One system can have up to 100 sensors connected at a time. SimpliSafe even offers extra panic buttons (there's already one on the keypad), which you can press to trigger the alarm and dispatch police. You can add on tons of other sensors, like glass break sensors, which pick up the sound of smashed windows, carbon monoxide and smoke detectors, as well as temperature sensors to alert you if indoor temperatures drop below 41 degrees Fahrenheit (so you can take action to keep water pipes from freezing). In addition to the base station and keypad that come in every package, it includes three entry sensors and a motion sensor. SimpliSafe has bundles you can choose from with different types of sensors and hardware, but I suggest starting with the Essentials package ($259) if you're overwhelmed. There's another $25 per month plan that adds a few more perks, but it's geared toward people using SimpliCam video cameras (more on that later). They'll try to call you, and if you need help or don't respond they can choose to dispatch police, fire trucks, or ambulances to your home. Using SimpliSafe is free, but you can choose to pay $15 a month for monitored security, which gets you access to the company's monitoring centers operated by real people 24/7. Its batteries will last up to 24 hours and will recharge once the power comes back on. The base station also has replaceable batteries plus a cellular connection, so if the power goes out or you lose your Wi-Fi, the system will keep working. Everything else uses replaceable batteries, and those should last a few years. I like that there are no wires to run anywhere, except for the base station that plugs into the wall. When you move, it's easy to detach the sensors and take them with you. If you need more sensors, you can buy the ones you want, stick them to the wall, and press the pairing button. In a room with many windows, it's easier and cheaper to use a motion sensor.

Entry sensors work for doors and windows. Adding the sensors is just like setting up the keypad, and if you're not sure exactly where to place them, the included installation booklet has some helpful suggestions.

Simplisafe monitoring center