Depends on what you want to find out.
On the first tab, you'll punch in how many amp that would go through your cable; how long the cable are, one way ( the program calculates for 2 wires.), and how many volts you will accept as loss in the cable.
(Do not try 0 volts loss, because that's an impossibility.
For example, a short run of 1 metres, try 0.2V; then try 0.1V and see the difference of minimum cable area...)
The result will be the minimum size of recommended cable size, and how much loss in Watts in the cable at given Amps.
On the second tab you can calculate cable loss for your existing cables. Enter current Voltage, size of the cable, length of the cable (one way; the program calculates for a 2wire cable), and how much Amps that's running through the cable.
The result are given in Volts lost, Volts at target; Watts through the cable, and loss in the cable in Watts.
That should be it.
Edit:
You don't have to enter numbers on both tabs; only on the tab you're interested in the result.
