last updated 4 Oct 2019
Under Construction
Convert 18V DC to 12V DC to power an DC to 120V AC inverter.

A 100 Watt inverter uses 8 - 25 amps.
  General rule 10 Watts per Amp
  Cigarette lighter version 8 A (100 watts)
  Stand alone version 25 A
  
A modem and router will use 15-30 amps (180 - 360 Watts)
  
Switching converters (such as buck converters) provide much greater power efficiency as DC-to-DC converters than linear regulators, which are simpler circuits that lower voltages by dissipating power as heat. Amazon.com: DC Voltage Regulator, DROK DC-DC Buck Converter Module 10V-75V to 0-60V 12A Adjustable Power Supply Step Down Transformer Board DKP6012 CC CV Numerical Control Volt Reducer: Home Audio & Theater 720W $29
DROK.com