Current, Voltage and Power dependencies
The graph is showing you that the load cell has a significant terminal resistance. That means you lose volts at the terminals of the load cell the more current you take.



simulate this circuit &ndash; Schematic created using CircuitLab
This is typical of any power source.
Based on about a 10V drop at 20A, that gives you a ballpark resistance of 0.5 Ohms.
As for your power requirement. You use the graph to find the crossover point at which the thing will nominally be operating at the power you are requiring.

So for about 1000W, the voltage will be around 35V and the current will be @28A. Your load therefore needs to be @1.25 Ohms.