Posted on Thursday Oct 10, 2013 at 10:14 PM
It depends allot on what you want to run with it, and how it's going to be connected to the 12vdc source. If you're going to run a low wattage air pump, a few low wattage lights, or run an 150W electric knife, then a 400W converter will be fine - especially if you connect direct to the battery. I'm an electronics engineer and I tested a number converters this year, putting them through the paces to see whats really happening. Some models have lower standby current drain. Some have higher efficiency when they are running a heavy load. Again, it comes down to how big your load is and the quality of the 12vdc wiring. If you're going to connect it directly to a 12v battery posts, then a 400W converter will probably power up to 250W continuously, just fine. If you connect that same converter to a 12vdc cigarette lighter socket, then expect maybe half of that capacity, at best. Or, expect the converter to start out carrying the load, and then a while later activate it's overload buzzer and to shut down. They say that a cigarette lighter is good to a max of 10 amps. But most cigarette lighters don't have heavy gauge wires going to them, and the wire run from the battery to the socket is a long distance - and that 10 amps spec is pure fantasy.
Here's some notes I made, using bulbs as a load, using different converters:
Bulb load Amps pulled at the 12vdc source
52W bulb - - - - - - - - - 4 to 5 amps
100W bulb - - - - - - - - 8 to 10 amps
250W Floodlamp - - - - - 25 to 29 amps
If you run the converter off the cigarette lighter socket, your load better be very light.
Which 12vdc/120vac converter did I buy? Cobra CPI 480
It's not the very best - but it's good enough. They are all pretty comparable.
I thought the Stanley converters Lowe's sells to be pretty good quality, and have some neat bells and whistles.
In my configuration, I didn't want the converter to ever be able to discharge the main boat battery.
Something about the thought of being on the lake (in the dark) with an engine that won't turn over, is unpleasant.
So, I power my converter using the 24vdc trolling motor batteries.
I bought a dc to dc converter (Pyle PSWNV480) which converts 24vdc to 12vdc.
The more you practice, the luckier you get.