The general rule of thumb for determining the run time for any given amount of electronics is to combine the amps from all the batteries you will be running in parallel.
Normally you will only be using one unless you wish to maintain a larger power base for more equipment or to extend run time. Divide the number gathered by twelve and keep that number on hand for further use.

Acquire the amp numbers for each of the devices you intend to use and add them all together.
Many suggest adding another half an amp to the total for the inverter itself, but a full amp is best to be certain the manufacturers were not off slightly in their estimates for each device.

Take this new number and divide that into the original number this resultant number is the run time in hours for how long your equipment can run on that power supply.

If you are unable to determine the amperage on any given appliance then take the wattage and divide it by one hundred-twenty. This will equal the amperage.
If this is done for each device then the formula will work.

Similarly to the run time formula the amount of total wattage, or amperage, will need to be determined to make certain the inverter you purchase will have the necessary amount of output to meet your needs.
There are two factors to take into account when determining this step continuous power and starting load

The continuous power requirement is how much wattage the device requires to continually run.
The starting load, also known as peak load, is the initial start up requirement for the device.
Generally the start up cost is twice the continuous cost in power.
However, it should be noted that this is not always the case and each device should be read carefully to determine the true values for each.

At any rate to determine the amount of power required for a device it is best to base it off of the peak amount.
Add the peak wattage, or amperage if you prefer, of each device together and the total is the amount of required power your inverter must generate to allow for the usage of these devices.

The power inverter purchaser will buy one that is capable of generating at least twice the amount of what they think they will need. This allows for other equipment to be added later.