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.
That bit is incorrect rubbish on the other forum, apparently from someone who knows enough to be confused. (I've been in that state plenty of times!)
RMS vs peak and the square root of two have to do with averaging a sine wave and having the equivalent steady-state value over time--integrating over time, basically. AC voltages and currents are generally expressed as RMS values, and these are related to the peak values by the conversion factor. They're expressed as RMS values precisely because it enables one to use standard basic DC laws to compute things like power or Ohm's law and get the correct answers for resistive loads (i.e. if the power factor is 1.0). To do otherwise would basically require calculus rather than arithmetic to figure these things, as indeed often is the case with non-sinusoidal waveforms.
AC power in watts is AC Volts (RMS) times AC amps (RMS) for a resistive load. An AC watt can of course accomplish as much work in a given time as a DC watt, since they're both just watts.
brulaz wrote:
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.
RMS watts in HiFi equipment mean just the steady-state power that can be produced with a sine wave input, usually over the frequency range of the equipment and with some specified maximum amount of distortion. Sometimes "peak" wattages are thrown around which basically mean as big a number as possible that cannot be proven to be greater than the instantaneous transient power produced (or withstood, for a speaker) under ideal conditions, regardless of distortion and so forth.
For e.g. an electric heater, the power consumption is just the wattage, and would I suppose be RMS watts if you wanted to integrate the instantaneous power consumption over the course of one or more AC cycles. Nearly always for power we're talking about more or less steady-state operation, so such integration is assumed.
If the inverter is 100% efficient, by definition the power going in will be the same as the power going out, and your DC power consumption will be exactly the same as the AC power output.
It may help to think of an inverter as a sort of electrical transmission. If you put 50 horsepower into first gear, you get 50 horsepower out the other end, albeit at a different speed. In practice, of course, neither an inverter nor a transmission are perfectly efficient, but "lose" some of the power to things like heat due to friction or electrical resistance or whatever, and so the useful power output is a bit lower than the input; you may need to put in 51 horsepower to get 50 horsepower out.
As a rule of thumb, for a 12V inverter, the DC current required being one tenth the output power (which implies about 80% efficiency) is handy.
