AC Watts vs DC Watts

brulaz wrote:
1.414 is the SqRoot of 2, which, I was told on another forum, gives an "approximation" of the AC RMS Watts to DC Watts.

DC Watts = Voltage X Amperage
AC Watts expressed in terms of DC = Volts X Amps / sqrt2

RMS Watts (Root Mean Square - the sqrt2 in the AC calculation) is so you have a direct correlation to DC Watts otherwise you'd end up not comparing apples to apples.

Let me rephrase the question:

So to run a 1000W AC RMS load do I need 1414 DC watts?

And does that mean my DC cables should handle an ampacity of 1414/12=118 Amps? (assuming 100% inverter efficiency and battery V drops to 12V)

Or are inverter wattage ratings not RMS?

Thing is, I've only heard of RMS watts in discussions of HiFi equipment and speakers. Never w/r to inverters or other household items. So still confused.

No, you don't need the 1.414 factor, it's confusing you. If you know the current draw of the load mulitply it by 10 and that will tell you pretty close the DC current draw and then figure the wire size.

If you don't know the current draw but know the AC watts, 10X will get you in the ballpark for things like a toaster, coffee pot, etc. For a microwave it's current draw is higher than the watts listed, use 12 or so for the current draw.

These may be off a little, but will give you a good indication of wire size needed and ability of inverter to drive the load.