Look at your base load. (mines 0.6kw constant 24/7) then work out how much battery you need to run that on battery between 4pm and 8am (winter). That's where your bulk of capital expense is repaid.
Your usage above and beyond the base load should, in theory, be either picked up by excess solar or spare battery capacity during the longer days in Spring Summer and Autumn.
You need to either charge the battery at off-peak via grid or produce enough electricity via solar to charge it and power the base load at the same time. Keep in mind you need an inverter that matches that rate or a solid enough main fuse to support ramming it in during the 4 hour cheap window.
As the cost of electricity climbs (I don't for one minute think we'll ever see it drop down significantly again) the rate of return on your investment increases. Projections to completely pay off the investment may be 10 years to start with, but actually may end up lower, and after that point you're not at net zero because you're now making savings every month thereafter.
