efficiencies are irrelevant here.

the most straightforward method would be to insert a fused heavy duty
ammeter in series with the battery and then turn on appliances, perhaps
one at a time to avoid accidently blowing the ammeter fuse, and noting the
current with everything on. dividing the amp-hour rating of the battery by
this current will yield the maximum operating time in hours:

                     t = amp-hours/amps

note that a discharged battery is not out of electric charge, but out of
chemical energy. conversely, a charged battery has no net electric charge,
but chemical energy to do work on charges and force them through a load.

daniel

On Mon, 24 Nov 2003 09:39:47 -0800, Felder <felder@KNOLOGY.NET> wrote:

>Anyone want to offer an easy-to-follow, practical way to to figure how
>much battery it will take to run lights, fans, etc. in the real 12v
>world?
>
>What would be helpful is a way to determine the amperage of various
>appliances and then figure what the rule-of-thumb efficiencies would be
>to then determine how many hours you could run various combinations of
>things