Hi all. In a project I'm working on I'm needing to model a DC circuit to find the required power sources/storage to keep the system voltages up to a sufficient level. There's a solar cell array, a reversible fuel cell stack, various DC "appliance" loads, and wire runs with resistances. I dug up some Kirchoffs' law code that I wrote and tested ages ago, but I'm mainly thinking now about the "appliance" loads at the moment Often when you see a device it'll be rated for a certain voltage range and will have a specified current or power draw. But what I need for Kirchoffs' laws is internal resistances. Now, I can just get that from V=IR. Say we had a device rated for 10-15V DC with a power draw of 150W. For 15V we get the following: 150W: 150 = V * I = 15 * I; I = 150/15 = 10 V = I * R; V / I = R; 15 / 10 = 1.5. So 1.5 ohms. Now for 10V: 150W: 150 = V * I = 10 * I; I = 150/10 = 15 V = I * R; V / I = R; 10 / 15 = 0,667. So 2/3 of an ohm In short, we find double the internal resistance for the device if I assume the upper end of the voltage range rather than the lower end! But surely that's not going to happen in the real world, right? Would the safe bet be to assume that the power rating is only valid for some "nominal" voltage? Or should I use the upper or lower bound in calculating the internal resistance? I'm also pondering how I should determine the wire diameters/gauges. Right now, all that comes to mind is picking some initial values and then either manually or automatically iteratively converging to diameters that keep all "appliance" loads above their minimum voltage rating at all times and don't exceed their current ratings. An iterative refining approach is also all that comes to mind for determining the sizing on the solar cell array and fuel cell stack. Am I thinking about this wrong?