I followed the same rules that SMKETTNER says but it wasn't long before I was wanting to add one more item, then another item...
I have three GP24 Interstate batteries in parallel and that gives me a whopping 255AHs capacity. Lets me run my 600WATT PSW Inverter drawing around 300WATTS load just fine thru the night. By 8AM the next morning I am down to somewhere around 50% charge state on my batteries so this is when I recharge my batteries running my 2KW generator. Here on the East side of the US we have all these generator run time restrictions so being able to re-charge my batteries to their 90% charge state in as little as three hours generator time usually fits in with everyone rules.
Inverters love lots of battery power...
Roy Ken