manufacturers often make it hard to directly compared battery capacity as they often rate them in various measures such as Ah, mWh, etc, so this page is to make this more simple for you.
There are several aspects that need to be compared:
maximum capacity of the battery - this is best measured as Watt-hours (Wh)
usable capacity in Wh
recommended level of discharge
lead acid batteries should only be discharged to about 50% of their capacity to avoid permanent damage to the battery
LiFePO4 batteries can be discharged down to 0% charge but the more this happens, the less lifetime cycles you will get
depth of discharge lifetime cycles (DoD)
for LiFePO4 batteries, this is often rated at number of lifetime cycles of 80% DoD and this tends to range from 1000-4000 cycles depending upon the design and quality of the battery
how many DoD cycles has been done already
there is a gradual decline in maximal capacity over usage time of hundreds of cycles
Watt-hours
using watt-hours as a measure instead of amp-hours (Ah) means that you can better equate batteries of different voltages and you have a universal measure of how long a battery will last given use of a appliance of a known amount of Watts independent of the voltage
Watt-hours (Wh) = Amp-hours (Ah) x Voltage
How long will your appliance run?
this is easy to ascertain:
duration in hours = Usable power capacity in Watt-Hours / Power usage in Watts(W)