A lot of the following is (hopefully) "applied common sense 101". There is a fair bit of empirical twiddling suggested (a bigger capacitor here causes ... / a longer tome constant helps xxx but makes yyy harder ...). While this may seem to be far more complex that "just using a bench power supply" the same considerations apply whatever is used. If the power supply was created by ascended-masters such as HP or Tektronix it may already be able to deal with fed back noise and rapid current variations. If it was created by lesser-mortals as are many of the cheaper bench power supplies, it may be susceptible to load induced issues without this being apparent. I have seen the voltage indicated on two meter (current and voltage)supplies increase very substantially as loading was varied even though the supply was never in current limiting and voltage should have been constant and in fact more or less constant. Adding noise filtering between supply and load tends to fix such problems at the potential cost of adding "burden" resistance. This may be able to be overcome. See below.
The term "burden voltage" is often used to refer to the voltage drop across an ammeter. In tyhe examples below there is ZERO burden resistance.
Simple method: Input power can be measured adequately well by ensuring that the operating voltage is what is desired and then measuring the current in a manner that produces zero "burden voltage". Below are a simple and an even simpler way to achieve this.
The first diagram will require a few bits and pieces to finish it off (mainly a few capacitors) but is close to usable as is.
R1C1 and R2C2 are simply noise filters for the meters used. The requirements are discussed below.
Magic. Of sorts. R_Isense is used as a current sense resistor. Because the current is sensed before the voltage regulator of IC1/Q1 the voltage drop across it is unimportant. As long as Vin is adequate the drop across R_Isense may be 0.1 ohm or 1 ohm or 10 ohms or more. There is zero "burden voltage" - the voltage drop across the sense resistor is not reflected in a change in output voltage. Burden voltage = zero.
Rather than using a resistor at R_Isense an ammeter can be used. This also does not affect the output voltage and burden voltage is zero.
If the circuit switches between a sleep and awake mode with currents in the microamps range in the first case and 10's or 100's of mA in the latter I find it useful to use an ammeter set to auto-range io place of R_Isense OR an autoranging voltmeter across R_Isense. This allows current in either mode to be displayed and again/still there is zero burden voltage as the meter is on the input side of the voltage regulator formed by IC1/Q1.
Q1 and IC1 are a basic voltage regulator. The aim is to hold Vout at the same voltage as Vr. Say +5 VDC or whatever. To keep the very basic operation of the circuit clear I have not shown any noise filtering on Vout or in the opamp feedback loop, as discussed below. Filtering can be as heavy as is needed to get a clean Vout and as minimal as required to maintain response to load steps. A larger capacitor across Vout will make maintaining voltage easier BUT will prevent rapid current variations being seen across Isense. If Vout rises above Vr then op-amp output goes low turning Q1 off and reducing Vout as required. As shown the opamp is a comparator with open loop action and no feedback. While this would work OK, the user may wish to give the opamp finite gain by using negative feedback. An N Channel MOSFET is sused but this could be a P Channel MOSFEt with inverted drive to the opamp. Q1 could be bipolar but there is no obvious advantage in not using a MOSFET in typical cases.
As shown the noise from the buck regulator may (will) disrupt the opamp feedback loop. A capacitor can be added across Vout to source current peaks and rapid variations and reduce smps noise. A filter as per R1C1 nd R2C2 can be added between Vout and inverting input to reduce noise that may affect the opamp. An RC filter to the inverting input with a 1/time-constant several decades below the smps switchiong frequency should suffice. eg if the buck regulator operates at 100 kHz then a filter frequency of <= 1 kHz is a good starting point. eh 10k, 0.1 uF.
time constant t = RC = 10,000 x 1E-7 = 0.001 or Frc =~~~ 1 kHz.
Once you get Voltage supply "stable enough" as load varies you get some free magic. Supply current flows through R_Isense. Load current can be determined by measuring voltage across here. Thje more voltage you allow to drop across R_Isense the more accuracy (actually resolution) is available for determining current. If say I_load max = 100 mA. If R_Isense is 10 ohms it will drop 1 volt at 100 mA. If R_Isense = 100 ohms it will drop 10 Volt at 100 mA. Obviously Vin has to bve large enough to allow this. A 4 digit voltmeter will allow you to resolve 0.1 mA steps at 100 mA full scale. If available a 6 digit voltmeter of whatever accuracy it happens to be will allow you to resolve 1 uA steps. A meter with 6 digit "accuracy" is unlikely to be available. The use of a multi ranging meter, as mentioned above, effectively gives high accuracy and resolution.
An annoying to use but simpler and super low cost solution is as per the diagram below.
This is functionally equivalent to the prior arrangement but uses no active electronics and again has zero effective burden voltage.
Current is sensed with R_Isense or an ammeter at this position and Vout is then measured with the meter at Vout2. Filtering is often crucial for correct meter operation. As voltmeters are used R1C1 and R2 C2 time constants can be as high as required to remove smps noise at the expense of loss of response time.
Load power measurement is "more of the same."
Voltage measurement with filtered meter to reduce smps noise enough. "Enough" will vary with manufacturer and noise level but is "easy", as above
If Rload is constant Power out can be inferred.
If Rload is dynamic then a current sense resistor or equivalent is needed. Again - "adequate" filtering is essential - with "adequate depending on immunity of meter to smps noise.