"meirman" wrote in message
...
Why are some transformers and power supplies, rated in VA and some in
amperes, and some, I think, in watts?

For example,
1) I have a doorbell transformer whose secondary is rated AC 10V 5VA.

2) And I have a plug-in little-black-box transformer about the same
physical size that is rated AC 9.5V 750mA.

I guess for the first one, 5VA is the same as 5 watts?,
and the second one is .75 x 9.5 = 7.125 watts.

Is it that simple, or am I missing something?

It surprises me a little because the first one is heavier, but what
really confuses is me is their rating one as VA and the other as mA.


VA is only equal to watts if the power factor is 1 (resistive load) but if
you have an inductive load (such as a doorbell) or a capacitive load, then
the watts will be lower than you would expect by measuring the current.