Re: Power Measurent: Watts Vs. Volts

Batteries are rated by their terminal voltage AND by their capacity,
measured in Amp-Hours. Many AAA, AA, C, D cells have their capacity
printed on the side, as well as their terminal voltage. Since the
terminal voltage is known, and the capacity is known, the total
available energy is also known (Volts x Amps x seconds = energy, in
Joules). This is a simplistic answer -- the actual available energy
is different from V x Capacity x time, because the terminal voltage
drops with: current draw, rate of current draw, time, temperature, and
other environmental factors.

The rate at which you can draw that energy (Watts) from a battery is
also more complex -- if you need to know that you should consult the
manufacturer's data sheets. Automobile batteries list their "cold
cranking Amps" -- that is a pretty good indicator of how much power you
can draw from it (CCA x V).

In measuring radio emmision energy, the measurement is the electric
field strength. The units are Volts/meter (or microVolts/meter).
Power in free space is measured in Watts/meter^2.

Only when the RF energy is converted to RF current through an antenna,
can direct power be measured. Then other factors must be taken into
account: antenna impedance, antenna gain, antenna type. For a given
field strength different power levels may be obtained depending on the
antenna.

.