The answer
In most cases an electric opening rooflight simply stops where it is. A closed vent stays shut and stays weathertight. An open vent stays open, and the rain sensor cannot close it without power. Whether you can close it by hand or on battery depends on the actuator fitted, so ask before you buy. A rooflight with a battery backup or a manual release gives you a way to shut it in a storm.
The usual behaviour: it holds position
Electric rooflights use a motor driving a chain, spindle or pair of actuators. Those are designed to hold their position when the motor is not running. That is deliberate, because a vent should not drift open or shut by itself.
So when the power fails, nothing dramatic happens. A vent that was closed stays closed. The seal sits in place and the rooflight keeps out water as it did before. A vent that was open stays open, at the same angle, until power returns or you move it another way.
This is why the starting position matters. If you leave an electric vent open and a cut happens later, the rain sensor cannot do its job, because the sensor and the motor both need power. The sensible habit is to close the rooflight when you leave the house or go to bed, as you would a window.
Power cuts are usually brief, and most people never notice the effect on a rooflight. But a long cut during a wet spell, with the vent left wide open, is the one case that causes trouble. The planning below is for that case.
Some systems also lose their stored settings, such as timers or preset positions, when power drops. They come back to a default after a restart. It is worth checking them after a cut, as the controls guide explains.
Ways to close it without mains power
There are three common routes, and which applies depends on the unit.
Manual release or override. Some actuators let you disengage the drive and close the vent by hand, or turn a spindle with a tool, usually from inside the room. This is the simplest fallback, because it needs nothing but the right tool and a clear view of the unit. Ask where the release is and how it works before the rooflight is fitted, not during a storm.
Battery backup. Some control systems include a rechargeable battery that runs the motor for a limited number of cycles. When mains fails, the battery takes over and the vent can still close. The battery has a finite life, and the unit should signal when it needs attention, so it is something to check on the same autumn routine as the sensors.
Low-voltage design. Many electric rooflights run their motors from a low-voltage supply fed by a transformer. In some systems, a small backup supply can be attached to the low-voltage side. This is a product-specific option, so the maker's data sheet is the place to confirm it.
If a unit has none of these, the fallback is to reach the vent physically, which for a high rooflight is awkward or unsafe. That is the case for choosing a unit with a proper release, or for choosing carefully where an opening goes. How reach drives that choice is covered in choosing electric or manual.
Planning for a cut before the rooflight goes in
The best time to deal with power cuts is at the survey. Three questions settle most of it.
First, how high is the vent, and can you safely reach it? If the opening is above a stairwell, a worktop or a double-height space, you cannot get to it in an emergency. In that case a battery backup or a dependable manual release is close to essential.
Second, what happens in the room if the vent stays open? A rooflight over a hard-floored utility room tolerates a wet spell better than one over a carpeted lounge or a bedroom. Know the consequences and decide how much protection you want.
Third, how often are you away from the house? If the rooflight is usually closed when you go out, the risk is low. If you like to leave it ajar for background ventilation, backup matters more.
We write the answers into the quote, so you know what is included. The quote is fixed and in writing after the survey, and it names the unit and its controls, so you can compare a unit with a manual release against one with a battery.
A tidy habit also helps. Many homes keep a card with the release method taped inside a cupboard near the room. When a cut comes, the person in the house is not the person who was at the survey, and a card saves a search for instructions.
After the power comes back
When mains returns, most units wake up in a known state. Check them rather than assume.
Look at the vent position first. If you closed it by hand using a release, the motor may need to be re-engaged before it will run again. Follow the maker's instructions for this; forcing a drive that is still disengaged can damage it.
Test the open and close cycle once, with the room clear, and watch the vent seat against its seal. Then check the settings. Timers may need resetting, preset positions may have gone, and a handset may need re-pairing.
If the rooflight was open during rain, look for water on the sill, the reveal and the floor. A little moisture on the frame will dry. Water that has run into the ceiling lining, or a stain that appears later, is a reason to let us know. For a unit we fitted, tell us what happened, and the 10-year workmanship guarantee, which is in writing, applies to our installation work.
If power cuts are frequent where you live, and you want a rooflight regardless, tell us at the survey. We can plan the controls and the release point around it. What we fit is on the main electric opening rooflights page, and the survey is free.
What a power cut does not change
It is easy to worry about the wrong thing. A cut does not damage a closed rooflight, it does not weaken the seal and it does not affect the glass. The weathering of the unit, the upstand and the flashings are all passive. They do their job without power.
It also does not turn an electric rooflight into a hazard. The vent stays put, so there is no sudden movement when the power returns, except where a unit is set to run a stored command. In that case you hear it start and can stop it.
The thermal performance of the rooflight is likewise unchanged. Heat loss and condensation depend on the glazing and frame, and in a grey, cold month the difference between a power-off and power-on rooflight is nil.
What a cut does change is your ability to react. Without power you cannot vent a damp room, close a vent from a switch or rely on a sensor. For that reason, think of power cuts as a question of control, not of safety, and choose a unit that gives you a way to shut it. That is a reasonable request to make at the survey and a normal question for the people fitting the unit.