www.TheDoItYourselfWorld.com

So I don't quite understand why you are using an inverter on the 12 volt system. You claim that you are using it to run the modem, the laptop, the cell charger.
Correct me if I am wrong, but aren't those all dc devices? So you are wasting battery power to transform 12 dc to 120 ac to plug in 3 transformers to convert 120 ac back to 12 volts dc or less. Looking at data sheets for dsl modems, most of them require 12 volts dc. so just wire them into the 12 volt grid and get rid of the transformer. Most cell phones have a You Are A Spammer lighter adapter plug that uses "Guess What" 12 volts dc.
And most laptops also have charge cords that plug into a cars You Are A Spammer lighter at 12 volts.
Your inverter is say 90% efficient. most ac adapters are 80% efficient. So you are just wasting power.
But so far the project has been very interesting to watch. Keep up the good work.

I charge my cell phone and camera on 12 volts DC through a cig lighter socket.

My netbook runs on 12 volts DC as well.

I run my laptop and modem on 120 volts because I cannot find an adapter plug that fits them. I bought a laptop DC charger and found that it does not plug into my laptop. They are using very strange plugs these days to prevent us from using anything other than what they provide from the company.

Same issue with my modem. I have taken the power supply all over the place looking for a plug that fits it. No luck so far.

With the laptop I am stuck using wasteful energy.

With the modem I might just cut the wire and use DC anyway.

My Ryobi battery charger is 120 volts AC. So are most of my other chargers.

So I run a mix of AC and DC power for now.

Here is a vehicle charger for Ryobi.
I am sure you can get one for your laptop also.

The Ryobi 18V ONE+ In-Vehicle Charger is compatible with all past and present Ryobi 18V ONE+ Ni-Cd and Lithium-ion batteries. It plugs conveniently into a vehicle's 12-volt DC outlet and takes as little as 30 minutes to charge the batteries fully. See below for additional details.
Plugs into vehicle's 12-volt DC outlet for flexibility to charge batteries when power is limited
18V ONE+ battery charges in as little as 30 minutes
Protective circuitry audibly warns you and shuts down charger if vehicle's battery is too low
Works with all Ryobi 18-volt ONE+ batteries
ChargePlus technology gives battery and charging status using LEDs

http://www.homedepot.com/p/Ryobi-18-Vol ... /100342149" onclick="window.open(this.href);return false;

That is awesome. Thank you. I will have to get one of those. It will greatly help me save power in here.