it's weird, but legal for some reason. Giving back energy to the grid can cost money. Shy of just stacking a bunch of batteries, what could I do with the spare summer sunlight?
I'll second heating water. A water tank is a battery. Alternatively, if you want to get really techy with it, you can make a refrigerator that runs on ice.
Basically, it's the opposite of a water heater. When you have extra energy, you freeze water. When you don't have any extra energy, the ice you made keeps your food cold. How you freeze the water doesn't matter. Most efficiently is a heat pump like a normal fridge, though.