battery isolator matching

i have been searching this subject for days now. i have been able to find how to hook an isolator up, but how do i know what size isolator i need? i have a 95 nissan pickup 4x4. i am installing a second battery to run all of my accessories. ie....lights and siren, stereo, and multipule scene lights. i have considered upgrading my alternator, however that is very costly. the second battery that i have is a deep cycle gel. it has a phenomonal amp hour rating, i'm not positive of what it is right now. this is not your regular everyday gel cell battery either. my thing is, sometimes i leave my engine running with accessories on, however i would like to have the option of running some of my gear without the engine running.

any suggestions/directions would be great. to match up an isolator, does it go by what the second battery has to supply in amps, or does it go by what comes into the second battery amp wise. i'm considering a 90 amp isolator.

the majority of my warning lights are led, so amp draw shouldn't be a significant issue.  thanks a lot, i am enjoying taking in all of the knowledge on this site.  thanks.

ok, so if i decided to run my amp off my second battery, and let's say my amp draws 60 amps, i would obviously have 30 amps to play with right? (if i use a 90 amp isolator) my amp has 2- 30 amp fuses in it. an average siren would draw say 15 amps running. that would leave me with 15 amps to "play with". so as long as all of my other accessories do not draw more than 15 amps i should be safe?

now say, when i'm running lights and siren to a call, with my stereo on, but not "pounding" obviously, the amp would draw a lot less amperage correct?

i'm thinking that a 90 amp isolator could be basically the smallest i could run.

Your probably right on that if all your electronics draw that much. does your siren really draw 15? that seems like alot to me for a siren.

as for the amp with 2 30 amp fuses, 60 amps is the most  it can possibly draw, and, keep in mind that is only for a short period of time. you are also correct that the less volume your amplifier is outputing, the less current it draws.

hmmmmm. i'm going to be switching to a unitrol 9000 siren. according to federal, they suggest a 20amp fuse to be used.

so i guess that as long as i don't run everything at once, i should be ok.

thanks a lot for the advice. isolators aren't cheap, and with me having another baby on the way, i have to play mr. cheap skate. i just don't want to get burned. or burnt up!!