In article ,
"The Shadow" wrote:

wrote in message
...
I would be grateful for the advice of one of our technically minded
members:
The new radio I have coming merely states it's power consumption as
210VA, as it is a multivoltage radio.

What consumption in watts/amps does this translate to at 240 volts AC
mains?
Also. if I used an inverter to drive it off a 12 volt DC battery, what
would the power consumption in amps then be?

POWER CONSUMPTION IS IN WATTS NOT AMPS

A friend who helped me wrote:
Watts = Volts * Amperes so 210VA = 210 Watts
NOT TRUE - WATTS = V*A*COSINE OF THE PHASE ANGLE

210 Watts means 210 / 240 Amps which is .875 Amps.

If you want to use a 12V battery, 210 Watts means 210 / 12 Amps or
17.5 Amps. This assumes the AC supply is as efficient as the DC
supply. It won't actually be quite as efficient, so you're likely to
need fewer Amps, maybe 14 or 15 but that's a guess. Maybe you can ask
as the DC current depends on their design which is not known to us
yet.

Those numbers look very high to me. I suspect they've made a little
mistake.

I replied:
it does seem very high to me for 210VA. at 240 volts AC mains.
As just a poor guess I would estimate
0.5 amps at 240 volts AC mains
and 8 amps at 12v DC
but I cannot calculate it.
_______________________________

So what do the experts out there think?

Would be grateful for your help.

WHAT KIND OF RADIO IS IT -- 210 VA IS TOO HIGH FOR A RECEIVER -- MORE LIKE A
TRANSCEIVER

Volt-Amperes VA usually refers to AC apparent power - it takes into account
reacive components see URL:
http://www.allaboutcircuits.com/vol_2/chpt_11/2.html

True power in an AC circuit is IxE if there are no reactive components
otherwise it is I squared R or ExI x Cos theta

So True Power (Watts) does not equal VA unless the circuit is purely
resistive.

Assuming your device is resisive (or near to it) then see URL:
http://www.sengpielaudio.com/calculator-ohm.htm

which indeed gives 0.875 Amperes

Again using
http://www.sengpielaudio.com/calculator-ohm.htm

which indeed gives 17.5 Amperes

Allowing for efficiencies and loss etc I would allow for 20 to 25 amperes
Most inverters will specify a power rating

As you noted above 210 Watts is to much power for a receiver. Chances
are then the VA the OP refers to is apparent AC power or peak power. In
order to answer the PO's question if we assume AC 210VA peak power I
have found that for most power supplies, which are full wave rectified
followed by larger capacitance a figure of about a third works well.
Then the AC real power would be 74 watts. Then to get to DC we need the
RMS value of the AC real power, which is a factor of 2.828 so about 26
watts RMS. At 12 Volts to drive the inverter would be about 2 amps DC.
I'm making plenty of assumptions here. You will be somewhat more than
2.1 amps depending on the inverter efficiency. Lets use a figure of 80%
so the battery current draw would then be about 2.5 amps. 26 watts and
2.1 amps are rational numbers for todays solid state receivers. A lot of
assumptions but it seems to make sense.

--
Telamon
Ventura, California