I have found that generally people think by installing a backup system they will "save" electricity. I have some bad news, not only does it cost you money to keep the inverter running but also a chunk of money to recharge the battery.

3.5 hrs backup, then 3.5 hrs @ 20 amp to recharge the battery, it starts getting expensive for 3 hours of backup for as little as 394 watts

You have to ask yourself, do you really need a backup inverter? IF you work from home or run a business from home as many smart people do, then it might be an essential requirement.