Smoke and CO Detecters - System Alert

All sensors work without monitoring, but the only way you know they've gone off is if you're in the house when they go off and the base station (and external siren if you buy that) go off.  Some sensors won't trigger the external siren based on what I've read, but I haven't done extensive testing of mine.

The keypad will tell you what has gone off.  In addition, environmental sensors trigger a different sounding siren from the base station than a entry/motion sensor.  

Carbon monoxide detectors should be installed in living spaces, not in the basement near a furnace or water heater.  You'll find that info in the installation guides, which you can read before you buy.  https://simplisafe.com/manual

While you can install a smoke detector in the basement, you'll also want them outside living areas.  They are interconnected, so when one goes off they all go off.  

If you didn't know already, having central station monitoring qualifies you for discounts from your insurance company, one for burglary/fire, and perhaps others for water, and for freeze.  Depending on what you pay for homeowner's insurance, the resulting discount may cover most/all of the monitoring fee ($14.99 one, not $24.99 one).

Thanks for the information, I didn't realize they were interconnected, that's good to know. I plan on getting the smoke for the basement once it's finished, then look at replacing all of my alarms with SimpliSafe one's.

Last question: are the smoke/CO detectors all battery only, or can they be hard-wired?

They are designed to be battery only.  However, if you are technically competent, you could jury rig up a hard wired power supply, but in that case, you would lose the battery powered function in case the power goes out.  If you are really technically competent, you  could probably rig up a solution which would power it from hard wire with battery backup.