help on designing a 12v UPS?

we have a problem with kids coming into our garden, vandalising our plants, breaking into our shed etc.
we have decided that installing a cctv system in our home is a good idea as even if it doesn't stop them coming in, it will be solid evidence to take to their parents.
problem is, we have regular power cuts and i would like the cctv to be recording 24/7/365 (especially in the shed because that is where our bikes get stored) i have two 12v 7ah SLA batteries that i am donating to the project, iknow that they need 13.8v and around 500ma to charge.
i need 16 camera outputs, and then a couple of spare outputs for aux devices (router to keep it connected to the internet, mics, infra red lamps etc)
the dvr its self wants 12v 5 amps input so i think for that i will make another psu just for that with maybe 2 or 3 12v 7ah sla batteries.
i have baisic electronics knowledge, have access to pcb printing facilities and can use eagle.
any help at all would be amazing, the main things i need to know is: how to get 13.8v from 240v ac, how to power all the cameras from 240v ac and have a smooth switchover from mains to battery.
thanks in advance - luke

the answers 12v UPS ! http://www.eleccircuit.com/simple-ups-12v/ !!
Mini UPS system circuit diagram http://www.2electronic.com/mini-ups-system/ BEST

Re-design6 years ago
Just go to a used computer store and get a computer ups and power your camera with a wall wart.

Designing and building your own will cost more money and might not work when you need it to.
ljarrald1 (author)  Re-design6 years ago
a ups would be the worst option, because it uses a 12v battery inside and then inverts the power to 240vac, then with a "wall wart" it would be converted back down to dc 12v. also ups's only last for around 20 mins