you are right, the DC-DC converter is what continuously refills the 12v system. in the 2008 highlander hybrid its around 1kw, but i was fairly conservative always when pulling out power, so stayed more around 700-800 watts. a couple of pointers for you... (i manufacture a product that does exactly this, turns a car into a generator, so have significant knowledge in this space) 1) you can do a simple test with any vehicle to see how much power they have engineered it to provide as a baseline. buy a DC clamp amp meter, put it on the 12v battery, switch on the car systems but WITHOUT the engine or hybrid system running, and then switch on all the accessories one by one. on my 08 hihy it gets up to around 70-80 amps. this shows you how much power they have engineered it to sustain safely in order to support all systems and accessories running. 2) if you get a big inverter like 2000-3000 watts just simply to do a test, watch for how much load you can pull on the AC side of the inverter, until the DC side car voltage gets down to 13.3 or 13.4 range. We instruct our users to make sure the DC vehicle voltage always stays above 13.3 volts because that generally indicates the alternator or DC-DC converter is keeping up supplying voltage. 3) I would be careful to not try and max it out continuously for a long time. you never want to risk harming your vehicle systems. Our sweet spot for most vehicles is 1000 watts, with 1500 and 2000 watt models for if your vehicle has the alternator or DC capacity. 4) make sure to use pure copper cable, minimum 4 guage, and ideally as short as possible. our product is 7 feet from battery clamps to the inverter post terminals. 5) above 1000 watts you must use direct bolt on lugs and/or anderson high amp DC quick disconnect plugs to sustain continuous transfer. if you are using clamps it will overheat above 1000 watts. Hope this helps, feel free to ask anything, i'm glad to help with 5-8 years of solid experience in this function of turning your vehicle into a generator.