If you know your Peukert # for your battery you can use this formula to see how long your battery will last for a particular load. This is kinda technical so stop reading here if ya don't wanna go nuts!
Peukerts Equation: I n x T = C
The AMPS use (I) to the Peukert # exponent (N) multiplied by time (T) = the battery amp hour capacity (at the 100 hour discharge rate...or 1.1 times the 20 hour capacity)
Example...let imagine a 200 a/h battery (at 20 hr. rating) with a typical flooded battery Peukert rating of 1.2 and you want to know how long you can run your 5amp refrigerator.
Well 5 amps raised to the 1.2 power is almost 7.
The 200 amp at 20 hr battery needs to be raised by 10% to 220
So we have 7(T) =220 or a bit over 31 hours to total discharge of the battery...so figure on 15 hours before you need to recharge.
If everything remained the same BUT you changed to a 10 amp fridge...
the 10 to the 1.2 is almost 16 and you thus get
16T=220...or 13.5 hours till total discharge and just 6-7 hours before you need to recharge.
So...you can see that as current draw increases...the capacity of the battery in hours drops even more quickly...and a higher Peukert number accelerates this.
Now conversely...a low Puekert # means the capacity is relatively unnaffected by the rate of amp draw. This is one of the big advantages of AGM batteries as they can have P numbers in the 1.08 type range.
So....if you are looking at building a large battery bank with big amp draws...checking into your PARTICULAR brand and model battery choices Peukert # may lead you to one brand and one type over another. Peukert #'s are generally hard to find and you may need to write to the mfr. you are considering.
Hope a few find the above interesting.