What we have

We have 5 amps at 12 volts out of our engine while under way. We have about 1 amp at idle, and about 2 amps in neutral. It seems safe to say we have 2 amps on average.

We also have a 65 amp-hour lead-acid dual-purpose battery, which can both start the engine and be discharged down to 70% without damage. We thus have 45 amp-hours available stored up.

What we need

Laptop, at 12 Watts
GPS 1 Watt
Radio 1 Watt standby, 5 Watts transmitting
Lights ? Watts
Random battery charging for another 3-5 occasional watts

We will probably be using an inverter for powering the laptop
We will probably have a battery charger on-board for rapid recharge when wall current is available.