The best way to explain it would be giving an example: a common laptop. Many traditional laptops run on 2.5 up to 4 amps depending on what brand and model of-course. Their input in voltage can be up to 12 volts. 12 volts multiplied by the ampage of the laptop (in this case let's say 2.5 amps) would be equivalent of 30 watts continuous with an equivalent of 60 watts peak. So even a smaller 400 watt inverter (800 watt peak) running on a 12V battery would be more than capable of handling just one laptop itself. You could likely run a small flat screen LCD TV along side it just fine, maybe even a lamp with a 75 watt bulb too. The bigger the inverter the more wattage they can handle, thus the more utilities they can run, or the bigger utilities they can run in moderation. Typically, to be on the safe side, running a box freezer you'd want a 2,500 watt inverter with 5,000 peak because a box freezer would use a lot heavier load with the peak typically significantly higher than the continuous wattage unlike average utilities. Same would go for electrical water pumps or air conditioners too.