12V DC UPS

my apartment has automatic generator backup (yay!), but the power goes down for around 10s before it powers up. A similar thing happens when the power comes back online. on a few days there is large number of power cuts and comebacks. i use a direct fibre router which takes around 3-4mins to startup and so i wanted a circuit like this. the router uses 12v power, so i plan to use non-rechargeable or small lead-acid battery. i have an nmos with me so a designed this circuit after looking at another similar pmos circuit. but when i open the dc switch, the output is 18kV!! what is the problem with this circuit?