I’ve actually made a simple calculator[1] for this, but you don’t really need it.
What you want to do is get a good picture of your actual usage per day in kWh. You can use a Kill-a-Watt plug or similar (a device that kWh that pass through it).
Once you have your daily kWh use, you can multiply by however many days you want to run without charging the battery. A lot of off grid solar people go for 2-4 days.
That gives you the useable battery size in kWh you need.
LiFePo4 batteries (arguably the best for this setup) typically have 80-90% useable capacity. Lead acid (the more traditional kind) have around 50%.
Adjust for that and you know what size of battery to buy.
A lot of batteries are defined as Amp hours and voltage. Multiply them together to get watt hours.
Max_kWh = (Ah * V) / 1000
Useable_kWh = Max_kWh * useable_percent