This
https://en.wikipedia.org/wiki/Impedance_matching wikipedia article is of interest.
I need to caution you in reading the article though. This quote:
[quote-wukipedia]Devices intended to present an apparent source resistance as close to zero as possible, or presenting an apparent source voltage as high as possible. This is the only way to maximize energy efficiency, and so it is used at the beginning of electrical power lines. Such an
impedance bridging connection also minimizes
distortion and
electromagnetic interference; it is also used in modern audio amplifiers and signal-processing devices.[/quote]
I think, is much more relevant. An ideal voltage source has an output Z of zero and an ideal current source has an output Z of infinity.
An indirect way of measuring a DC impedance is to place a variable resistor in series with the output and adjust that variable resistance until the voltage is 1/2. Take the resistor out of circuit and measure it. That will be the DC output resistance. That's not practical for DC power.
If you have a few I-V pairs, you can fit a line through those pairs. The slope will be the resistance.
An electronic load can operate at constant voltage, current power or resistance.
In AC sinusoidal signals, the cosine of the angle of the phase difference between voltage and current is the power factor. For pure resistive loads, it is 1.
You have three powers: Real, reactive and apparent power. See:
https://en.wikipedia.org/wiki/AC_power
Max power point tracking in solar cells is an entirely different animal. A solar cell has a max power point and that's where you want to operate.
I'm not sure where the max power point of a thermal electric generator (TEG) is. Highest temperature difference or some particular temperature difference.
I could be missing something too.