I know a thing or two about security and can say there's nowt better than a big chain and proper lock. Thick chain is good but heavy however works anywhere. Chain your bike to something or another bike at least, if nothing else round back wheel and frame. keep padlocks off the floor.
A bike alarm is great and I would suggest getting one, however be familiar with the extra wiring if it goes wrong andyou need to get rid of it. Ive been left immobilised due to this!
My present bike has no alarm but I put a £5 rape alarm under the seat and attach the pull out pin to the back spokes(use fishing wire so they can't see), anyone tries to wheel it away I can hear 100%
Alarm and batteries are only an issue if you leave the bke stood for a while, a battery will not drain over a week.