If there are power bricks or wall warts, these will spec an DC output voltage and amperage.
This does not mean the device always draws this amount, but the maximum it can draw.
Many devices, like many laptops, require 19.5vDC.
An inverter takes 12vDC converts it to 115vac, then the powerbrick converts the 115Vac down to 19.5vDC.
This double conversion is hardly efficient.
A simple DC voltage Booster can be used in many instances and is usually much more efficient, electrically.
https://www.amazon.com/Gowoops-10-32V-Co...voltage+booster
Lowering battery voltage to less than 12 would require a voltage bucker instead of a booster.
The Inverter is often seen as the solution for all ones portable power needs, but little thought is given to how long a battery can power it, and how long it actually takes to recharge the battery. If the lead acid battery is not regularly fully recharged after being deeply cycled, it is not going to last for more than a few dozen cycles.
A group 27 battery, 9.25 tall 12.5 long and 6.5 inches wide has about 100 Amp hours of capacity.
This means it can support a 5 amp load for 20 hours before voltage falls to 10.5v which is considered fully discharged, though some AGMS say 100% discharged is in the 11+ volt range.
Loads over 5 amps will reduce the total available capacity, loads lower than 5 amps will increase it, but not as much as loads over it will reduce it.
a 100 watt load, at 12.2v, which is the voltage a new healthy fully charged group 27 battery can be expected to hold, is about an 8.1 amp load.
Ignoring Peukert component, 8.1 amps of load will drain the healthy group27 battery to 50% state of charge in a few hours, and the lower one draws a battery below 50% the harder it is on the battery, even if promptly and fully recharged, which is unlikely.
But Add 20% more to that amp figure for inverter inefficiency, and 8.1 amps is a greater load than the 5 amps at which the battery earned its 100Ah rating. So at 8.1 amps of load on that new healthy fully charged battery is more like an 87Ah capacity battery, and that 8.1 amp load is more like 10 amps once inverter inefficiency is factored in, and that 10 amp load is double the rate at which the battery earned its 100Ah rating and is now more like a 75AH battery. These capacities assume a battery temperature of 77F. Lowert tmeps also reduce capacity and higher temps raise it, but not as much as lower temps reduce it.
So if the inverter is to be powered from the battery alone, with No engine running, it will not take long for it to be drained into the danger terrirory of not being able to start the vehicle.
Carry a jumper pack and or cables. Get a good charger capable of at least 10 amps, and have a way to top up the battery when you are home parked overnight, and know that even an overnight charge might not have the battery near full, especially as it ages and the time it takes to fully charge grows, and when higher voltages held longer become necessary to keep the capacity from jumping off a cliff.
Deep cycling lead acid batteries, and getting anywhere their rated cycle life, requires prompt and proper full recharges