@ wiztech,
I think from your intentions what you need is not a battery but an Uninterruptable Power Supply unit (UPS)...this is a back up unit with integral battery arrays,charger,inverter and protection circuitry...when the mains supply goes dead the integral batteries supply the needed power...they come in different sizes in terms of load and back-up time. UPS's are mainly used to back up power to the connected appliances in your office which are sensitive to any power outages eg computers,servers,large scale data storage,cctv workstations etc.
the very first step to determine the size of the UPS is to know how much power in total you want connected to it...simply put determine which appliances which should be connected to the UPS. secondly convert the real power into its respective apparent power component. thirdly,with your KVA rating and the back-up time you have already dtermined the size of the UPS...you can now confidently go a dealer and request for a quotation.
one more thing you may wish to compare the price of a UPS and that of an AMF unit. AMF is the short form of AutoMains Failure. this unit integrates both generator and the mains (KPLC) supply in such a way that when the mains goes dead,the load is transfered directly to the generator. this means that the generator starts immediately and automatically (you dont have to go and switch on the generator yourself). AMFs are also manufactured in different sizes in terms of load (Ampereage). its determination follows the same sequence just like for the UPS,and the current drawn by the appliances is calculated which is the basic ordering information in addition to supply voltage.
BTW what appliances are you talking about here?
NEVER TALK OF A RHINO IF THERE IS NO TREE NEAREBY - ZULU PROVERB
...besides, the presence of a safe alone does not signify that there is money inside...