Frank asked, "Installed a fridge on my boat that draws 1.5 amps ac
current, is fed thru a 800 watt inverter, from the 12 volt battery. Is
there a formula to determine how many amps its using from the 12 volt
battery?"

Hi Frank

Since no one else has yet replied, the answer is that a watt is the same
in any system, i.e. volts x amps. So, if your fridge draws 1.5 amps in a
110 volt system, that equals 110 x 1.5 = 165 watts. To convert that to
amps in the 12 volt system, amps = watts / volts, so 165 / 12 = 13.75
amps.

Your unasked question, I assume, is how long can you safely run that
fridge from your inverter? As with everything in boating, the answer is,
"It depends...?

If nothing else is running off the batteries - and that's never the case

• and you follow the "never discharge below 50%" rule, divide your rated
  battery capacity in amps by the refrigerator's draw. Although the fridge
  will not run constantly, each time it cycles on, compressor startup
  consumes a lot more amperage than just running, so using 1.5 amps is
  somewhat conservative but provides a margin for error, since the
  manufacturer's amp rating is for perfect conditions.

One more unrequested suggestion: do a consumption inventory, i.e. list
everything on the boat that uses electricity in a vertical column eg
appliances, lamps, electronics, winches, and so on. Beside each item,
note its amperage requirement and the estimated running time in a 24
hour cruising period, then multiply the two numbers to get amp-hours.
(Even though a winch might only run for a few minutes, its draw is quite
significant.) Add the amp-hours column, and you know how much you expect
to use in a typical day. Now divide that into the capacity of your
battery bank, and you know how long you've got before your batteries go
dead. Divide it by 2 and you know how long you have safely before you
need to recharge.

Cheers, Garrett

Garrett's suggestion of a consumption inventory is a good idea, but I really
think a monitoring device like a Link 1000 is almost mandatory. At the
least, it makes excellent financial sense.

Power consumption is so dependant on circumstances that averages are of
little use.  Here in the Pacific Northwest, we use a lot of power in the
wintertime.  We get about 8 hours of daylight, so we use a lot of lighting.
We need to run our diesel heaters, and they take an amazing amount of
electrical power to run fans and pumps.  Summer is so dramatically the
opposite.  There's no time to sit around and watch TV, powered by the
inverter.  You are out and about and only use more power than winter for
refrigeration.  Sunrise today is 0521 and sunset 2104.

I am amazed how few of my friends on the water understand how to take care
of a battery bank.  Most common is the idea that it is "good for a battery"
to discharge it completely and then recharge it.  When asked about it, they
say they "never had a problem."  When asked how long their batteries last,
the answer is usually about five years.  With a monitoring system like my
Link 2000, you always know where you stand with your batteries and the
effect you have when you choose to run various appliances.  You know when
you need to run your generator and when you might as well shut her down.
When you think about it, you only have the use of about 35% of your battery
bank's total capacity.  You don't want to run below 50%, and it makes no
sense to try to recharge beyond about 85% with a generator because of the
smart charge the inverter supplies.

I admit to being a major gadget freak, but a monitoring system is not a
luxury in my opinion.

Usual disclaimers, I have no financial interest in these gizmos.

Todd Mains
M/V Pinguino
50' O/A Pilothouse
Portland, Oregon