I'm very pleased with this fan. Xpelair LV100PIR Toilet/Bathroom Low Voltage Extractor Fan with Pir Sensor
I'd not used one before, but we really needed a fan that would come on whenever the room was used, rather than being linked to the light switch. In other words, a fan which would come on in daylight without having to switch the light on.
The installation guide was adequate, covering all the key points.
It's a 12V device and comes supplied with a suitable transformer, which I located in the roof space above the bathroom. The power is supplied via a conventional 13 amp plug with a 3A fuse.
Installation in my case required a 107mm diameter hole to be drilled through an external wall, lined with a 100mm plastic tube and then the supplied anti-back draught vent fitted outside, some 5m up. I was perhaps fortunate to have a large SDS drill and ladder, so only needed to hire the hole cutter from the local builders merchant.
Ideally the PIR sensor (i.e. the fan) needs to be positioned so it covers movement anywhere in the room, preferably facing the door, but this is not always possible. However, the detection zone is good, covering most of the bathroom even it's fitted in a corner.
The length of time the fan runs for after the last detected movement is adjustable - after some trial and error, I now have the micro-potentiometer set to about 5 minutes. It does switch off if someone is in the shower for longer than 5 minutes, but it switches back on as soon as they open the shower door and step out.
The fan is not particularly noisy but it is noticeable. Visitors have commented that it's a bit spooky initially as the fan goes on automatically a few moments after entering without them having consciously switched it on.