1kW inverters for free!!!!!!!!

i see (and answer) a lot of posts both here and elsewhere about 1kW inverters. my immediate answer is usually something along the lines of "there's no such thing as a free lunch". first of all do the math. 1000W/12V=83A. assuming about 15% in losses, that's pretty close to 100A. not only are we talking "heavy iron" here for a transformer, we're talking heavy copper too, since the wire used in the primary circuit needs to have negligible copper losses at 100A. you also need about 20 MOSFETs per side (of a split winding on the transformer) assuming 10A devices, and a safety factor of 2:1. that also makes for a very large heat sink. so basically we're looking for a "welding" transformer with a 12Vct @100A winding, 00ga wire (or at the very very least 2ga kept very very short) 40 MOSFETs, and a very big heat sink.

like i said above, there are 1000W inverters that can be had for free, but it takes a bit of looking to find one that works from 12V. these would be computer UPS supplies. 90% of these run from 24 or 48V batteries, but once in a while you will see a 12V one. 99% of all UPS supplies get scrapped because of sulphated batteries, and the fact it's usually cheaper to buy a new one than pay somebody to replace the old batteries. these are a good "starter" for a 1kW inverter, since the heavy iron, wire, transistors, heatsink and driver board are already there, plus it's already in a chassis with AC outlets. however once you replace the batteries, you will soon find that you can't get it to run on demand. the reason for this is that the control logic wants to see the device pluggedin to line voltage when idle. you will usually want to find this with a schematic, but there is an AC detector circuit that wants AC present, then see the AC go away before it triggers the inverter into operation. the best thing to do here is find the actual logic line that starts the inverter and put a switch there, bypassing the AC detector. every UPS is different, so this is only a generality. you WILL need a schematic to find this, unless the signal is marked on the board somewhere (try looking for a signal marked something like START/STOP, or ACDET).