How to measure the voltage an appliance is actually drawing when connected to a solar panel?
If I understand you correctly, you have a solar panel connected to a battery (technically, a whole battery/charging circuit combination) and you wish to measure the voltage while it is receiving sunlight.

This is as simple as setting the multimeter to measure volts and attaching the meter in parallel with the load.

After looking at the solar panel and that power bank, it seems that the challenge may be finding points to probe while the unit is operating. You may have to make a short passthrough cable (female to male DC jack/plug with cable in the middle) with probe points so that you can measure voltage without destroying the existing cable. This may also prove handy if you decide you want to measure current (you can cut the cable and put your meter in series with the power bank).

Edit: in more detail, here's how I would do it. Electrically, you have the schematic below. The schematic editor makes it brutally difficult to show connectors and cable assemblies though, so I'll have to describe the setup. Basically, get yourself a female DC power jack that mates with any of the plugs that come with the solar charger, and a male power plug that mates with the power bank. Solder a cable between them, making sure to get the correct polarity (you can check with your multimeter!), and leave the terminals on at least one end somewhat exposed. Then you can insert that cable between the charger and power bank, and probe the voltage while the solar panel is charging the power bank. 







simulate this circuit &ndash; Schematic created using CircuitLab