Bear with me, I’ll try to be detailed as possible.
Let me answer you by the #
#1 Power stuff
Most of my sensors are Honeywell 5800 wireless series. They came with my ADT when I first puchased the house. That ADT system had a wireless main controller panel, that had no means of integration (no comms). I ditched the ADT service after the contract and I purchased a Vista alarm panel. The vista has a rs232 so it was easy to integrate with an arduino (total cost to integrate was like $10). It worked fine but since this is a SECURITY thing, I purchased an AlarmDecoder module (not the AD2Pi) since I already had the pi. AlarmDecoder is one of most respected modules out there. I replaced ADT monitoring service with an EyezOn service. I kept all sensors, and added few myself. With ADT, it will cost you about $100+ just to add a single window sensor and ADT will want to install them themselves. Sensors are cheap and adding it to Vista panel takes 10 minutes. A Vista20p has a maximum of 40wireless zones. A Vista 128 has 128 wireless zones. Here’s a good chart: http://www.security.honeywell.com/hsc/solutions/commercial/documents/PanelComparison6_4_4.pdf or https://www.alarmgrid.com/documents/honeywell-vista-series-commercial-comparison
Heres a good chart for the battery for these sensors:
https://www.alarmgrid.com/faq/how-long-does-a-cr123-last-in-honeywell-sensors
THey last 3-5 years!!
For the above, I am talking about the alarm sensors, such as window contacts, door contacts, PIR motion sensors, garage tilt sensor, glass break sensor, etc. All these can be integrated with the Vista since they are all Honeywell products. I also use AlarmDecoder module. With Alarm Decoder module, you can use ANY of your DIY stuff to trigger the alarm using “Virtual Zones” or Virtual Relays in AlarmDecoder: http://www.alarmdecoder.com/wiki/index.php/Protocol#Zone_Emulation
So, on top of the honeywell sensors, I added my own sensors using ESP8266: Gun safe door contacts, and so on. Basically, you can send a command to the AlarmDecoder a command to make the zone thing it’s ‘open’ just like any honeywell sensor would do.
For the smoke detectors, I used the existing smoke detectors that are already in the house (the ones attached to the ceiling). These smoke detectors are wired to the mains with a 9V battery backup. It has 3 wires: Black, white and red. Black and white are the leads for the 110V, and the red wire is the intercom, which means if a smoke alarm is trigerred, the red wire sends a voltage to the other alarms. THis is why when you have a smoke sensor trigerred, all the alarms in the house go off. Using an esp8266, you can monitor the voltage of this red wire: (from memory) 10.5 volts for fire, and 2.5 volts for CO2. Since there’s the black and white wire already for 110V, I used that to power the esp8266 using a simple stepdown voltage regulator.
WARNING
Since you are messing with a security/safety system, please consult with your home insurance company/provider. Some companies will DENY your claim if you ever had a fire if you messed with the system EVEN IF it wasn’t your DIY stuff that caused the fire. Or even if you DIY has completely nothing to do with. Just having them “in-line” with your safety equipment could be grounds for them to categorize them as ‘tampered.’
Because of this I used a UL certified relay instead: http://www.brkelectronics.com/product/RM4 Same concept here, except you use the relay to trigger your output. Just connect the relay to ground a pin on your ESP8266. The only difference is that with this relay, you cannot tell if it’s a smoke/fire or a co2 alarm since relay will just activate on either.
For most of my DIY sensors, they are powered by cr123 and in deep sleep, not connected to WiFi at all. THey only connect to Wifi when triggered (interrupt) to send data. Data is sent twice for redundancy. an esp01, assuming you use bare chip or desolder any LED, should last more than 2 years. I have most of them on original cr123 still. I do, however, coded a ‘heartbeat’ mechanism to ping openHAB once every day so I know the sensors are still alive (good battery). Note that an esp8266 uses only 200uA in deep sleep.
#2 Security Cameras
Yes, I use POE on cameras I cannot reach, i.e., outdoor cameras mounted near the roof 
For the security camera (NVR) I use a dedicated Windows server running BlueIRIS. I cannot use a Pi here since some of my cameras need transcoding and takes a lot of CPU. BlueIRIS is amazing, I highly recommend it. It is also easy to integrate BlueIris with any home automation software. Want openHAB to be notified on motion? Yes, just have BlueIris execute a command when a motion is detected (much like an exec binding of openHAB). Want BlueIris to do something when something occurred in openHAB? Yes, BlueIRIS has virtual DIOs that you can trigger. BlueIRIS also has a REST service. One thing I wanted to point out is that I DO NOT use WIFI for any of my security cameras. My cameras feed BlueIRIS using 4096 bitrate and most of them are 1080p (2MP) streams. If you use WIFI here, your IP cameras will eat your bandwidth, and slow down your entire network (or the VLAN, or the WIFI, depending on how you set it up). I initially tried an isolated AP but that only congested the air too much so every camera in the house is hardwired (ethernet). I setup BlueIRIS to ONLY record on motion since that’s the only thing that actually mattered. Every night at a specific time, I have scheduled code in the Windows server to upload these videos to Youtube (private, unlisted) for backup BlueIRIS can keep X number of videos and only delete oldest ones if needed (space). I have my videos stored on a 5TB drive. BlueIRIS is one of those that you set and forget 
#3 Power Monitoring
Go to your PG&E, enable API access (Yes, PG&E has an API!) but this is not real time. You can checkout your meter. If it is a SmartMeter, you can monitor your usage in real time using a simple and cheap SDR to read the meter. You also have to know if SmartMeter is actually enabled on the device. Talking about cheap, an SDR dongle can be had for like $15!
#4 Light Switches
90% of switches around my home can be controlled by openHAB. I primarily use only 2 ways to communicate: 433mhz and zwave. An esp8266-01 is like $2, a 433mhz pair can be had from China for 50 cents! You will need the pair (receiver, transmitter) if you want to communicate both ways. So some switches are connected to the esp8266 communicating to openHAB via Wifi. Some switches (via relay) communicate to openHAB via 433mhz connected to an ATMega168, some switches are Zwave (if I find a good deal in slickdeals lol). Note that for 433Mhz and other relay-based DIY switches, you have to code and persist the state OR use a circuit to determine that. With zwave devices, you wont need to.
MQTT
Most of my IoT stuff interfaces with an MQTT server (private cloud server). For the cloud server, I recommend NFP Hosting. Some of their VPSes can be had for $5 a year. YES, that’s FIVE BUCKS a year! Make it light, most of them are plenty enough for MQTT + other light services.
Pardon the lengthy response. Hope I didn’t bore you.
Again as a WARNING
Consult with your Home Insurance if you want to mess with Safety stuff. Use certified devices a much as possible.