It relies on a CR123 battery.

The battery is supposed to last for up to two years. 

You can use up to six Nest Temperature Sensors in a house. 

Nest also sells three-packs for $99 (£70/AU$125 converted).

You can name each sensor you buy and schedule them straight from the Nest app. 

That way, you can optimize the heat and air conditioning in certain rooms at certain times of the day. 

The Nest x Yale lock costs $249 (roughly £180/AU$315 converted).

Like other Yale locks, the Nest x Yale version replaces your existing deadbolt. 

This lock uses four AA batteries. 

You should get an alert on your phone when the batteries are low.

The low battery alert is important, since this lock doesn't have a keyhole. 

The Nest Hello doorbell costs $229 (roughly £165/AU$290 converted).

The hardwired doorbell replaces your existing doorbell wiring. 

Like Nest's IQ cams, the Hello has 1080p HD live streaming, person alerts and facial recognition (if you pay for the Nest Aware service). 

This doorbell also has two-way audio so you can chat with whoever's at your door straight from your phone.