goober

how do you up the wattage coming out of a power inverter? it has an input of 12v dc & an output of 120v ac. The transformer ups the voltage, so why do 300 watt inverters cost so much less than lets say 1500 watt inverters? Do i put a bigger transformer in an inverter to increase volts? Also i need more watts to run things, not more volts. I want to make an inverter that will put out a lot of watts to run more than one thing. Thanks ??? ??? ???