitches, split roofs |
| Tether | Continuous | 2,000+ m2 per shift | Large flat atria, hangars, airports |
Which is safer on glass?
A tether removes the dead-battery risk. If a battery robot loses charge mid-panel on a 30 degree roof, suction holds it, but the recovery drill is slow – you need to walk it down manually or winch it. Tether units keep suction at full power until you switch them off. For roofs above 12 m with no walkway access, that reliability is worth the cable.
The trade-off is mechanical. A dragging cable can snag on frame upstands, and over years it scuffs the glass where it touches. Route it along a rail or use a cable guide. Untended cables on public-facing atria are also a fall hazard for anyone on the roof; plan the layout before commissioning.
What does the power choice cost?
Battery packs are the hidden line. A 48 V pack rated for 1,200 cycles costs roughly 600-900 USD to replace, and a heavy schedule consumes one to two packs a year for each machine. Tether units carry no pack cost but need a 16 A supply at roof level, which existing plant rooms often cannot give without an upgrade.
Rough payback: on a 1,500 m2 roof cleaned every 6 weeks, battery wins because you never touch the electrical infrastructure. Above 4,000 m2, tether wins because run time stops being the bottleneck. The crossover sits around 2,000-2,500 m2.
Who should avoid battery machines?
Sites with no safe roof access for a battery swap, and buildings where a mid-panel stop would block a public atrium. Also, operators who will not manage charge discipline. A flat pack on a hot glass roof in summer is the most common field failure we hear about, and it is entirely preventable with a rotation of two packs and a rule of never starting a pass below 30 percent.
Hybrid setups are appearing in 2026 – battery drive with an optional tether for long shifts. That flexibility costs more upfront but removes the 2,500 m2 gamble. Compare it against your fleet sizing plan before deciding.
Which power type suits a split or multi-level roof?
Battery, almost always. Split roofs force the machine to travel between levels, and a tether either cannot follow or drags across a change in plane, which risks snagging on an upstand. Move a battery unit between zones and you never think about the cable again. The extra pack cost is far less than the time lost untangling a line.
Tether still wins on one large uninterrupted plane – a hangar, a warehouse northlight roof, an airport concourse. There, the machine never leaves the panel, and continuous power means it can run a full shift without a swap. Match the power type to the geometry, not to the brochure.
Key Takeaways
- Battery: 3.5-5 h per charge, 700-1,100 m2. Tether: continuous, 2,000+ m2 per shift.
- The cost crossover is around 2,000-2,500 m2 of roof.
- Cold below 5 degrees Celsius cuts battery time by 20-25 percent.
- Tether is safer for high roofs, but route the cable to avoid scuffs and trips.
- Rules out battery if a mid-panel stop would block a public space.
Not sure which fits your building? Send us the roof layout and we will recommend a power setup.
How does the tether affect route planning?
A tether changes how you plan the run. Work from the far edge back toward the power point so the cable trails behind rather than crossing fresh glass. Add a cable guide at each direction change above 90 degrees, and keep the line off any drainage channel where water pools. A loose cable in a channel becomes a wet extension of the machine and can snag.
Battery machines plan differently: you map zones to a single pack’s range and place the charging spot where the last pass ends, not where it is convenient. On a 1,200 m2 office roof, the Y3 does two zones per pack, so the charging point sits at the roof access door and the run always finishes there.
What does a battery swap really take?
About 4-6 minutes on a hot-swap design, 20-30 minutes if the pack is internal and the machine must come off the glass. That difference decides whether two packs cover a full day. Ask before buying. A site that assumed hot-swap and got an internal pack quietly loses 90 minutes a day, which is a fifth of a shift.
Charging at roof level is worth the electrical work if you clean more than twice a month. A 48 V 20 A charger draws under 1 kW, so a standard circuit handles it. Running packs down to the van to charge wastes more labour over a year than the socket costs to fit.