wrote in message
...

want to use it pretty much just
for my refrigerator (about 800W),

That is either a very large refrigerator, a very inefficent one, or the
rating is way above what it actually draws. If this is a "frost free"
refrigerator then you would be best to get rid of it and get one that uses
lots less power. Frost Free refrigerators use more energy creating heat
(directly, not counting the byproducts) then they do "cold".

and my TV/VCR
etc. It seems like it would be cheaper to get two
less powerful inverters, than one powerful one.

It is possible that dedicating a smaller inverter to the refrigerator might
be more efficient. An inverter uses power even when there is nothing
drawing AC power. This "idle" power can be significant, especially on the
larger inverters. There are advantages to a good expensive inverter,
however.


seems like the best deal for me. That would leave
me plenty of head room--especially on the TV
circuit--for including other things. Does anyone
know of a better deal?

I would go with a minimum of 1500 watts, then you can run just about
anything (coffee maker, blow drier, belnder, etc.). You can decide if it is
wise to run such things, but at least you will have the option.



How do I figure what size batteries to use? Should
I keep the batteries separate for each inverter, or
is it better to connect two or more batteries and go
from there? If so, how to connect the batteries
together, and then how to go from there to the
inverters?

One large battery bank is much better than two smaller ones.

The first step will be to establish how much capacity that you need.

The refrigerator is going to be the biggest consumer. 800 watts will force
the inverter to draw about 76 amps out of the battery system, after
adjusting for the inverter efficiency loss. I would guess that the
refrigerator would run about 50% of the time, which means that you would
need 919 amp-hours per day.

That's a lot! If you want to last a three day weekend you would need 2700
amp-hours. To treat the batteries reasonably well you don't want to
discharge them below 20%, so you really would need ~3300 amp-hours. That's
a bunch of some very serious batteries. A lot of weight, a lot of room, a
lot of cost. Since you started off saying that you were concerned about
cost I would think that this would be a major issue.


I'm lead to believe that any inverters I can afford
would produce a square wave instead of a sine
wave.

There are sine wave inverters, square wave inverters, and modified sine
wave.

A square wave inverter goes from +60 volts to -60 volts in one step.

A modified square wave would go from somthing larger than 60 volts, to zero,
then to somethng larger (magnitude) than - 60 volts. The pause in the middle
would approximate the zero crossing of the sine wave and would make
inductive loads much happier.


How to know what influence that would
have on my TV's performance?

If the TV was made in the last 20 years it probably would be very happy with
the output of any inverter. Most electronics these days use switching power
supplies that would actually have better efficiency off a square wave than
they would off a sine wave. Where you can get into trouble is with
inductive loads (some motors) and transformers.

On a sine wave AC, the peak voltage is ~1.4 times the RMS voltage. Thus, if
you had a device that uses a "wall wart" transformer it would be expecting a
" 6 volt" output to actually peak at 8.4 volts. When rectified, this would
produce a useable amount of 8 volts DC, which could be used recharge a 6
volt battery operated device. If you connected this up to a square wave
inverter than the rectified output would only be 6 volts, which might not be
enough for the circuit to operate. The modified sine wave would certainly
be better and they probably make the "modification" such that the peak is
the same.

Square wave inverters are great for running series wound motors (drill
motors, blenders, saws, etc.). They will run most electronic items that
have swicthing power supplies or that aren't too sensitive on the voltage.
They are a lot cheaper and more efficient than modified square wave or sine
wave.

There are other factors to consider for the total installation.

If you go with something like a Xantrex marine inverter you will get the
following features (which will not be available from the inverters you have
indicated)

1) Automatic AC input cross over. When you have 110 Volts available you
will not want to be running off the batteries. The Xantrex will sense the
incoming AC and pass it through, switching the load over without ever
powering it down. With the cheap inverter you will have to pull the plug
out of the inverter and plug it into the normal AC outlet. (that can be a
real hassle if the refrigerator/TV is recess mounted).

2) The Xantrex inverter includes a battery charger, suitably sized for the
job. If you install that 3300 amp-hour battery bank you will need a way to
charge it. If you don't have a really good charger it could take a week to
get the batteries recharged!

3) I have some amount of faith in Xantrex products. They have been around
for a while and I know many people who have used them. Some fly by night
company off the internet is a gamble.


My recommendations:

1) Get a refrigerator that uses a lot less power. The "old" style, where the
cooling coils are the floor of the freezer, and you have to manually defrost
them occasionally, are the best for this application.

2) Install a battery bank that will provide at least twice what you think
you will need for the length of time you will be away from shore power.

3) install a single inverter that will cover all your needs.

Rod McInnis