With a windmill you connect the load to the battery because you're not trying to eat the power as it is generated - maybe occasionally it's coming in too fast for a few seconds and you can't dump enough, sometimes it's coming too slowly and you can use that opportunity to catch up on what you missed (or just turn the dump load off when you've dumped enough) - you're just trying to keep ahead of the mill on the average. Also, if your dump load is doing something useful you give it clean power.
Windmill dump loading is like a reservoir:
- Windmill is the incoming river and rain, with wildly varying flow.
- Battery is the dam and reservoir. Let it get too full and the dam is damaged.
- Dump load is like draining off excess water before the reservoir gets too full and the dam breaks. Unfortunately it's not like a spillway, which gives higher flow with more water. It's like a fixed-size drain. So you turn it on when the reservoir is getting just to the desired level and try to keep it from being too overfilled too long.
You could use the same system for solar. But solar has advantages: You can safely short the panels upstream of the blocking diode and let the panels themselves be the dump load, getting slightly hotter than they would if they were delivering power (but no hotter than if they were just sitting in the sun unconnected). The max current is limited by the sun's illumination. So if your switching transistor can handle that current you don't need the dump load resistor-or-whatever, don't have to dissipate the heat somewhere, and don't have damage to the battery if the (nonexistent) dump load resistance fails.
