I think a little bit of additional understanding is needed.  Power = Volts X
Watts.  A 300 Watt inverter puts out 120 volts at 2.5 amps, (theoretically,
certain loads such as motors changes this). However, the input needs to be
25 amps at 12 volts. Add another 5 or 10% for Inverter inefficiency, (Heat).
Thus, a 300 watt inverter is actually 50% of this batteries 1 hour capacity.
Another consideration, the amp/hour rating of most batteries is rated over
20 hours of use. Using the current at a faster rate will cause the voltage
to fall off faster due to internal heating of the battery. This is why it is
recommended to only use 20% of the amp hour rating (~5 hours). My
recommendations would be as follows:

Lights, Hi efficiency Fluorescent. 2 15 watt fixtures should be adequate(~ 3
amp/hour)
Stereo, use dash unit. Avoid hi-powered amps if extended play is needed
without charge source.
Computer- Use Laptop with 12 volt cord. Late models will go 3 or 4 hours on
internal batteries.
Heater-probably draws 4 or 5 amp hours running. Determine duty cycles to
keep van warm. Cooler inside temps will mean less run time. Keep in mind
that this heater probably needs full battery voltage to run so only figure
50 or 60% amp hour rating to be available, assuming full charge at sleep
time. My experience, you will need 2 of the optima's to get you through a
long cold night. More are needed for weekends.

Battery charging~will take about 4 hours to fully recharge most any battery
using the engine alternator. An 8-10 hour charge is best for the life of the
battery. Adding additional batteries does not increase charge time unless
the alternator or wiring can not maintain voltage.

Hope some of this helps.

Dennis
----- Original Message -----
From: "Turinsky" <kturk@ALASKA.NET>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Saturday, December 18, 1999 12:53 PM
Subject: What Size Inverter Do I Need?

> Like many of you have already done, I'd like to install an onboard
> electrical system for some interior lights, Propex heater, stereo, 'puter,
> etc. all run off of an Optima battery.
>
> The Optima battery rates at 52 amp/hrs. I've calculated my max daily
> consumption at roughly 35 amp/hrs. According to West Marine you should
have
> at least 20% of your inverter's rating in amp-hrs of battery capacity.
> Therefore, I come up with about a 300 watt inverter (20% of 300 watts = 60
> amp/hrs. compared to the Optima's rating of 52 amp/hrs.).
>
> West Marine says that this calculation roughly "allows you to run your
> inverter for two hours at half power while consuming 50% of the battery's
> energy." Huh? I don't get it. Is this overkill, or not enough? Can a 300
> watt inverter handle lights, heater, stereo, and 'puter running all at the
> same time for a few hours?
>
> Now one of the ftp files I got off of gerry.vanagon.com says your battery
> capacity should be double your expected daily amp/hrs. consumption.
> According to this, I'd have to be driving around with 90 pounds of Optima
> batteries, i.e. two Optimas! Yikes! And who has room for two Optimas?
>
> I'd also like this setup to recharge itself as I'm motoring down the road.
>
> I'm really flying blind here. I've just read enough to make myself
> dangerous. Any guidance will be appreciated.
>
> Hope you're all enjoying a big vat of figgy pudding.
>
> Kevin
> '89 Syncro Westie
> "Anchored, down in Anchorage - Alaska"