Is there a way to hook up a multimeter to see what total watts are going to a breaker box? (I'm not wanting to see amount used but amount that could be used there)
If your residence is up to code, you should just be able to calculate the wattage based on your breakers and your country's standard voltage. Volts x Amps = Watts. Depending on what country you live in, you use 110/120V or 220-240V and standard breakers are 15/20 Amp or 40 Amp for utility breakers.
A standard house in the US for example will have plenty of 15 Amp breakers, at 110V and can supply a max of 1650 Watts. Although, like all things electrical, its not a good idea to use a max for any period of time.