You recharge your phone's battery. :) Best WiFi today will consume about ~2mA when in standby. They are using ElectricImp which is a little worse. People ofte

You recharge your phone's battery. :) Best WiFi today will consume about ~2mA when in standby. They are using ElectricImp which is a little worse. People often look at "sleep" current of a WiFi module and think "oh boy I can run this for 2 years at like 100 micro amps". That is true if the WiFi module is turned off; useful if you are waking up, taking a reading, pushing data to the cloud, and then going back to sleep. But for a door you need to be listening on the network for an incoming packet all the time. How else would you know that the user wants you to open the door right now? Alkaline AA batteries AT BEST have ~2600mAh of capacity. They run at 1.5V. You have 2600 x 1.5v = 3900 mWh of energy in each. A Wifi module consuming ~3mA and 3.3V runs through that power at around 3900 / (3.3 * 3) = ~394 hours or about 16 days. 4 AA batteries and you'll get about 64 days or little over 2 months of usage. Of course you'll get FAR LESS because: A) You have to convert battery voltage to 3.3V and you'll get about 85% efficiency for a high current (300mA peak) needed for a WiFi device B) You burn more energy when you actually send packets C) You need quite a bit of power from those 4 AAs to actually turn the lock so you can't do the math assuming all your power is available for WiFi.