Hi emmy-lou
There are both mains operated (Petporte) and battery, but I would strongly recommend the battery operated Sureflap. They really are first rate. Sureflap have a real commitment to what they do, and customer care and service are their number one priority.
I've bought three of their flaps - one of the regular Sureflap catflaps (which we donated to a neighbour), one of the larger pet door types (because Ross was a big lad and struggled with the smaller sureflap) and one of the new dualscan flaps. They all work off batteries, and battery life is good.
The pet door has a curfew function, so you can set the door to lock during certain times - handy if you like to keep cats in overnight. The dualscan means you can elect to give certain cats unrestricted access to the outside whilst keeping others as indoor cats only. Moray is also growing into quite a large cat, and yet he manages the dualscan flap without difficulty.
All are simple to fit, and with the exception of the Pet Door (which is larger), will fit an aperture from the standard Staywell flap.
All of them can be used as fully manually operated flaps should you prefer . If you decide to buy one, I'd recommend buying direct from the Sureflap site, as you'll then be supplied with their most up to date model.
http://www.sureflap.co.uk/They're pricier than ordinary cat flaps, but they are worth every single darned penny. There are a few of us on Purrs who've bought these flaps and they don't disappoint. (Now there's a company I'd be proud to work for. If ever they wanted a north east rep, I'd be delighted to oblige
)