If you are new to the concept of switching from a lead acid battery to a lithium battery, you may be confused about what size or capacity of battery you need. There are some easy rules of thumb but they don't tell the whole story and should not be used as a perfect measuring stick.
The best way to size a battery is based off of your actual, real-world usage. Using a power meter, measure the consumption of energy from the battery you are using now (lead acid) and then use those numbers to select a lithium battery that can handle that load. Always add some cushion, say 20%, to your calculations to make sure you can handle occasional spikes in demand.
For example, lets say you have a travel trailer and your data logs say you are averaging about 2kWh of usage per day with your current 12V lead acid battery. So I would recommend that you use 2.4kWh of capacity as your target, which means you need a 12V 200Ah battery or battery bank (2.4kWh = 2,400Wh = 12V x 200Ah).
But what if you don't have a battery now or don't have any measuring equipment to accurately log your usage? The next best method is to estimate usage based on your loads. Loads are all the devices that you are going to be using that require electricity from the battery. To estimate usage, you will have to look up the manufacturer specifications for that battery to find the electrical consumption ratings. Then you will have to decide on how many hours per day that you will run that device, on average. Do this for each load and then add all of those numbers together to get a total. Again, make sure you add in some cushion. It is never bad to have a little extra capacity in reserve. But it is a very expensive mistake to miss the mark on the low side.
For example, lets say you have a boat and you will be running a trolling motor that uses 500W for 1 hours, a GPS system that uses 12W for 4 hours, and a bilge pump that uses 24W for 1 hour. 500W x 1 = 500Wh, 12W x 4 = 48Wh, and 24W x 1 = 24Wh for a total of 572Wh. Add in an extra 20% cushion and you get 686.4Wh of capacity required. On a 12V system that would mean you would need at least 57Ah of capacity (12V x 57Ah = 684Wh).
Pretty straightforward, right? Well, there is more to the story. We have been using 100% of the lithium battery's capacity for our calculations and selection. In other words, if a battery is sized as a 100Ah battery at 12V then it can provide 100Ah of power before it will be 100% depleted. But on any lithium battery, you will slightly shorten the life expectancy, called cycle life, of the battery by discharging it to 0% state of charge every time you use it. You will get more life out of the battery by discharging the battery less each cycle. For example, MillerTech's Premium series of LiFePO4 deep cycle batteries are rated to provide 2,000 cycles at 100% discharge. But they are rated to provide 3,000 cycles at 80% discharge. So by leaving 20% of capacity in the battery each time, we can get 33% increased life out of the battery.
Now that may not be a big consideration for some people, but you may want to consider re-rating the batteries based on 80% DoD when shopping for a battery. That is my personal recommendation. That also gives you the added benefit of always having emergency capacity in reserve for unexpected scenarios, which can be critical for boats or people living off the grid. Not only that, a larger battery will usually have better discharge characteristics such as a higher continuous discharge rating (the maximum Amps that the battery can provide on a steady basis) and a higher max discharge rating (the maximum Amps that the battery can provide for a brief period, usually 5 seconds).
So the last example to take advantage of this principle would be that you know you need 1.2kWh of battery storage and you know you need a 12V battery. In our previous examples, we would have used a 100Ah battery and called it a day. But if you want to only discharge the battery 80%, you need to re-rate the battery as a 80Ah battery and then it is too small. To get at least 100Ah @ 80% you would need a 125Ah battery (125Ah x 80% = 100Ah). So by doing this you will spend a little bit more up front, but your investment will last about 1/3 longer and give you a lot more bang for your buck!