Dallas -- sorry I beat you to the punch. As Jerry noted, the page I posted cautions against the "simple" version of Peukert's equation -- the only "capacity" that works in that equation is the "Peukert Capacity" -- the amp-hours available at a 1-amp draw rate.
Jerry -- Peukert refers only to drawing energy out of the batteries. Putting energy back in also has a "fudge factor" that must be applied, often called the "Charge Efficiency Factor (CEF)." This measures how much of the input energy actually gets translated to chemically stored energy in the battery. (To answer the obvious question, "where does the rest of the energy go?" -- it is heat, as can be easily demonstrated by measuring the temperature of the batteries as they are charging.)
The meter I use (which I highly recommend), the Xantrex Link-10 (not to be confused with the very similar but lower-quality Xantrex Battery Monitor), has settings for both. The Peukert Exponent must be entered by the user -- the factory default of 1.25 (not 1.15 as I guessed earlier) is sufficient for most users if the exponent is not known.
The CEF may be manually entered, but the recommended setting is to allow the meter to calculate it on a rolling basis. It does this by using a set of user-entered parameters to know when the charging process if finished, including final voltage of the system and amount of current flowing from the charger. If you set these correctly, then every time the batteries charge up fully, the meter resets the amp-hours "used" to zero, and re-figures the CEF (since it knows how depleted they were, and how much energy was put back in).
The Link-10 also keeps a running total of the number of cycles your batteries have been through, what the average Depth-of-Discharge (DOD) is, and what the deepest-ever DOD was. Very handy information. I think I paid around $135 for mine on eBay -- a worthwhile investment.
-Sean
http://OurOdyssey.BlogSpot.com
