We've got two 50-gallon natural gas water heaters (piped in series with one another -- which my plumber says is something they'd never do any more). In the summer, they account for the bulk of our gas bill, which will hover around $25/month. I'm looking to replace them with a new high-tech hybrid tank or a tankless, either of which would be radically more efficient. So storing excess electricity in hot water isn't particularly attractive.

What is attractive is over-air-conditioning the house. While the usual environmentally friendly thing to do is to run the AC less during the day when you're not home, then have it come up to speed when you're on your way home, I could instead keep the house cool all day, perhaps even cooler than I want it when I'm home, consuming a big chunk of my excess solar production. This is better for the house, in some regards (cooler air = drier air => less mildew/mold), but would put more wear on the AC compressor/motor.

If I were super high tech, I'd bury a cold water tank in the backyard and I'd store chilled water, for the AC to use whenever I needed it. Chilled water distribution is commonly used in commercial system, where they might chill water all night, when power is cheaper, and then use it all day. If I were building a house from scratch, I'd look into this. Needless to say, it doesn't appear to be an easy retrofit.