wrote:

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It says on it, 400 Watts continuous, 800 Watts Peak. Your calculation
matches the formula you posted. In that case, if it needs 33.33A to
operate at full load, that built in 25A fuse wont handle it.

But I connected it to my car with NO load. Why did it blow the 15A fuse
in my car? Obviously it's not dead shorted. My multimeter shows around
500ohms on the 12V input, (which climbs over time).

Since I posted this message, I connected it directly to a car battery
using jumper cables. It threw a large spark (with power switch turned
on), as I touched the cable to the battery. Then it appeared to work. I
did not attach any 120VAC device to it yet, because the weather is too
rainy to mess with it outdoors. But the LED indicator showed it to be
working.



** Likely an "inrush" surge with the first cycle of drive to the output transformer. You said it was "old" so I bet it's heavy too, using an iron transformer to produce 120V AC output.

When 12V DC is applied, the drive circuit applies it directly to the low voltage primary causing the core to briefly until steady AC drive conditions arrive.


..... Phil