I'm not sure how to figure this out, but I would think it is pretty
straightforward.A 12 volt appliance uses 15 watts. what size battery
(deep cycle, golf cart??) would be needed to run it 24hr per day 7
days per week for 2-3 weeks. How is this figured

Hello jmanly,

You have asked a very good question. 

Power (Watts) = Volts X Amps

In your case: Amps = Watts/Volts or Amps = 15 Watts/12 Volts = 1.25
Amps

Batteries are rated in Amp Hours (Amps X Hours)
A 100 Amp Hour battery will provide 1 Amp for 100 Hours.
Therefore, if you want to supply 1.25 amps for 24 Hours/Day X 7
Days/Week X 2 Weeks you would need 1.25 X 24 X 7 X 2 = 420 Amp Hours

I doubt that you will want to buy a battery rated at 420 Amp Hours, so
you might consider using multiple batteries and change them more
often. Another possibility would be to hook multiple batteries up in
parallel to still provide 12 Volts, but increase your Amp Hour
Capacity.

I think this should answer your question and get you headed in the
right direction.

Good luck, Redhoss

Good answer and quickly too. Thanks

A really easy way to remember this formula for future use - think of
homemade apple pie.


P = Power
I = Current
E = Voltage

P = I x E

;-)

SgtCory

This is just to Add to the answer by redhoss-ga:
The calculation seems to be perfect, except that it has to take
the following into account:
i)If the batteries are used at a higher rate of discharge, they
may significantly produce lower power(a 100Ah battery WILL NOT
produce 100A for 1 hr(10-20minutes is realistic)).
ii) Since the duration of the discharge is rather long, you should
also take into account self discharge(which might change the calculation
a bit, and depends mostly on the type of batteries used)).
iii) Since the size of the battery is rather large(12V 420Ah), it is
ideal to have a lower wattage batteries with a battery capacity indicator
and a simple diode reverse discharge preventer(needed for changeovers without
interruptions)
iv) While calculating the capacity, you might also take into account
the charging rate(yes it could significantly change the cycle life), aging
induced capacity reduction(memory effect in nicad's being an example).
v) In order to be fairly accurate in the calculations, you should also account
for any additional wire resistance(esp. if the battery is located at a distance
and connected to the equipment through long wires).

Regards
ldavinci-ga